LRF or attempting them

[johnholmes]

Thank you Mr Holmes, a top class read as always. :o

well hello Mr Ipswich, I'll drop you a pm sometime tomorrow

[Solar Cycles]

Always look forward to these LRF of yours John, again a top forecast, which is easy to digest for all.

[johnholmes]

No 2 lrf on 13 February for 26 February-6 March

and into Spring we go!

Tropical connections

There has been no further IRI update since the last one in lrf 1.

The noaa update on 9 February said this

•Atmospheric and oceanic conditions reflect La Niña.

•Negative equatorial SST anomalies persist across the central and east-central Pacific Ocean.

•Based on recent trends in the observations and model forecasts, La Niña conditions are likely to continue into Northern Hemisphere Spring 2009.

AO

The predicted fall on 8 February issue has occurred and over the past week the AO has shown a more definite suggestion of a rise towards zero with some suggestion of another fall later, towards the third week in February.

NAO

The NAO has fallen somewhat, as predicted, and is now expected to stay just below zero although edging a little more upwards.

The 30mb temperature

I normally take this at the end but so much interest has been generated since it moved rapidly upwards around 20 January that I'll take it here.

It has dropped quite a lot over the past few days but the very latest shows a slight increase again. It is still well above the average value for this time in February. In my estimate it will not be at that level until about the same time as the AO looks to be predicted to back at zero. Whether the two are connected I do not know.

NOAA 500mb anomaly charts

The first one shows the expected situation between 15 and 21 February, just a couple of days ahead. I'll make further comment about this under the heading GFS and ECMWF T+240 and beyond.

The one on the right is for 18-24 February and again I'll try and link that in to standard forecast chart for the mid point of those dates.

These charts have continued to show a +ve anomaly west to west south west of Alaska. A second one has been centred just west of Greenland with an extension showing, at times, towards the south west of the UK. The main centre has tended to edge a little further east. The overall 500mb flow showing a deep 500mb trough over the eastern side of north America with the ridge just west of the UK tending to edge slightly further west.

The ECMWF and GFS charts, the latest on the right is for 22 February, a similar time to the NOAA version above on the right.

Are there any similarities?

Both ECMWF (the left hand of the two charts each time) and GFS show the +ve anomaly in the Pacific along with the –ve anomaly east of it. Both also show the +ve area in mid Atlantic up towards Greenland. The actual 500mb flow of each is about the same, from a point north of west, over the UK, and each of them appears also to have edged both these ridges slightly further west. So a good deal of similarity between the various versions of 500mb flow and the height anomalies.

Here I will, for ease of comparison insert the ECMWF and GFS T+00 and T+240 charts to compare with the NOAA, ECMWF and GFS 500mb anomaly charts and 500mb flows predicted to see how they each compare.

noaa comments.

Not easy to be even subjective so bear with me if you see things differently.

Only GFS is available for Saturday 14 February from the 12z today.

All I am trying to compare is the 500mb flow. On the GFS chart below this is indicated by the black line, the 552DM line.

Some similarity, in that they all show a 500mb ridge pretty well over the UK with a marked trough south west of Greenland. Not a perfect match but not too bad, so perhaps the initialisation of GFS on this run is okay.

Turning to the GFS for a similar date to the charts on the right, for 22 February or around that date, what do we have?

ECMWF below

The first comment is that has a lot off differences from its GFS version above on the right. Has it any similarities with the anomaly charts above, GFS/ECMWF/NOAA?

It is a flatter 552DM line than GFS but is not that different to both ECMWF and GFS anomaly charts. So in that instance it is less like the NOAA chart which has quite a more marked 500mb ridge south of Iceland and south west of the UK.

Overall both suggest below average temperatures for the UK but nothing dramatic.

NOAA comments over the past week

Their comment this evening sums up their views in regards to model performance and marking along with the general upper air pattern from the Pacific out east of the States.

FAST ZONAL FLOW IS EXPECTED TO DOMINATE THE SOUTHERN PART OF THE COUNTRY. TROUGHS ARE FORECAST NEAR THE PACIFIC NORTHWEST EXTENDING SOUTHWESTWARD TO NEAR HAWAII, AND OVER EASTERN CANADA, WHILE RIDGES ARE PROGGED OVER THE NORTHERN ROCKIES/NORTHERN PLAINS, SOUTHERN MEXICO, AND AN AMPLIFIED RIDGE NORTHWEST OF HAWAII. The PNA INDEX WHICH RECENTLY HAS BEEN NEGATIVE IS FORECAST TO REMAIN NEGATIVE THROUGH DAY 14. THE NAO INDEX WHICH RECENTLY HAS BEEN WEAKLY NEGATIVE IS FORECAST TO REMAIN WEAKLY NEGATIVE THROUGH DAY 14.

and mark the whole period out to day 14 as 4/5.

Overall Summary

Yet again the signals from the various factors taken into account are not in total agreement by any mark. The first one to comment on is the 30mb temperature. I'm totally new to this and like many others who may read this not really sure what to make of it. Other evidence suggests that the Polar vortex is not in the right place for any major blocking in the north Atlantic or indeed north or north east of the UK. How that may be brought about is open to much discussion. It does seem unlikely that the very marked warming will not be without the blocking in those areas sometime over the next 2-3 weeks before it goes negative again, assuming it does as I've suggested above.

The overall upper pattern suggests that over this period colder rather than milder is most likely with a flow generally in the upper air north of west. How far north of west, or indeed east of north is something I cannot predict.

Forecast Summary

On the above evidence I would go for temperatures below their late February/early March average for the whole country. This for most of that period. In terms of extremes I would suggest 75% for the cold side and 25% for the mild side.

Again I'm unsure if there will be a major outbreak of deep cold air from the necessary directions, north west to east. Snow and frost are probably going to end up about or just a shade above the 1971-2000 average, especially in northern and eastern areas. The possibility of some kind of major low is I think no higher than 25% so March may come in not too much like a lion in terms of wind.

Much depends on IF there is major blocking related to the fairly unusual, for recently, 30mb temperature warming, and the relocation of the Arctic vortex as a result.

Edited February 13, 2009 by johnholmes

[Snowaddict]

An excellent read as always John.

Very understabale all the way through.

Fair play to you for producing such a comprehensive forecast, given all of the current conflicting background signals.

[ukmoose]

Extremely comprehensive and very informative as always John. Great to see an old pro at work. (And I mean that in the nicest possible way!)

Thanks for all that effort.

Moose

[johnholmes]

LRF No 3 issued Friday 20 February for the period 5-15 March 2009

Taking the usual list of possible cause/effects. This time with the 30mb temperatures first. Below is the latest one showing temperatures falling again but still well above the average line.

In spite of this there is absolutely no sign in any other signal of any cold affecting the UK other than very transitory. Obviously the question has to be why? I confess I have no real idea but would expect that several of us will be pondering this question over the coming months. It worked quite well 3 times previously, 2 for cold and one for a more disturbed less cold spell.

Overall comments on global patterns from NOAA for March

PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK

NWS CLIMATE PREDICTION CENTER CAMP SPRINGS MD

830 AM EST THU FEB 19 2009

30-DAY OUTLOOK DISCUSSION FOR MARCH 2009

SEA SURFACE TEMPERATURE (SST) ANOMALIES IN THE EQUATORIAL PACIFIC HAVE RISEN

SLIGHTLY OVER THE EASTERN PACIFIC DURING THE LAST SEVERAL WEEKS WHILE REMAINING

FAIRLY CONSTANT OVER THE WESTERN AND CENTRAL PACIFIC. LA NINA CONDITIONS

CONTINUE TO BE PRESENT. THE SST ANOMALY FOR THE NINO 3.4 REGION (170W TO 120W

AND 5N TO 5S) IS CURRENTLY ABOUT -0.8 DEGREES CELSIUS. SST FORECASTS FOR THE

NINO 3.4 REGION INDICATE GRADUAL WARMING OF THE CENTRAL PACIFIC THOUGH LA NINA

CONDITIONS ARE EXPECTED TO CONTINUE THROUGH AT LEAST MARCH 2009. ATMOSPHERIC

CONDITIONS IN THE TROPICAL PACIFIC OCEAN ARE ALSO CONSISTENT WITH A LA NINA

EVENT. SUPPRESSED CONVECTION HAS CONTINUED NEAR THE DATE LINE THROUGH THE LAST

MONTH AND LOW-LEVEL EASTERLY AND UPPER-LEVEL WESTERLY WIND ANOMALIES ARE ALSO

PRESENT OVER THE EQUATORIAL PACIFIC OCEAN. THE MADDEN-JULIAN OSCILLATION (MJO)

HAS WEAKENED IN RECENT DAYS AND CANNOT BE CONSIDERED IN THE MARCH FORECAST.

and below the IRI assessment on the same date

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -0.7 C, indicating weak La Niña conditions in the tropical Pacific. What are the probabilities for achieving a La Niña event (i.e. an "event" refers to La Niña conditions persisting for approximately 5 months) in early 2009? Persistent and large-scale easterly wind anomalies along the equator in the western-to-central Pacific since early October now have become coupled to changes in the upper ocean, but only after two to three months of relative oceanic unresponsiveness from September to November 2008. The thermocline, which constitutes the main element of the tropical Pacific system imparting ENSO predictability, is shallow in the eastern part of the Pacific, but deep anomalies in the west loom ready to reverse the current cooling.

It is possible that the negative anomalies in the east-central Pacific could persist or even strengthen slightly as the recently stronger sub-surface anomaly structure influences the surface in the coming month. However, the intensity of enhancement of the Trade Winds has not increased, and even appears to have slightly weakened, since its late-November maximum and recent late January secondary maximum. Additionally, as the ITCZ moves closer to the equator, the Trade Winds weaken and the seasonal warming of the eastern Pacific ensues, it will become more difficult for the atmosphere to maintain cold oceanic conditions. The models, both dynamical and statistical, suggest a tendency towards neutral conditions beginning from the first season.

Currently, the models indicate probabilities greater than 50% for La Niña conditions, and very little possibility of developing El Niña conditions for the Feb-Mar-Apr season in progress. All things considered, during the Feb-Mar-Apr season there is very nearly a 60% probability for La Niña conditions, 40% probability for ENSO-neutral conditions, and a near-zero probability for El Niño conditions.

Finally the CFS chart for 2m temperatures shows the whole of Europe –ve in March, with the UK around -0.5C from average; areas in the north of Europe/Scandinavia are -2C or more.

AO

These show that there has been quite a change in the prediction over the past few days. Much more +ve than it previously showed.

NAO

The NAO seems to be going the opposite way, showing more –ve than previously but still above the zero line for most ensembles.

