LRF or attempting them

[johnholmes]

No 31 lrf for period 1-7 November issued 22 October 2008

Average temperatures across the country for early November are

9C in the Northern Isles to 11C for Scotland and Northern Ireland; 12-13C for much of England and Wales with 13-14C for London

Looking at the various factors which can influence the weather in the weeks ahead.

ENSO and its last update on 18 October

Currently, the models see very little possibility of developing either El Niño or La Niña conditions, although the model probabilities for La Niña are slightly higher. The tiny sub-set of the models that do see the possibility of developing La Niña conditions are primarily the dynamical models, rather than the statistical models. All things considered, during the Sep-Oct-Nov-Dec season there is a 90% probability for ENSO-neutral conditions.

NAO

Below is how the NAO prediction has changed over the past week. It still shows a trend to –ve values but with quite a wide scatter and some still +ve.

Not shown here but the AO also shows a trend towards –ve values, more so than NAO but still not totally definite. One interesting fact is the AO through October has been highly +ve, one of the highest I should imagine. This can lead to cold winters but the jury is out on how much truth there is in that. (One day I'll try and sift through both the warm winters and the cold ones to see if there is a link – heaven only knows how many times I've searched for this Holy Grail and am yet to find it!)

Other events around the Tropics, QBO etc do tend to show events developing that may support surface pressure heights rising to the north of the UK. For the longer term, looking at Stratospheric temperatures, there is as yet little sign of any values differing from the long term average for this time of year. (these can impact several weeks further down the forecast time scale, so the initial outlook for late November is not showing much sign of anything noteworthy?)

NOAA

Their recent appraisal of the various models is positive with marks fairly high for a long wave pattern over North America which would involve a trough near Alaska and one near/off the eastern seaboard with a ridge between. This would indicate (out to day 14 or so (5/6 November) a long wave pattern that would seem to favour a marked trough over the region of the UK with a ridge (both at 500mb) in mid Atlantic towards Greenland and probably again around 20 east.

Their 500mb anomaly charts have consistently shown a fairly large +ve area developing in the Greenland/Iceland area, as we see below, tending to move slowly east or east south east, late October into early November.

The ECMWF and GFS version of this is shown below, for 1 November; this also supports the idea shown on the NOAA chart above, with a flow from north of west into the UK.

Turning to the daily GFS and ECMWF outputs at their longer range and we have, as is often the case, a split in how the charts look, using the 00z outputs

GFS has had 3 versions of its idea today with the 12Z finally coming into line with the ECMWF version; its 00z and 12Z whilst not the same are fairly similar, and do suggest that the cold spell may last longer than first thought.

Its 12z version, which to me seems more consistent with how events are likely to turn out; shows a cold rather than mild period right out to the end of its run. It also has the idea of almost an Omega pattern. This is one happening which is certainly a possibility in my view. It would fit with the NOAA idea of just two major troughs over North America, indeed some of the hemispheric models are hinting at just 4 round the hemisphere. This is very long wave and usually slow to change.

So the summary, obviously some aspects of the data above will be given more weight than others. It is best to try and stand back from day to day let alone run to run outputs by the everyday models and look just for major trends. Then to take more account of what events further afield seem to point towards, although here again they can and do change over several days. Perhaps not with quite the major changes seen in day to day modelling though.

Summary

The cold snap from Sunday into next week seems fairly well set. Beyond that and looking a week further down the line to the start of this lrf, what then?

Far from clear once more but overall I would suggest that mild is probably less likely then a rather colder spell, say 30 to 70 in favour of cold rather than mild. There are conflicting signals between items like AO and NAO and the ENSO area with what is probable in the Tropics over the next week or two. On balance the data from these sources seems to suggest to me that 70:30 in favour of Pm type air rather than Tm type air will affect most of the UK for most of that period. Just what guise that will take, northerly or more ridge type conditions is hard to say. It will sound easy if I say a mix but that is probably how it will pan out. Temporary northerly winds being replaced by surface highs moving east over the Uk to be in turn displaced by another trough in the major 500mb flow. This does seem likely to be what meteorologists call the Long Wave pattern, possibly even an Omega block for a time, that is with about 60-80 degrees of longitude between each major trough. This tends to set the major weather pattern but still allows short wave features to move through.

The final summing up in percentages then is

45% of the time with surface winds between north west and north east; and 35% of the time under a high.

20% of the time with winds between south east and south west. Put another way 80% of the time on the cold side of average for temperatures. Fog and frost seem likely and with snow for many northern hills.

I'll post the charts in asap

Edited October 22, 2008 by johnholmes

[johnholmes]

No 32- lrf issued on 29 October for period 7-14 November

The average temperatures for about mid November show this:

From 8C over the Northern Isles to 9C for Northern Ireland and Scotland and 10-11C for much of England including the London area. Signs of the south west being shade higher and eastern areas a touch under these numbers.

As usual taking the main components in turn

Equator and Tropics

There are signs here of events beginning to show indications of some coupling of those happenings with further north. The forcing occurring over the se Asian area is impacting further upstream and the possibility of both the QBO and MJO (for meanings of these please go to Guides and definitions) causing changes in the long wave pattern further north. Effectively IF this happens then we again get a situation of the major Rossby wave lengths setting up to a very slow moving situation. Just where the surface high would reposition if this occurs I am still unsure of. Certainly it will be north of the UK, be it north west or north east. Again whether there may be some link up between the two is again outside my ability to predict. If one looks at the northern hemisphere predictions from about T+216 on both ECMWF and GFS there does seem some evidence of this idea. (More under ECMWF and GFS).

AO

The latest pattern prediction is below, certainly downward into –ve but then some sign of uncertainty at the end.

Turning to the NAO, and comparing two from several days ago with that shown today.

This, to me illustrates my comments that no branch of meteorology, short or longer term is anything other than full of changes. Trying to pick the right signal as to events 24 hours ahead or 2-3 weeks ahead throws up similar difficulties. So do be aware of this when you see predictions of these long term parameters.

NOAA comments

They have been on a yo-yo ride! On Friday they marked the models at 1/5 out to 14 days. By Tuesday they gave two versions, the early morning comment blamed inconsistencies on a 200+ knot jet coming off Korea. A later comment gave marks of 3/5 and said this

THE ENSEMBLE MEAN FORECAST SOLUTIONS ARE IN VERY GOOD AGREEMENT DURING THE 6-10 DAY PERIOD AND FORECAST AN AMPLIFIED 500-HPA FLOW PATTERN OVER THE DOMAIN. ALL

SOLUTIONS FORECAST A RIDGE ACROSS THE EASTERN TWO-THIRDS OF THE CONUS WITH A TROUGH JUST OFF THE WEST COAST. A STRONG JET IS EXPECTED ACROSS