If you look further at the AO, the part of the link shown every day with the accuracy marking its clear to see that 7 days is pretty accurate but the accuracy falls of beyond that.

NOAA 500mb anomaly charts

On the left side an early issue in the last week with the one from Thursday on the right.

Both continue with the idea of the +ve anomaly, that had been showing around 50N in the Atlantic to disappear with a smaller +ve just off western Greenland. The large +ve area south west of Alaska persists. A –ve area close to the western side of the UK is a trend that they have shown over the past week. If anything the 500mb flow is predicted to back very slightly to just about south of west. The overall pattern forecasts have, that is the 500mb flow, in my view, been quite accurate at the time scales involved through most of the winter period.

ECMWF and GFS version

On the left the version for 26 February several days ago and the one from this morning on the right (for 2 March)

GFS now has a more ridgy type of flow for the UK with ECMWF showing less of that and almost a flow south of west. This has been its position over the last 3-4 runs. Differences also show in the Greenland area. If anything the ECMWF favours the NOAA pattern rather more than GFS does.

NOAA comments

They were happy to accept most model guidance for a ridge around 50N 30W into the back end of February. Their latter comments suggest that the flow over north America is likely to be fairly zonal and with no comment, understandably, about what happens to their east.

GFS and ECMWF charts

In the time scale being dealt with here, March, then they have shown variations with one another and themselves, so nothing unusual in that. The overall idea does seem to suggest an Atlantic flow with their 500mb charts showing a mix of south of west and north of west but little, if any, indication of a northerly other than the odd run showing a very brief one behind a surface low. That said, GFS has on its 12z run today with support from the other 2 out to 144 shown a flow from a northerly point. It then carries this on, using a surface high over western USA at T+48, to end up beyond T+216 with a surface and upper ridge quite well north of 50N giving a cold north west pattern for the UK. How realistic this is we all have to wait and see as its tried this on several occasions in the last 10 days.

Summary

Going beyond 5 March, some 2 weeks on, is as usual at this range pretty dicey with not a lot to go on other than some of the more exotic type charts, by this I mean upper atmosphere type. They still indicate the warmth at high level and that it does appear to be finally propagating down to nearer the earth.

Forecast for 5-15 March

The flow seems about 70/30 to be more Polar than Tropical for most of this period. So colder than average for most for much of the time. Briefer interludes when a surface low tracks in from the Atlantic as at times it looks reasonably mobile. Behind each one, probably with a preferred track north of Scotland, then colder with snow and frost for many areas away from the south, chiefly but not exclusively for higher ground for snow. Trying to predict if there will be a major polar outbreak is difficult as the signals available are mixed. I would say its about 40/60 at the moment. Currently there seems to be only a very low, 15% or less, chance of any major storm.

Edited February 20, 2009 by johnholmes

[johnholmes]

No 4 issued Friday 27 February for 15-25 March 2009

Taking the usual list of teleconnections etc

Tropical, as of 23 February from NOAA

•Atmospheric and oceanic conditions reflect La Niña.

•Negative equatorial SST anomalies continue to persist across the central and east-central Pacific Ocean, but have recently weakened.

•Based on recent trends in the observations and model forecasts, La Niña conditions are likely to continue into Northern Hemisphere Spring 2009.

AO

The predicted increase, on 20 February, his occurring. The predictive accuracy over 7 days is quite good, thus the predicted fall now being shown on the issue for today seems quite plausible.

NAO

Here it predicted a rise, slight, but it soon levelled out. Its predicting a fall but less pronounced today than over the past week.

500mb NAA anomaly charts

The +ve anomaly east of the states has moved around in its predicted position over the past 7 days. From a centre around 50N 40W to just south of Greenland and latterly between Canada and Greenland. The advice its given over the past 3-4 weeks has not been as good as in the previous weeks of this winter. Below are two charts towards the start of the forecast period. 4-10 March on the left and 6-12 March. Both show the +ve area west of Greenland and a –ve area over and near the UK. Again both suggest a 500mb flow, with below normal contour heights, from north of west into the UK with an upper trough to the east.

ECMWF and GFS similar charts

There have been noticeable differences in how the two have dealt with things. GFS tending to make more of a possible upper ridge than ECMWF. Both though giving a flow north of west over the UK in the time period, again with somewhat lower heights than normal.

NOAA comments

This was their comment this evening and tends to follow previous comments on northern hemisphere patterns over the past week.

WELL ABOVE NORMAL HEIGHTS ARE ALSO ANTICIPATED FROM THE FAR NORTHERN ATLANTIC AND GREENLAND WESTWARD TO CENTRAL CANADA, WITH THE OPERATIONAL 00Z GFS EXTENDING THIS ANOMALOUS RIDGING COMPLETELY ACROSS CANADA.

Overall 10 day+ runs from GFS and ECMWF overall patterns shown

These have both shown similarities into about T+120-168 at times but thereafter have been rather different. This is not surprising given the differences shown over the past 7 days with their respective 500mb anomaly charts.

And finally

The 30mb temperature chart

As can be seen below its heading back towards the normal curve. As to what this means I must confess I do not know. There some who suggest that a return to below normal temperatures will somehow allow a renewed westerly wind to force, finally, the warming effects into the lower Troposphere. I do not know enough about this subject to give a view.

Summary

Aside from the 30mb comment most drivers at long and short time scales do seem to be more suggestive of a flow north of west across our part of the Atlantic. Whether there will be any significant/major northern blocking is unclear to me.

Forecast for period 15-25 March

It would suggest that below normal temperatures are more likely than above normal. Perhaps a rather typical March spell. Unsettled with some milder interludes but more cold spells than mild ones. Relatively windy at times, especially in the north. Frost and snow about average to a bit above in the north to rather below average in the south. No sign of any major snow situation nor for that matter any major mild spell.

Edited February 27, 2009 by johnholmes

[carpo21]

John, I don't post on here much but follow all of the winter threads avidly, and have to say, top banana! It is very refreshing to have an ex-professional forecaster (is this accurate?) actively seeking/researching new methods of predicting weather. I think too many on here dismiss the value in researching what is still, despite super-computer processing and data input, a hit and miss type of science. I read what the likes of Stratos Ferric write, and can't help but think their time would be better spent analysing a science where the variables involved are absolute and irrefutable. Unfortunately, with meteorlogy, you can only really analyse once the weather's been and gone - positive-hindsight? This isn't really conducive to improving the longer term forecasting ability (beyond 3-5 days - 1-2 days for GFS ;o)). You, and other key members of this forum, are really trying to expand the envelope of forecasting - d'you know, I think, collectively, you may all be onto something. Keep pushing the boundaries (and sceptics)!

Edited February 27, 2009 by carpo21

[johnholmes]

thanks carpo and yes I was a senior forecaster with UK Met.

I enjoy the idea of trying to get a general outlook, it can really never be more than that, beyond the usual time frames, say beyond T+240 and sometimes beyond T+384. Its a fascinating aspect and certainly something I knew nothing about when in the Met O. I'd be interested to know if any of the latest ideas for lrf work are given in any of the many courses forecasters attend through their career.

memo to self, send e mail to contacts in the Met O to see if they can tell me anything?

The last two have been rubbish, a non technical term, for my last 2 lrf's. But overall about a 70% score over 14 months so I'm encouraged by that. I too read in detail some other posters on here with a good deal more knowledge than me in this sphere.

Together we can all learn from one another.

Okay the model thread gets heated at times but overall there is a good exchange of views and knowledge. The climate thread also generates a fair amount of heat at times.

Still we are all entitled to our views, if only some would make it a little less personal but never mind.

welcome to this site by the way. Lots to see, read and browse over.

Edited February 27, 2009 by johnholmes

[johnholmes]

Lrf No 5 issued Friday 7 March for the period 25 March-1 April

Tropical

Issued by CPC on 28 February-their summary of how events may develop through March

30-DAY OUTLOOK DISCUSSION FOR MARCH 2009

LA NINA CONDITIONS CONTINUE OVER THE EQUATORIAL PACIFIC, THOUGH SEA SURFACE

TEMPERATURE (SST) ANOMALIES CONTINUE TO RISE IN RECENT WEEKS. THE MOST RECENT

WEEKLY SST ANOMALY FOR THE NINO 3.4 REGION (170W TO 120W AND 5N TO 5S) IS NOW

ABOUT -0.5 DEGREES CELSIUS, THOUGH LA NINA CONDITIONS ARE EXPECTED TO CONTINUE

THROUGH AT LEAST MARCH 2009. IF THE CURRENT LA NINA EVENT ENDS, LA NINA

ATMOSPHERIC CONDITIONS MAY CONTINUE FOR SEVERAL WEEKS THEREAFTER. THE

MADDEN-JULIAN OSCILLATION (MJO) IS CURRENTLY WEAK AND NOT EXPECTED TO BE A

FACTOR DURING THE MARCH FORECAST PERIOD.

And the ENSO update on 4 March

•Atmospheric and oceanic conditions reflect La Niña.

•Negative equatorial SST anomalies continue to persist across the central and east-central Pacific Ocean, but have recently weakened.

•Based on recent trends in the observations and model forecasts, La Niña conditions are likely to continue into Northern Hemisphere Spring 2009.

AO

Just two charts, to save time downloading items, if anyone wants to see the intervening ones please pm me and I'll post them.

The first shows the current +ve with the last one showing the fall and a predicted minor peak before the majority of ensemble members suggest a fall below zero, most to levels not seen, (IF its correct), since early February.

NAO

As with the AO.

All along its been showing the slowish fall on both the plots, the predicted part that is.

Remember my comments on the last lrf. They are pretty accurate out to 7 days, less so after that.

500mb NOAA anomaly charts

The same thing with these chart just two and comments on how they have changed over the past week

On the left the 8-14 day issue for 10-16 March; well over a week before the start of the forecast period. On the right side that for the period 13-19 still almost a week before it starts. So its trends, remember, we are looking for.

Ignoring the +ve south west of Alaska, a semi permanent feature the whole of this winter (perhaps it is every winter?), then the major +ve area in our area of interest is that around southern Greenland with its subsequent creation downwind of the north west 500mb flow over the UK. This has been a consistent prediction, supported by NOAA comments over the past week for the time frame the two charts cover. Also the –ve area south of the +ve one and the –ve indication east of the UK. Reasonable confidence then that this pattern seems a feasible one. The output this evening (Friday) is very similar extending into 14-20 March.

ECMWF and GFS similar issue

Same procedure with these charts

On the left is the issue, 10 days ahead from 28 February for 10 days ahead=10 March, even further back from the forecast period. Remember its trends we are looking for to get an idea of reliability.

These charts have been much less consistent, both of them. Both have shown a largish +ve area west of the UK, sometimes north of about 50+N sometimes south of it. The trough in the 500mb flow shown on the left chart, east of the UK, has also come and gone and moved position. Currently ECMWF shows a large +ve near southern Greenland with a large –ve area south of it, giving a 500mb flow just north of west over the UK. GFS is back favouring the largish +ve area near south Greenland with a double –ve area south west and south of this. It has tended, through its changes, to keep a –ve area east of the UK as it has this time. The upshot is that the 500mb flow is much more north westerly than the ECMWF.

NOAA comments

They have stuck with their view, in our area of interest, that the 'block' seems reasonable near southern Greenland for their expectation of the flow over the States. Their belief in the models has been very high, rarely at 4 even for the 6-10 day period and sometimes 2 for the 6-10 and 8-14 day period. Last evening (Thursday) they rated them 2 then 3, so not that much belief it would seem. It should be added that much of this concern relates to how they see the models dealing with short wave features across their area of interest. The major pattern they seem content with out to 19 March, the furthest they go at the moment.

30mb temperature chart

This is now almost back to the normal curve, I'm quite pleased as some time ago I did suggest it would be well into the first week that this happened. What to make of it I must confess to having little idea!

Comments on the T+240 ECMWF and GFS charts and out to T+384 with GFS (now at 20 March)

Nothing changes with them, be it mid winter or mid summer. They differ often with one another and themselves. To be fair they have both, not necessarily on the same run, suggested that cold rather than mild was likely with blocking out to the north west or sometimes west of the UK. This has been so on several runs from GFS out into its furthest reaches.

Summary

Overall, to me, the pattern being suggested is more blocked than mobile and with an upper flow north of west into the UK and western Europe. Little sign of any pressure rise over Scandinavia. Quite the reverse, the indications are for an upper trough to be east of the UK and an upper ridge west of the UK, chiefly to the north west.

Forecasts for 25 March – 1 April

I would lean heavily; say 80-20% for cold rather than mild. As to whether its an overall cold and unsettled or cold and settled again, most of the time, unsettled, in a time scale of about 70:30.

It is getting well into Spring; we pass the change of clock times in this period so the sun is higher and the nights noticeably shorter.

Thus the probability of prolonged cold such as early February is very slight if at all. What is more likely is above average frosts, moderate n intensity at times, with above average days with snow falling. Lying snow, as the comments above show, is not something to expect in terms of days for lowland areas, especially out of central and northern Scotland. More in terms of perhaps a day here and there with snow on the ground for much of one day, anywhere could see this but the further north, on low ground, obviously the more likely this must be. It could well be a good spell for the Scottish ski resorts.

To sum up, cold rather than mild most of the time, with just brief milder interludes occurring. No sign, to me of really well above normal temperatures other than isolated days in the south. Little, at this range, to suggest any very windy spells although, as depressions run west to east, brief windy spells near a low centre are likely.

charts to be added asap

Edited March 6, 2009 by johnholmes

[Snowaddict]

Thank you very much John.

Clear, understandable and well reasoned as always.

Mark.

[johnholmes]

No 6 LRF issued on 13 March for 1-8 April!

Admin note

I seem to have got further and further out in the time scales over the past couple of weeks or so. These are supposed to start 10 days from the start date and go out 10-14 days beyond that!

So the issue tonight is for 23 March-04 April (12 days)

This is an identical period, almost, to the one I issued for last week, so it will be interesting to compare the two!!

Going through the usual checks

Tropical sources

No ENSO update since 19 February and the 30 day outlook is the same as in No 5, issued 28 February. The NOAA update on 9 March gave this assessment

•Atmospheric and oceanic conditions reflect La Niña.

•Negative equatorial SST anomalies persist across the central and east-central Pacific Ocean.

•Based on recent trends in the observations and model forecasts, La Niña conditions are expected to gradually weaken during the Northern Hemisphere Spring 2009.

30mb temperature

AO

It has shown some variations in the last week and is now less definite in its trend for below zero.

NAO

It too, after spending several runs showing a tendency to below zero, now seems less certain of this.

NB

Remember the predictions for 7 days hence have usually got a fairly high level of accuracy, less so beyond that.

NOAA 500mb anomaly charts

The 500mb flow keeps the flow into the UK as a 500mb ridge. Over the 7 days it has slowly progressed the ridge into the UK with some rise in contour height as it has done so. The upper low has also edged a little further east from its position, or rather its prediction from 7 days ago.

NOAA comments

They had given their okay to the 500mb +ve anomaly in the south Greenland area from the beginning. Through the week they have been less than happy with much of the extended outputs from any of the sources. Perhaps a glimpse at how they rate the models over a period of time might be useful here. Below is the link to look at a whole variety of ways they have of presenting this data. Throughout almost all of it, short term check or longer term, ECMWF beats GFS with UK Met vying with GFS for second place. The actual chart I show is for the past 31 days. As I've posted elsewhere on the site there has, for some reason, been a noticeable drop in ALL the models accuracy over the past month.

The link is http://www.emc.ncep.noaa.gov/gmb/STATS/STATS.html

The ECMWF and GFS 10 day forecast 500mb and anomaly charts

Rather more 'chopping and changing' than usual is the observation I would make over the past week with ECMWF making rather more of the +ve area and hence a more north of west flow over the UK in 10 days time.

Comments on the main GFS and ECMWF outputs

Both have shown differences from each other, using the same initial time/date and 144 and 240 hours ahead. They have also shown variations in run to run with each other. Beyond the T+240 then GFS has, I would say, on balance, shown more of a 'cold' rather than a 'mild' outlook post T+240

Summary

As so often occurs we have some conflicting evidence. I really have little idea of what the 30mb temperature will lead to now its back around the normal and looking like heading below. Other information gives mixed signals as well, AO not now showing a real decrease nor is the AO. NOAA charts showing a 500mb flow from a cold rather than a mild direction. GFS and ECMWF versions showing changes over the past week with ECMWF more in favour of the NOAA idea. The actual model runs from GFS and ECMWF for up to T+240 showing a continuing variability between each other and from run to run themselves.

Forecast for 1-8 April

The overall trend to me suggests colder rather than milder but no major sign to me of a prolonged and deeply cold pattern. I suspect it's a case of a fairly typical coldish March spell = some milder interludes but with occasional outbreaks of colder air. Temperatures north to south showing differences from their respective average by differing amounts. Lower than normal for the north rising to about normal, over the period, for the south. Its impossible to say from this far out if there will be above average windiness. I suspect unsettled rather than settled for most of the time. So snow for northern hills on about or a little above average and possibly for northern lower levels along with frosts a bit above normal. By the time we get to the south coast then I would expect this to be below average for both, thus milder than usual for the period but not excluding average mornings of frost but with snow largely absent. Mild days will be mostly confined to southern counties of England and possibly south Wales with that term rare for more northern areas. Total rainfall ending up for most at about average amounts.

Edited March 13, 2009 by johnholmes

[johnholmes]

a bit late this week and only posting the text, if I get time I'll add the charts over the weekend

No 7 issued 20 March for 30 March-9 April

Tropics

Issued by NOAA on 16 March

•Atmospheric and oceanic conditions reflect La Niña.

•Negative equatorial SST anomalies persist across the central and eastern Pacific Ocean.

•Based on recent trends in the observations and model forecasts, La Niña conditions are expected to gradually weaken during the Northern Hemisphere Spring 2009.

AO

This has generally over the past week tended to show a more –ve phase although rather less than half maintain the value above zero

NAO

This has trended down but with a mix holding some above and some below zero

NOAA 500mb anomaly charts

the tendency has been for the +ve area to move westwards and with the 500mb flow to become ‘flatter’ across the Atlantic but with 500mb heights remaining about the same value over the UK

for 24-30 and and 27 March to 2 April on the right

for 27/03-02/04

500 mb anomaly charts from ECMWF and GFS over a similar period

25 March on the left and 30 March on the right

Overall model comments on GFS and ECMWF around the T+240 period

none made as not had time

NOAA comments

Rather variable although for their area relatively well marked, 3-4/5

and below is their comment on Thursday

THE PNA INDEX WHICH RECENTLY HAS BEEN WEAKLY NEGATIVE IS

FORECAST TO STAY WEAKLY NEGATIVE OR BE NEAR ZERO THROUGH DAY 10, AND REMAIN

NEAR ZERO THROUGH DAY 14. THE NAO INDEX WHICH RECENTLY HAS BEEN WEAKLY POSITIVE

IS FORECAST TO REMAIN WEAKLY POSITIVE THROUGH DAY 7, TREND TOWARDS ZERO BY DAY

10, AND BECOME NEGATIVE BY DAY 14

8-14 DAY OUTLOOK FOR MAR 27 - APR 02, 2009

FOR WEEK 2 THE ENSEMBLE MEANS INDICATE A PROGRESSION TO THE CIRCULATION PATTERN

FROM THAT FORECAST FOR THE 6-10 DAY PERIOD. THE GFS-BASED ENSEMBLE MEANS ARE

MORE PROGRESSIVE COMPARED TO THE CANADIAN ENSEMBLE MEAN. TROUGHS ARE FORECAST

OVER THE EASTERN CONUS AND ALASKA WHILE RIDGES ARE PROGGED FOR THE NORTHERN

GREAT BASIN, NORTHERN ROCKIES AND THE ALEUTIANS. THE OPERATIONAL 0Z AND 6Z GFS

ARE SIMILAR IN PHASE TO THE ENEMBLE MEAN SOLUTIONS, ALTHOUGH THE OPERATIONAL

RUNS ARE SIGNIFICANTLY MORE AMPLIFIED WITH THE TROUGH EXPECTED OVER THE EASTERN

CONUS.

30mb

Summary

As almost always there are conflicting signals. Aside from eh Tropical outlook from NOAA the 30mb temperature suggests less blocking rather than more blocking. AO suggests some form of blocking. NAO seems less keen on the idea.

The 500mb anomaly charts from NOAA seem to indicate less blocking at the start of the period (they only cover out to 2 April). The ECMWF and GFS version foes no further than 30 March at the moment. They have been fairly different over the past 7 days with ECMWF more in favour of a meridional type than GFS, perhaps indicative of the varying ideas we have seen in the normal N Atlantic outputs from these two models whilst trying to decide what the actual weather might be for the period this Sunday (21st for the following week!

The latest chart, the one on the right, is again a different version really from them both over what they have each shown over the past 7 days.

Forecast for 30 March to 9 April

To me the indications are that the weather is less likely to be blocked than mobile. The typical weather of late March into April in other words. A fairly average mix of mild and rather colder interludes, about even in terms of time for each. Windy at times as deepening depressions track across a fairly mobile upper pattern in the Atlantic. At least one of these to allow colder Arctic air to flood south behind it. Overall the temperatures will probably end up about normal in the north to somewhat above normal in the south.

Unsettled sums it up in a word for the weather. No sign of any prolonged surface high affecting the UK either with a southerly flow or a northerly one.

[johnholmes]

No 8 issued Friday 27 March for the period 6-13 April

Tropics

A long input from the IRI update on 19 March; not a bad idea to read it all as we move into spring then summer. Reading this will help show the variability in predicting this area of longer term meteorology.

Expected Conditions

The most recent weekly SST anomaly in the NINO3.4 region is -0.4 C, indicating borderline La Niña conditions in the tropical Pacific. What are the probabilities for achieving a La Niña event (i.e. an "event" refers to La Niña conditions persisting for approximately 5 months) in early 2009? Persistent and large-scale easterly wind anomalies along the equator in the western-to-central Pacific have weakened in the last couple weeks. The thermocline, which constitutes the main element of the tropical Pacific system imparting ENSO predictability, is shallow in the eastern part of the Pacific, but deep anomalies in the west loom ready to reverse the current cooling. The shallow thermocline anomalies in the east have already abated considerably since February.

It is possible that the negative anomalies in the east-central Pacific could persist or even strengthen slightly if the low level zonal winds were to increase again in the coming month. However, the winds have actually been weakening recently. Additionally, as the ITCZ moves closer to the equator at this time of year, the Trade Winds will weaken further as the seasonal warming of the eastern Pacific ensues, and it will become more difficult for the atmosphere to maintain cold oceanic conditions. The models, both dynamical and statistical, suggest a tendency towards neutral conditions beginning from the first season.

Currently, the models indicate probabilities of about 50% for La Niña conditions, and very little possibility of developing El Niño conditions for the Mar-Apr-May season in progress. All things considered, during the Mar-Apr-May season there is very nearly an approximate 50-50% split in the probabilities for La Niña and ENSO-neutral conditions, and a near-zero probability for El Niño conditions.

The above assessment was made in part on the basis of an examination of the current forecasts of ENSO prediction models as well as the observed conditions. For purposes of this discussion, El Niño SST conditions are defined as SSTs in the NINO3.4 region being in the warmest 25% of their climatological distribution for the 3-month period in question over the 1950-present timeframe. The corresponding cut-off in terms of degrees C of SST anomaly varies seasonally, being close to 0.40 degrees C in northern late-spring/early-summer season and as high as 0.75 degrees C in late northern autumn. La Niña conditions are defined as NINO3.4 region SSTs being in the coolest 25% of the climatological distribution. Neutral conditions occupy the remaining 50% of the distribution. These definitions were developed such that the most commonly accepted El Niño and La Niña episodes are reproduced.

The models are showing fair agreement in their ENSO forecasts through the 10-month forecast period. For the current Mar-Apr-May season, just under half of the models are predicting La Niña conditions, and the rest predict weakly below-average ENSO-neutral conditions. In fact, throughout 2009, the models favour ENSO-neutral conditions. For the Apr-May-Jun 2009 season, 6 of 22 models (27%) predict La Niña conditions and none predicts El Niño development. At lead times of more than 4 months into the future, statistical and dynamical models that incorporate information about the ocean's observed sub-surface thermal structure generally exhibit higher predictive skill than those that do not. Among models that do use sub-surface temperature information, only 3 of 15 (20%) indicates the existence of La Niña, while 3 of 15 (20%) indicate El Niño conditions for the Jul-Aug-Sep season; 9 of 15 (60%) predict that SSTs will be ENSO-neutral (Note 1). Caution is advised in interpreting the distribution of model forecasts as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolved with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in the long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versus another has not been established using uniform validation procedures, which may cause a difference in the true probability distribution from that taken verbatim from the raw model predictions.

An alternative way to assess the probabilities of the three possible ENSO conditions is to use the mean of the forecasts of all models, and to construct a standard error function cantered on that mean. The standard error would be Gaussian in shape, and would have its width determined by an estimate of overall expected model skill for the season of the year and the lead time. Higher skill would result in a relatively narrower error distribution, while low skill would result in an error distribution with width approaching that of the historical observed distribution. This method shows probabilities favouring La Niña near 58% for Mar-Apr-May and 32% for Apr-May-Jun, and favouring ENSO-neutral as being consistently most likely from Apr-May-Jun onward. The same cautions mentioned above for the distribution of model forecasts apply to this alternative method of inferring probabilities, due to differing model biases and skills. In particular, this approach considers only the mean of the predictions, and not the range across the models, nor the ensemble range within individual models.

The IRI's probabilistic ENSO forecast takes into account the indications of this set of models, the outcome of the standard error approach described above, and additional factors such as the very latest observations that may have developed after the initialization times of some of the models. It indicates a 50% probability for La Niña conditions in the Mar-Apr-May season in progress, declining to 32% for Apr-May-Jun, and to 25% by Jun-Jul-Aug and beyond.

The NOAA summary on 23 March says this

•Atmospheric and oceanic conditions reflect La Niña.

•Negative equatorial SST anomalies persist across the central and eastern Pacific Ocean.

•Based on recent trends in the observations and model forecasts, La Niña conditions are expected to gradually weaken during the Northern Hemisphere Spring 2009.

AO

A more definite signal over the last few days for a rise then a fall.

NAO

This has gone from its mainly falling idea to a more definite suggestion of a rise at the end. It remains to be seen if this is just a blip or a trend.

NB: - remember that out to 7 days the verification on AO and NAO is very high, but less so beyond that.

NOAA 500mb anomaly charts (8-14 days ahead)

30/03-05/04 with the 04-10 April on the right

This set of charts has not shown over much consistency over the past 7 days, which is a bit unusual, other than the spell when in early February it persistently predicted a cold outlook for the UK. The +ve area has 'wobbled' about somewhat although to be fair it has generally shown a move east and a tendency to try and show a separate +ve area SW then S of the UK. Now it has the main +ve area over and SE of the UK.

ECMWF-GFS 500mb anomaly charts (10 days ahead)

Issued 23 for 2 April and for 27 March for 6 April

Quite a change from a few days ago for both; more meridional

The EC is still meridional but progressive! The +ve area/ridge in the 500mb flow has moved east slightly. GFS has intensified its idea of a –ve area/upper trough just about over the UK and seems to have pulled it a little west from its position on the left hand chart.

NOAA comments

For Friday, and they are even less happy with the models in the 6-10 and 8-14 day period than before; and

That has been a feature all week, or at least for much of the time

6-10 DAY OUTLOOK FOR APR 02 - 06 2009

THE OUTLOOK FOR THE 6-10 DAY PERIOD CONTINUES TO REMAIN SOMEWHAT UNCERTAIN, AS TWO POSSIBLE SOLUTIONS ARE POSED BY THE SUITE OF ENSEMBLE AND OPERATIONAL MODELS.

and for the 8-14 DAY OUTLOOK FOR APR 04 - 10 2009

THE OUTLOOK FOR THE 8-14 DAY PERIOD IS ALSO MARRED WITH SOME DEGREE OF UNCERTAINTY

GFS-ECMWF overall model patterns T+240 onwards

As so often they have been sometimes similar to one another, sometimes not; sometimes looking like a reasonable 24 hour change comparing their own last chart, sometimes not. If anything, to me, the GFS version has been more consistent but whether it will be right or not is, as ever, something we won't know for 10 days!

Summary

More conflicting ideas from most sources making an attempt at a prediction at least 2 weeks plus even more uncertain than usual.

Forecast for 6-13 April 2009

To start the period it seems most likely that a spell of anticyclonic weather is most probable 80/20 in % terms, and with the centre SW of the UK. Beyond that and I would opt for a slow edging west or possible NW of the centre with a less settled end to the period.

In terms of temperatures then the usual north-south split by day, but night time values will be governed more by cloud clearances with the south being more favoured for at least some ground frost, less so further north at first. No major radiation fog problems expected anywhere. Day time values could almost be in the rather warm category for some southern parts, nearer normal in the north. Little rain away from the NW.

The second half and a slow and rather erratic change, from the north to a less mild and more unsettled spell. Just how far south this will get is open to considerable doubt with high pressure hanging on more in the south than elsewhere. Rain and wind frequency about normal in the north but a shade below for the south. Possibly some snow for the Scottish mountains towards the end of the period along with some frost in overnight cloud clearances.

Edited March 27, 2009 by johnholmes

[johnholmes]

No 9 issued on Friday 3 April for 13-20 April

Tropics

From NOAA 30 March

•Atmospheric and oceanic conditions reflect La Niña.

•Negative equatorial SST anomalies persist across the central and eastern Pacific Ocean, but have recently weakened.

•Based on recent trends in the observations and model forecasts, La Niña conditions are expected to continue weakening during the Northern Hemisphere Spring 2009.

AO

The wide spread at the start has gradually been done away with (in the latter stages) with now a secondary 'mini' peak with a wide spread showing now around zero towards the end of the forecast period

NAO

This has consistently shown the expected rise to just above zero over the past 7 days; some ensembles again suggesting well above zero

NOAA 500mb anomaly charts

They have generally been quite consistent in showing the developing +ve area east of the UK with a general trough, now more marked, in the 500mb flow to the SW of the UK and just about a westerly flow over most of the country at 500mb.

Issued 31 March for 8-14 April and on 3 April for 10-16th

NOAA comments

Their marks, for their area of interest, have ranged from high to moderate and down to 2 as last evening for the 8-14 day outlook. They seem happy with the continuing +ve area south of Alaska (I can't remember when it was not shown on their charts), with a suggestion of troughs in the 500mb flow both over west and east N America. Overall they have tended to favour the ECMWF idea rather than GFS but not all the time.

500mb ECMWF-GFS anomaly charts

Over the past 7 days ECMWF has shown a desire to transfer its +ve area stretching from Canada/USA-Iberia out east so that it now shows as a flattish +ve area from Newfoundland towards SW UK and Iberia.

GFS has mostly kept its idea of a _ve area and 500mb trough near/just west of the UK throughout. Hence much of the reason for the variation in T+240 charts seen from these models. At the same time both have consistently shown a 500mb flow into the UK from north of west, in spite of their +/-ve differences. Both have some suggestion of height rises to the east, not unlike, although less pronounced, than the NOAA version.

28 March for 7 April on left with 3 April for 13 April on the right

Summary

Most indicators to me suggest that there is no major sign of a cold or warm spell of any duration likely in the forecast period. Differences between ECMWF and GFS persist all through the past 7 days. AO and NAO seem to be using the same hymn sheet. The Tropics signal is fairly consistent.

Forecast for 13-20 April

On the above I would suggest a fairly typical early/mid April type of weather. A mix of sunshine and showers, mostly of the coldish variety rather than a southerly thundery type. Snow for the Scottish hills but only briefly and even less so for the mountains further south. Days when it seems spring, in terms of warmth, is really here, especially for southern areas. Still some frost around under any clear skies at night, mainly ground rather than air, but a couple of air frosts is possible even away from the Glens. Winds will chiefly be between south through west, occasionally round into north west. A small chance, as shown on the NOAA 500 charts of a build of pressure to the north east of the UK, I would rate it about 30% at the moment. A 40% risk of one depression giving gale force winds for western and especially northern areas in the time period.

To sum up, a fairly typical April mix with temperatures around the average, a bit above to a bit below for most parts.

Edited April 3, 2009 by johnholmes

[johnholmes]

No 10 issued on 10 April for 20-30 April

Tropics

•Atmospheric and oceanic conditions reflect La Niña.

•Small negative equatorial SST anomalies persist in parts of the eastern Pacific Ocean and continue to weaken.

•Based on recent trends in the observations and model forecasts, La Niña conditions are expected to continue weakening during the Northern Hemisphere Spring 2009.

and the CPC output on 31 March for April

30-DAY OUTLOOK DISCUSSION FOR APRIL 2009

SEA SURFACE TEMPERATURE (SST) ANOMALIES IN THE EQUATORIAL PACIFIC HAVE RISEN

SLIGHTLY OVER THE EASTERN AND WESTERN PACIFIC DURING THE LAST SEVERAL WEEKS

WHILE REMAINING FAIRLY CONSTANT OVER THE CENTRAL PACIFIC. CURRENT LA NINA

CONDITIONS ARE EXPECTED TO GRADUALLY GIVE WAY TO ENSO-NEUTRAL CONDITIONS DURING

THE NORTHERN HEMISPHERE SPRING. ATMOSPHERIC CONDITIONS IN THE TROPICAL PACIFIC

OCEAN ARE ALSO CONSISTENT WITH A LA NINA EVENT. SUPPRESSED CONVECTION HAS

CONTINUED NEAR THE DATE LINE THROUGH THE LAST MONTH AND ENHANCED CONVECTION HAS

PERSISTED ACROSS THE MARITIME CONTINENT. LOW-LEVEL EASTERLY AND UPPER-LEVEL

WESTERLY WIND ANOMALIES ARE ALSO PRESENT OVER THE EQUATORIAL PACIFIC OCEAN. THE

MADDEN-JULIAN OSCILLATION (MJO) IS WEAK AND INCOHERENT AND IS NOT EXPECTED TO

BE A SIGNIFICANT FACTOR DURING THE APRIL 2009 FORECAST PERIOD.

AO

This has continued to show generally above zero values but also rather variable ensemble views

NAO

All forecasts show the expected value as being at or above zero, again with the usual fairly wide spread

NOAA 500mb anomaly charts

7 for 15-21 April and on the right 9 for 17-23 April

Each, and through the period, show +ve to the NE of the UK and in the Atlantic, (this area has moved NW over the period). The –ve area(s) from Greenland into Iberia also have been kept.

NOAA comments

They have generally tended, as they often do, to favour ECMWF over the other models. They have also on most days not awarded marks above 3 for either their 6-10 or 8-14 day periods, sometimes only 2.

ECMWF-GFS 500mb anomaly charts

These show more changes, both ECMWF and GFS, than their NOAA equivalents. But tend to show +ve areas over the north Atlantic with this now transferred north of the UK, with GFS now making more of +ve area NE of the UK, whilst both suggest a flow continuing, at 500mb, to be north of west, and with a trough near the west of the UK.

3 for 13 April 10 for 20 April

Overall ECMWF-GFS outputs through T+240 and beyond for GFS

At 240 varying ideas from both models, rarely consistent or similar to one another, (typed on Tue), and differences between each other and from day to day with themselves persists (Friday). There does seem to be an overall trend to pressure being higher to the north and lower to the south of the country.

Summary

Most indications seem to suggest that anticyclonic rather than cyclonic weather patterns are most likely, about 70:30.

Forecast for 20-30 April

The period seems likely to start with the cyclonic conditions of Easter having given way to a more anticyclonic type. The best idea of where the anticyclone might be is north of the UK with a slight favour to be to the NE. With this signal then we can expect the Atlantic to be relatively quiet in UK latitudes with gales less than normal even in the north and NW. Little sign of any pronounced northerly or southerly. Below average rainfall and with frost still likely to occur in prone places under any clear skies (mainly ground frost). Sunshine is probably, given the punt for the high to be NE, better in the west than the east. Temperature levels again could show an east-west split, near to average near the east coast and average to rather above in places away from that coast.

Confidence in this forecast is perhaps lower than usual. By the end of the forecast period a more robust flow pattern is anticipated from the Atlantic but this has an even lower probability than the main forecast.

Edited April 10, 2009 by johnholmes

[johnholmes]

No 11 issued on 17 April for 30 April-10 May

Tropics

No ENSO update since 19 March or CPC monthly since 31 March, see earlier LRF's for their updates.

ENSO/NOAA update on 13 April

•A transition from La Niña to ENSO-neutral is underway in the equatorial Pacific Ocean.

•Equatorial SSTs in the central Pacific Ocean remain below average, but negative departures have weakened considerably since February 2009.

•The patterns of tropical convection and winds continue to reflect La Niña.

•Based on recent trends in the observations and model forecasts, a transition to ENSO-neutral conditions will continue.

AO

Considerable variations have been shown over the past 7 days in the longer term outlook-initially it suggested it falling as the first one shows-it then showed a rapid climb (reaching between +3 and 4), yesterday this was changed to a suggested fall again at the end, but still mainly above zero

NAO

Changes in this were less marked but still some variation in the further outlook, now less marked above zero with a more modest rise suggested

NOAA 500mb anomaly charts

10 for 18-24 April and the second one issued 16 for 24-30

These have moved the main +ve area from south of Greenland (well south) to just SW of the UK. The –ve area over the northern Atlantic has grown in the same period

NOAA Summaries

This is the text of their 6-10 day summation from Thursday

TODAYS NUMERICAL MODELS ARE IN POOR AGREEMENT ON THE PREDICTED 500-HPA HEIGHT PATTERN OVER NORTH AMERICA AND ADJACENT OCEAN AREAS. WHILE TODAYS ENSEMBLE MEAN SOLUTIONS ARE IN AGREEMENT WITH YESTERDAYS SOLUTIONS, THE HIGH RESOLUTION RUNS OF BOTH THE GFS AND ECMWF ARE SUBSTANTIALLY DIFFERENT THAN YESTERDAYS RUNS. HOWEVER, THE HIGH RESOLUTION RUNS AGREE WELL WITH ONE ANOTHER, GIVING CONFIDENCE TO THEIR PREDICTIONS. THE HIGH RESOLUTION MODELS FORECAST A SUBSTANTIAL CHANGE IN PATTERN BEGINNING EARLY IN THE 6-10 DAY PERIOD, WITH A DEEP TROUGH MOVING INTO THE WESTERN STATES AND A RIDGE BUILDING IN THE EAST. THIS IS A PATTERN REVERSAL FROM YESTERDAYS RUNS WHICH PREDICTED A TROUGH IN THE EAST AND A RIDGE IN THE WEST.

They mark it 1 and the 8-14 day at 2. This is a great change from most of the past 7 days with 3-5 being awarded to both 6-10 and 8-14 day outlooks at times. Perhaps an indication of uncertain major synoptic patterns upwind of our area of interest?

ECMWF-GFS 500mb anomaly charts

11th for 21 April and 17 for 27th

ECMWF has shown throughout its idea of a +ve area in the Atlantic with its main centre south of 50N with GFS not that far different in this aspect. They are rather different in their treatment of the –ve area north of this-having a different shape with GFS now suggesting a more meridional developing over the far western Atlantic-both suggest this over the States. But see the NOAA comments above about this.

Overall GFS-ECMWF T+240 onwards

They have as usual shown some wide variations, more so with ECMWF than GFS. GFS has largely stuck with its idea that out to T+240 there will be a 500mb flow from S of W with a surface ridge to the SW of the UK. ECMWF has varied between a flattish westerly 500mb flow and one showing N of W flow.

Summary

As almost always we have conflicting evidence from the various sources. NOAA seems at 6's and 7's with their most recent comment, and events further out are also indicating differences. (For example the reference to La Nina conditions changing to neutral ENSO conditions). It is possible that this is due to the models, of whatever type, being unsure of how to deal with the major change due at this time. That is as the northern hemisphere becomes 'summer' orientated rather than 'winter' orientated. It does seem to happen each spring, to me, although the actual dates do vary, this year seems a little later than last.

Forecast for 30 April-10 May

Essentially there are two options which the two main models give a view of. A 500mb flow west of north=rather changeable and never far from normal temperatures. Or the flow being mainly S of W and high pressure being closer with the possibility at times of quite warm conditions especially in the south. To favour this view would be to side with the GFS version. In all honesty I am very unsure which variation to take!

I tend to lean towards the idea of high pressure to the SW at the start giving mainly settled conditions for southern areas, less so for the NW where frontal troughs will skirt close by. Temperatures from above average in the SE to about average in the NW. Looking further into the period and I cannot see this persisting. At some point there has to be a flow (at 500mb) from N of W, just when this will occur, and how marked it will be, is impossible to say. I do think it will become more unsettled over most of the country with a small chance, maybe 10-15% of a marked NW-N burst, however brief. Hopefully in 7 days time that will be much clearer. Indeed I suspect there is a 50% chance it may be starting to show by then.

charts asap-seems a problem re uploading at the moment

[johnholmes]

No 12 issued on 24 April for 4 May- 14 May

Tropical

The update issued on 20 April

•A transition from La Niña to ENSO-neutral is underway in the equatorial Pacific Ocean.

•Equatorial SSTs in the east-central Pacific Ocean remain slightly below average, but negative departures have weakened considerably since February 2009.

•The patterns of tropical convection and winds continue to reflect weak La Niña conditions.

•Based on recent trends in the observations and model forecasts, a transition to ENSO-neutral conditions will continue.

AO

The rise was correctly predicted for mid April. It shows this continuing until late in the period when most members show a return to around or just below zero. (end period=about mid May)

NAO

This also showed correctly the expected rise although it was not as great as had been predicted. Over the past week it has predicted that, by mid May, it will be nearer the zero line but most members keep it above zero, in fact there is a tendency now to show another rise.

NOAA 500mb anomaly charts

issued Monday 20 for 28 April-4 May 22 April for 30 April-6 May

The 500mb flow pattern has been predicted to stay much the same with the flow into the UK from S of W, and the main +ve and –ve areas.

It is as well to remember that even the right hand chart is only a prediction to a point two days into this attempted prediction!

NOAA comments

They have been not impressed with any of the models recently, for their area of concern, scoring below 3 over the past week and even down to 1 on two occasions, that is until this evening (Thursday). Looking at their verification tables below, the latest, one can perhaps see why. None are doing all that well although they are better at day 6 than they were in late February into early March.

ECMWF-GFS 500mb anomaly charts

20th for 30th April and below 23 April for 3 May

EC has kept the ridge SW of the UK although decreasing the +ve value; GFS never had it there to start with. For the major –ve area likely to affect us then they do both now seem to suggest a similar idea with GFS having lost its meridionality. Both have the 500mb flow from S of W, much as the NOAA version does. The flow shown is stronger though than the NOAA version, and somewhat flatter. NOAA show a hint at a diffluent area near the UK, EC also tends to hint at it.

Overall GFS-ECMWF 240 onwards

Once again their ability to chop and change from day to day has been shown this week. Rarely have they had a similar set up, although today, admittedly with GFS at 12z and ECMWF at 00z, as I write this, they are more similar than for some time. Both show a trough (500mb) well west of the UK with the Azores surface ridge building in towards the UK. Again remember this is for T+240 10 days from now=just the start of this forecast.

Summary

Overall there are certainly differences but the tendency does seem weighted in favour of a possible anticyclonic start with a suggestion that it could become more cyclonic with time. The 30mb temperature curve shows it climbing towards the normal curve. What impact that may have 2-3 weeks down the line I have to say honestly I have no idea!

Forecast for 4-14 May

A dryish start with light winds and reasonable temperatures for most. The actual position of any high being crucial to the temperature levels. Probably from rather warm in the SE to about normal in the NW.

As we go through the period then a more cyclonic spell seems the higher probability. Possibly breaking down to start with as the upper trough moves in with perhaps a thundery breakdown. Thereafter a rather unsettled spell with a fairly active Atlantic, this more pronounced, as per normal, the further NW one lives. Perhaps one fairly active depression to the north of Scotland in that time. In this regime then temperatures would fluctuate from just above to about or slightly below normal and with above average rainfall in the period.

[johnholmes]

No 13 issued 1 May for 11 -21 May

CPC output on 30 April

30-DAY OUTLOOK DISCUSSION FOR MAY 2009

SEA SURFACE TEMPERATURE (SST) ANOMALIES ALONG THE EQUATOR IN THE CENTRAL

PACIFIC OCEAN HAVE CONTINUED TO WEAKEN IN LATE APRIL AND CURRENTLY AVERAGE NEAR

ZERO FROM ABOUT 160E TO THE SOUTH AMERICAN COAST. THE MADDEN-JULIAN OSCILLATION

(MJO) IS CURRENTLY ACTIVE WITH THE AREA OF ENHANCED CONVECTION IN THE EASTERN

PACIFIC OCEAN BUT IS NOT EXPECTED TO PLAY A MAJOR ROLE OUTSIDE OF THE TROPICS.

WITH ENSO NEUTRAL CONDITIONS EXPECTED IN MAY, THIS OUTLOOK PRIMARILY REFLECTS

THE FORECAST FROM DYNAMICAL WEATHER PREDICTION MODELS IN THE FIRST TWO WEEKS OF

THE MONTH, THE CFS MONTHLY TEMPERATURE AND PRECIPITATION PREDICTIONS, SOIL

MOISTURE ANOMALIES, AND LOCAL SSTS ALONG COASTAL AREAS.

Tropics

The ENSO update on 27 April

•A transition from La Niña to ENSO-neutral is underway in the equatorial Pacific Ocean.

•Equatorial SSTs in the east-central Pacific Ocean remain slightly below average, but negative departures have weakened considerably since February 2009.

•The patterns of tropical convection and winds continue to reflect weak La Niña conditions.

•Based on recent trends in the observations and model forecasts a transition to ENSO-neutral conditions will continue.

AO

This continues to suggest through the past 7 days a fairly marked downward trend, initially to above zero, but latterly to some going below zero

NAO

Is almost a mirror of the AO although its downward prediction has been both more recent and more marked.

NOAA 500mb anomaly charts

23 April for 1-7 May 1 May for 9-15

The changes over the past week can be seen. Still the 500mb trough is shown as being just west of the UK with a flow from south of west, less strong than a week ago prediction and with a more marked trough.

NOAA comments

On wed= and marked 3 becoming 2

8-14 DAY OUTLOOK FOR MAY 07 - 13 2009

THE EXPECTED 500-HPA HEIGHT PATTERN FOR THE 8-14 DAY PERIOD INDICATES MOSTLY ZONAL FLOW ACROSS MUCH OF THE PACIFIC NORTH AMERICAN SECTOR

Thur=4 becoming 3 with an amplified flow 6-10 but more zonal 8-14 over the US area

The NOAA checks on the models are below (for 6 day outputs) and they show a marked improvement very recently. Whether this will be sustained now we have about left the seasonal transition remains to be seen.

ECMWF-GFS 500mb anomaly charts

Differences, as almost always, between EC and GFS. EC, until today, showing less ridging of the 500 flow than GFS. Today they both suggest 500mb ridging in about the same area.

24 April for 4 May

and 1 May for 11th

General T+240 onwards ECMWF-GFS

EC started with a ridge from the Atlantic into Scandinavia, then had the centre over the UK and has ended with it, this evening, in much the same place. The associated 500mb flow has the ridge but its got a trough nearer the UK than initially.

GFS went for the high being east of the UK with a marked 500mb trough into the far west, then warmed that out to give a generally westerly upper flow and has now put the ridge and surface high not too different to its initial idea. Remember that the charts I'm describing are 3-4 days apart.

30mb

This continues to move back towards the long term average and again I am unable to suggest its longer term effect. Anyone care to comment I'd be happy to read their views?

sorry about the above chart, hope you can see the 30mb in there!

Summary

We have both the AO and NAO suggesting some kind of blocking being likely around the period of the forecast. Some support from CPC and ENSO for that? Perhaps the NOAA 500mb anomaly charts just starting to show a blocking set up starting east of the UK. EC and GFS also suggesting a similar idea. The 240 EC and GFS charts, with some variations, also tend to support that idea.

Forecast for 11-21 May

From the summary, it may be a wrong one by me, but I have no alternative other than to go for a less active Atlantic with a 500mb block, of sorts, trying to establish itself, probably, east, perhaps north east of the UK. Assuming this blocking idea is correct then its position is crucial to any fine detail. I tend to prefer the 'east of the UK' idea. Thus the forecast should start with a flow from somewhere between south east and south west, generally above average temperatures and below average rainfall. Some increase in the rainfall the further North West you are but not that much. There is perhaps a 60% probability of it becoming very warm for southern and central areas for a time. Thereafter as we head out further into May I suspect the 500mb trough will move in, possibly as a thundery breakdown with less settled weather to end the period.

Edited May 1, 2009 by johnholmes

[Glacier Point]

As ever John, thanks.

One feature I would disagree with you is the forecast 500mb ridge to our east.

Composites for the current hemispheric arrangement of SSTA suggest the mean ridge positioned to our south west:-

The state of the polar atmosphere makes a northward correction of these anomalies, placing the ridge to our west perhaps becoming centred over the UK by the mid month period. It's interesting to compare both current 30mb and 200b - 500mb temperatures and zonal winds in terms of recent years:-

current zonal winds:

http://www.cpc.ncep.noaa.gov/products/stra...ALL_NH_2009.gif

compare to 2007:

http://www.cpc.ncep.noaa.gov/products/stra...ALL_NH_2007.gif

notice how the usual easterly wind reversal in the upper atmosphere has yet to occur this year. Put into climatological perspective:

This I think is a very good explanation for the much above average Spring (this anomaly extended throughout the stratosphere and into the troposphere).

Current 30mb temperatures are cooling off again:

http://ds.data.jma.go.jp/tcc/tcc/products/...e/pole30_n.html

which is favouring a strong mid latitude ridge into the Summer as westerly winds will persist across the polar region at 200mb-500mb and favour a +AO for the most part.

[johnholmes]

thanks Stewart, I would not disagree with you as I mulled over just where it might end up, but in the end plumped for the position I did-you could well be right and for sure the components you mention do support that idea.

[johnholmes]

No 14 issued 8 May for 18-28 May

Tropics

Enso update on 4 May

•A transition from La Niña to ENSO-neutral is underway in the equatorial Pacific Ocean.

•Negative equatorial SSTs have weakened considerably since February 2009.

•The patterns of tropical convection and winds continue to reflect weak La Niña conditions.

•Based on recent trends in the observations and model forecasts, a transition to ENSO-neutral conditions will continue.

AO

This forecast continues to predict a fall although it's a bit mixed towards the end. The most reliable part, 7days, does suggest a fall to near zero.

NAO

As with the AO this continues to predict a fall to lower +ve values than for some time

NOAA comments

8-14 DAY OUTLOOK FOR MAY 14 - 20 2009

INSPECTION OF THE DYNAMICAL MODELS FOR WEEK 2 REVEALS IMPORTANT TIMING DIFFERENCES FOR THE WESTERN CONUS. THE FORECAST PROBLEM INVOLVES THE SPEED OF A MID-LATITUDE RIDGE (NOT PHASED WITH THE ALEUTIAN RIDGE) MOVING ACROSS THE FAR EASTERN PACIFIC AND INTO THE PACIFIC NORTHWEST. SOME MODELS AND TOOLS APPEAR TO BE TOO FAST REGARDING THE ANTICIPATED TRANSITION OF THE PATTERN ACROSS T NORTHWEST FROM THE EARLIER 6-10 DAY PERIOD. THE OFFICIAL BLENDED HEIGHT FIELD SUGGESTS A REASONABLE COMPROMISE, WITH THE APPROACHING RIDGE AND ASSOCIATED

HEIGHT RISES BEGINNING TO AFFECT THE WEST COAST LATER IN THE WEEK 2 PERIOD. THE

BROAD SCALE RIDGE PREDICTED OVER THE EASTERN CONUS AND THE TROUGH EXPECTED OVER

THE INTERIOR WEST SHOULD LEAD TO A WET PATTERN AND INCREASED CHANCES FOR SEVERE

WEATHER FROM THE SOUTHERN HIGH PLAINS TO THE GREAT LAKES.

scores 3/5 with 6-10 4/5 but now expect pattern change

Their comments on Thursday are interesting. In the 6-10 day outlook they back the Euro ensembles and models pointing out their better scores over the past 60 days than north American models. This is what they posted.

6-10 DAY OUTLOOK FOR MAY 13 - 17 2009

TODAYS NUMERICAL FORECAST GUIDANCE FOR THE 500-HPA HEIGHT PATTERN IS IN FAIRLY GOOD AGREEMENT FOR THE LARGE-SCALE MEAN FLOW OVER THE PACIFIC-NORTH AMERICAN REGION. TODAYS CHALLENGE IS TO ACCURATELY PREDICT THE AMPLITUDE OF THE WAVETRAIN EXPECTED OVER THE FORECAST DOMAIN.

YESTERDAYS 12Z EUROPEAN ENSEMBLE MEAN SOLUTION IS PREFERRED - IN PART DUE TO ITS SUPERIOR PERFORMANCE OVER THE LAST 60 DAYS. ALL OF THE ENSEMBLE MEANS AND DETERMINISTIC RUNS CONTINUE TO PREDICT A SIGNIFICANT ANOMALOUS RIDGE NEAR THE ALEUTIANS FOR THE 6-10 DAY PERIOD. TO ITS EAST, OVER THE CANADIAN ARCTIC LIES THE POLAR VORTEX, GENERALLY CONFINED BY HIGH LATITUDE BLOCKING TO THE WEST,

NORTH AND EAST. IN SOME MODEL RUNS, SUCH AS YESTERDAYS EUROPEAN ENSEMBLE MEAN (LATEST AVAILABLE) AND TODAYS 06Z OPERATIONAL GFS, THE SOUTHWEST PORTION OF THE POLAR VORTEX PHASES WITH A MID-LATITUDE TROUGH ACROSS THE WESTERN CONUS. OTHER SOLUTIONS, SUCH AS THE GFS SUPERENSEMBLE MEAN AND THE 00Z GFS ENSEMBLE MEAN, DEPICT A DISTINCTLY SEPARATE POLAR VORTEX TO THE NORTH WITH FAST UNDERCUTTING ZONAL FLOW ACROSS THE LOWER 48 STATES.

and they scored 4/5.

Once we turn to their 10-14 day extension there is no ECMWF available so they have to rely on GFS and the Canadian models, both of which, regularly score lower than that by ECMWF or indeed UK Met at 6 days

It is not definite of course but it does lead one to believe that their own are not necessarily that good beyond here. It's the first time I've made much of this but it does tend to agree with a sort of gut feeling that, perhaps, after 10 days, looking at other outputs and applying at least as much weight to their predictions is worthy of more time. Say AO/NAO etc along with Tropics and other outputs.

Anyway their comment for the 10-14 days suggests that

ALL RUNS (BOTH DETERMINISTIC AND ENSEMBLE MEANS) AGREE ON RISING 500-HPA HEIGHTS FOR THE WEST COAST, AND A RIDGE OF AT LEAST MODERATE INTENSITY PERSISTING IN THE VICINITY OF THE ALEUTIANS. THE LINGERING OF THIS LATTER FEATURE IMPLIES THAT MOST MODEL RUNS FOR THE PAST FEW DAYS MAY HAVE BEEN TOO FAST IN LOWERING HEIGHTS ACROSS WESTERN ALASKA.

and they score it 3/5.

Remember the comments above when we come to look at the NOAA 500mb anomaly charts for out to 14 days.

NOAA 500 mb anomaly charts

A reasonable consistency in their output over the past few days, even if you look at the last lrf for longer than that. Their chart valid for 9-15 May shows a fairly similar 500mb pattern and distribution of +ve and –ve areas, considering the time scale, almost a week. What has happened in that time is they now predict a sharper 500mb trough edging into the far west of the UK. Can this be believed from other sources?

6th for 14-20 May and on 7th for 15-21May

EC-GFS 500mb anomaly charts

As usual there are differences between the two models although they have tended to keep with their own idea of how the 500mb flow will develop. GFS has made quite a feature of the 500mb low moving into towards the SW. ECMWF is much less inclined towards this idea. It has a general N of W flow with its main +ve area remaining well out over the Atlantic. GFS, somewhat like the NOAA version going much more for the developing upper trough, maybe even a marked cut off centre. Which is more likely?

On left 1 for 11th and on right 8 for 18th

EC-GFS T+240 charts

00z issue on Tuesday; EC on the left each time

This is the 12z Fri for 10 days time=Monday 18 May

So, the sequence over 7 days, what does it show?

EC goes with a 500mb feature and surface low which it seems to transfer east of the UK suggesting a northerly. GFS is not that different really, a smaller 500mb low to start with but it too transfers this and its surface low but SSE, so it lies over the UK rather than Scandinavia. What about surface high pressure? Both suggest its likely to be somewhere north west of the UK. So there is both similarity of each model with itself from run to run and to a degree with one another.

Summary

AO and NAO both suggest consistently a fall in the ensemble values. The Tropics data and that from last week is suggesting the change from La Nina to ENSO neutral is underway. NOAA charts show +ve anomalies for areas north of the UK and also in the Atlantic. The ECMWF and GFS equivalent charts (but for a few days earlier) are not so certain about this, but remember their cut off date is before the NOAA charts. The comment from NOAA was, for their 6-10 day period, that the Euros had a better handle on the pattern over the states and Pacific than their own. They were then happy with the polar vortex being shown over the Canadian Arctic. Most of the subsequent anomaly charts have this. Thus the pattern APPEARS to me to suggest the period starting with some kind of block at high latitudes and a 500mb flow N of W for the UK from ECMWF and from the SW with GFS. The problem which one is right?

Forecast for period 18-28 May

It's a difficult one to call. Do I side with ECMWF or the GFS, possibly NOAA 500mb chart version? I tend to favour the ECMWF idea but it's a very close call, pretty evenly balanced but it will make a big difference to the weather pattern we start with, and possibly end with.

I would suggest a SW upper flow soon changing to a more WNW flow as the upper pattern allows pressure to rise north of the UK. Some kind of block to the WNW or NW of the UK for a time. Little sign of any major Atlantic system coming through into the UK in this period. Temperatures after a rather warm and humid start dropping away again, so summer will still be on hold. As to whether it's a cyclonic pattern or an anticyclonic one then it would suggest to me, and I'm not sitting on the fence, both. Perhaps cyclonic at the start turning more anticyclonic during the period. Rainfall tending to be below average after the first few days. Sunshine about or possibly rather above for central areas in particular. Low cloud and sea fog for SW areas at first then a spell with eastern areas affected as the flow turns to one from east of north.

apologies about charts on top of others

Edited May 8, 2009 by johnholmes

[johnholmes]

rather later than usual due to varying other commitments

No 15 issued 16 May for 25 May-4 June

Tropics

No IRI update since 14 April.

For NOAA they say, on 11 May

•ENSO-neutral conditions are present in the equatorial Pacific Ocean.

•Equatorial SSTs in the central and Pacific Ocean have returned to near-average, while positive SST anomalies have strengthened in the eastern Pacific.

•The patterns of tropical convection and winds weakly reflect La Niña conditions, but recently have been dominated by Madden Julian Oscillation (MJO) activity.

•Based on recent trends in the observations and model forecasts, ENSO-neutral conditions are expected to continue into the Northern Hemisphere Summer.

AO

The predicted fall to near zero or even below on the 7 May has now changed to an anticipated rise, during the first 7 days (usually the most reliable) and a levelling out with a few showing a trend towards zero at the end.

NAO

This has also tended to downplay, with time, its predicted fall to at or below zero with a more modest decline from its current high +ve values.

It has to be noted that both the AO and NAO have been rather less 'stable' in their predictions over the past 7 days than they usually are. Perhaps this is linked to the NOAA comments on the Tropics and the arrival at the 'normal' curve of the 30mb temperature. The Tropics wind flow has also now started to reverse its direction.

NOAA comments

Their enthusiasm for the models has waxed and waned through the week, starting off happy and finishing up (Thursday) as overall content with the predicted set up. See below

3 out of 5 for 6-10 days ahead and 8-14 given 3/5 and this comment

8-14 DAY OUTLOOK FOR MAY 22 - 28 2009

FOR WEEK 2, THE MEAN UPPER AIR CIRCULATION PATTERN DIFFERS LITTLE FROM THE 6-10 PERIOD, FEATURING A TROUGH NEAR THE PACIFIC COAST, WEAK RIDGING NEAR THE CENTER OF THE CONUS, AND A TROUGH NEAR THE EASTERN GULF OF MEXICO. MODELS DIFFER CONSIDERABLY ON THE LOCATION AND TRACK OF ANY TROPICAL LOW PRESSURE CENTER THAT MAY AFFECT FLORIDA OR THE SOUTHEAST, BUT THERE IS A CONSENSUS THAT MOIST FLOW WILL DOMINATE THE REGION DURING THIS PERIOD.

NOAA 500 mb anomaly charts

13 for 21-27 May and on the right 14 for 22-28 May

The shape of the 500mb trough just west of the UK is predicted to change and also its moved further west. The –ve height anomaly has also been pulled further west. Upstream over north America then most things are about the same, position/depth of troughs and +ve and –ve areas. The overall prediction of a flow south of west at 500mb for the UK has been sustained for some time. Some suggestion also through the period of a small +ve area east of the UK.

EC-GFS 500mb anomaly charts

How consistent have they been through the week?

EC has gone from an initial idea of a 500mb flow N of W, via some not very consistent charts, to a position over the past couple of days showing a flow S of W with a block somewhere north of the UK. Its latest idea is for a meridional, almost reversed omega, type flow in the vicinity of the UK; see the chart issued Thursday 15 for 25 May.

GFS has been rather more consistent in its 500mb flow. It has kept its idea of a deepish cut off centre (low) W or SW of the UK throughout. Along with this has been the idea of a 500mb block N, and chiefly NW, of the UK. For its latest positions see the same chart as quoted for ECMWF on the right below this text.

8 may for 18 may and 15 may for 25 may on the right

GFS and ECMFW T+240

Anyone who has been following the EC and GFS T+240 charts over the past week is aware that they have at times appeared different to each other and other times fairly similar. The 12z issues on Friday show their differences again. One, EC, has switched from its idea of an upper low west of the UK, much as its 500mb anomaly charts have suggested to an omega style block! The other, GFS, has continued with its idea of an elongated high pressure developing from the central Atlantic out towards northern Scandinavia and north Russia. Again which is going to right? The GFS idea is not at all like its 500mb anomaly chart for about the same date, see above.

And finally the

30mb chart

It shows that the temperature is now at the normal level and probably going to continue climbing, at least for a time. We need to refer back to the Tropical link for ideas there. The other question is does this link in with the now suggested return to +ve for the AO?

Summary

So to sum up. Changes through the past week in both AO and NAO predictions. NOAA 500mb charts very similar over the States and pretty similar over/west of the UK. They move the 500mb trough a little west but keep a flow from S of W into the UK. The ECMWF and GFS charts, albeit for some 5 days previous in the forecast period, so more like the first NOAA chart, show a rather similar pattern. That is with the flow being S of W into the UK and with the trough west of the country. Further out, the Tropics and the 30mb charts are perhaps saying a similar thing to each other. In turn the whole lot may be trying to show a similar idea-unusual if this is so, in my experience. How realistic is this? Well we all remember early February when most things suggested a continuation of the early month cold! The T+240 EC and GFS versions are not a lot of help either.

Forecast for period 25 May – 2 June

Most factors support the idea of a 500mb trough west of the UK with a flow at 500mb from S of W. Just how high surface pressure is likely to be a little ahead of this trough near the UK is very hard to decide. This is the key to the start of the weather for the UK.

Overall I suspect that it will be relatively high with fairly settled and rather warm conditions in the south. Less settled and more normal temperatures in the north. Within a few days expect this to change as the upper trough advances, perhaps as another thundery breakdown with air from a southerly source ahead of it. Following on this more unsettled spell with normal or perhaps below normal temperatures then the more likely solution seems to be for surface high pressure to assert itself from a westerly point. This would lead to, within the period, often dry conditions but with a fair amount of cloud around its northern periphery, so warmer for the south than the north as is often the case. Just where the surface high is going to be located by early June is not at all clear. It could be way north allowing more unsettled conditions over the country or, about a 30% probability, be centred over or possibly just east of the country giving pleasantly summer conditions for many. I will leave that decision until the next update on Friday 22 May.

Edited May 16, 2009 by johnholmes

[johnholmes]

No 16 issued 22 May for 1-11 June

Tropics

This is the link to the IRI update on 21 May and well worth a read.

http://iri.columbia.edu/climate/ENSO/curre.../technical.html

just a couple of excerpts from them

May is near the middle of the so-called "predictability barrier" of boreal spring. It is typically easier to predict from this point forward than it was a few months ago and going through the boreal spring.

me=I would agree with that, look at the scores on my lrf as we came through that period compared to more recently!

and

Currently, the models indicate probabilities of about 80% for ENSO-neutral conditions, about 5% for the return of La Niña conditions, and about 15% for developing El Niño conditions for the May-Jun-Jul season in progress. Going forward the models indicate that maintenance of ENSO-neutral conditions is the most likely scenario, but with the possibility of developing El Niño conditions only slightly less likely, and the possibility of returning to La Niña conditions much smaller than either of the above.

and

It indicates a 70% probability for ENSO-neutral conditions in the May-Jun-Jul season in progress, declining to just below 50% by Jul-Aug-Sep and beyond as El Niño probabilities rise to a nearly matching 45%.

The NOAA update on 20 May gave

•ENSO-neutral conditions are present in the equatorial Pacific Ocean.

•SSTs are near-average or slightly warmer-than-average across the equatorial Pacific Ocean.

•Based on recent trends in the observations and model forecasts, ENSO-neutral conditions are expected to continue into the Northern Hemisphere Summer.

AO

Overall a similar pattern has been predicted through the last 7 days; rising values to start with a fall later but as usual a fairly wide spread.

NAO

It has consistently gone for a fall, rise and fall but tending to rise right at the end.

NOAA comments

They have, for the most part, seemed happy with the model outputs, both the synoptic ideas and those from AO and NAO. A degree of 'deamplification' is expected in the 8-14 day period over their area of interest.

They mark 4/5 for 6-10 days and for 8-14 days-quite high compared to many recently

NOAA 500 mb anomaly charts

The one on the left for 14 May for 22-29 May and the one on the right for 22 for 30 May-5 June

and this continues with the overall idea of upper trough roughly N-S over the eastern Sates, ridge out to the west and over our area a trough west of the UK with +ve anomaly that started east of the UK being centred just about over the eastern side of the country.

EC-GFS 500mb anomaly charts

on 16 for 26 May=left hand side and on right 22 May for 1 June

The EC version, comparing to the 500mb actual charts on 22 May was the more accurate for 22 May; both show a +ve area near the UK which is similar to the idea shown on the NOAA version for that date.

EC-GFS T+240 charts

As can be seen below they started with rather different ideas for 28 May.

EC on the left with GFS on the right.

Both have mainly kept to their ideas and rather different charts from each other. So both consistent with themselves but not too similar with one another at each time step. Having said that both do suggest upper and surface ridging rather than troughing.

Summary

Most inputs seem to suggest a 500mb flow from S of W rather than N f W, which in turn suggests air from Tropical not Arctic origins, so temperatures above rather than below normal at least for the start of the period.

Forecast for period 1-11 June

First to the back end of the last lrf where I said I would defer a final decision on the latter part of that, 'the beginning of June'. It does seem now that the start of this period is going to have air from Tropic rather than Polar origins direct or returning Polar air. Thus warm rather than cool for most areas. As is often the case the further south then the more above normal, so perhaps very warm for some southern and central parts with rather warm for the north. Fairly dry to start although in this air flow relatively minor thundery outbreaks affecting some parts, usually, from a southern point, cannot be ruled out. Any major breakdown seems unlikely for the first few days at least with the mostly fine and fairly warm end to May continuing for many areas.

Further ahead and we have very much less synoptic guidance of course. The AO and NAO both show a tendency for +ve values. Does this match with the Tropical output? The 30mb chart shows temperatures a little above normal but apparently levelling off.

I would say that the settled spell will come to an end through the second part of this lrf. Probably, given the initial direction of flow, as a thundery breakdown to start the change. Temperatures and rainfall returning to more normal levels countrywide. Probably a more NW-SE split again. The more unsettled for the NW with the SE perhaps holding more settled at least for a time. I personally don't see much indication of very windy weather for the NW nor of the Atlantic becoming very active.

Edited May 22, 2009 by johnholmes

[johnholmes]

No 17 issued 29 May for 8-18 June

Tropics

The only update is from NOAA on 26 May; see below; for other latest information please see lrf No 16

•ENSO-neutral conditions are present in the equatorial Pacific Ocean.

•Recently, SSTs have been increasing with slightly warmer-than-average conditions across the equatorial Pacific Ocean.

•The atmospheric pattern of winds and precipitation is near-average.

•Based on recent trends in the observations and model forecasts, ENSO-neutral conditions are expected to continue into the Northern Hemisphere Summer.

AO

The idea of a fall to near zero has been kept over the past week although the latter end idea of a return to +ve has now become a much less sure looking outcome than a day or two ago.

NAO

This has generally gone for a decrease after its slight +ve showing early on, although there is a remaining fairly wide degree of spread.

NOAA comments

They continue to be fairly happy 4/5 for 6-10 days and 3/5 for 8-14 days for the ideas as suggested below for their area.

ENSEMBLE MEAN SOLUTIONS ARE IN GOOD AGREEMENT ON THE EXPECTED 500-HPA CIRCULATION PATTERN OVER NORTH AMERICA FOR THE 6-10 DAY PERIOD. TROUGHS ARE

FORECAST OVER EASTERN CANADA AND THE EASTERN CONUS, THE ALEUTIANS, AND NEAR

CALIFORNIA, WHILE RIDGING IS FORECAST OVER THE WESTERN PART OF THE CONUS AND

ALASKA.

8-14 DAY OUTLOOK FOR JUN 06-12 2009

THE ENSEMBLE MEANS FORECAST A SIMILAR CIRCULATION PATTERN AS IN THE 6-10 DAY FORECAST

NOAA 500 mb anomaly charts

25 May for 2-8 June 28 May for 5-11 June

Through the week NOAA charts have tended to keep a weakness to the SW (possibly the cut off shown on EC/GFS charts) with the +ve area tending to migrate or relocate over/WNW of Greenland. It also suggests a –ve area NE of the UK. A general agreement between NOAA and the ECMWF GFS charts although remember there is a difference in valid dates-NOAA being for a somewhat later date.

EC-GFS 500mb anomaly charts

22May for 1 June on left issued 29 May for 8 June

EC has changed over the week from a largish +ve area over/ENE of the UK to a +ve area around Greenland with a marked upper trough from NE of the UK to way SW of us. It has had a –ve area to the SW (a cut off through much of the week).

GFS is not that dissimilar if one looks closely at the two models. Its largish +ve area E/over the UK has become a smallish +ve area WNW of Greenland. Its –ve area, the cut off to the SW (like EC) has now two centres, with one NE of the UK. So really both versions are going for much the same. No great flow through the Atlantic with upper lows NE and SW of the UK. The upper ridge over the UK area having given way to an upper low. The flow is now predicted to be more S of W than earlier, although that was not easy to decide if I'm honest about what it showed a week ago.

EC-GFS T+240 outputs (EC on the left)

GFS, initially, shows less of a northerly than ECMWF by Tuesday 9 June. It then proceeds to make the major issue the upper trough south of Greenland and by the run for today valid for Monday 8 June it is the dominant feature extending from northern Europe out to almost the east coast of USA/Canada. If it is correct then the surface feature it shows south of the main upper trough is likely to engage with that trough and swing across the uk-just how much it deepens is very far from clear.

ECMWF from its northerly flow with the surface high and its upper ridge over Greenland and Iceland only slowly changes this as it moves them both, much as the circumpolar outputs to WNW of Greenland, although its latest one shows hardly any sign of it. It makes quite a lot of the western part of the upper trough with a fairly marked surface low moving in towards the western UK, underneath the upper ridge So the later stages of the T+240 and there is some similarity with the two models. Both show the low I mentioned when commenting on the GFS, below the bottom of the upper trough.

Summary

A fair level of agreement in the 500mb circumpolar charts this week-not that often an occurrence. The AO and NAO also seem to be offering about the same idea over the same or similar time scale. NOAA feel that the 'neutral' ENSO conditions seem likely to continue. Finally the 30mb temperature is showing signs of trying to go below the average line again having just climbed above it.

Forecast for period 8-18 June

After the northerly flow of next week, the start of June; what then are the prospects for further out?

The overall consensus does seem to favour an upper trough, mostly, NE of the UK, with little real flow across the Atlantic. Having said that, both the T+240 models tend to disagree with that idea.

Overall I would suggest more anticyclonic than cyclonic to start with becoming a reverse fairly early on. Rather unsettled through much of the period with perhaps rather more in the way of Atlantic weather systems affecting the UK at times. I suspect the surface flow is going to be between SW and NW for much of the time. Around average amounts of rain and sun with rather above rainfall for some western/northern areas. Possibly one or two fairly windy days up there as well. Temperatures will be near or around the average for this time in June-no heat wave seems in the offing as yet. Whether some kind of ridge will develop just east of the UK late in the period to lift temperatures overall is very uncertain at the moment.

Edited May 29, 2009 by johnholmes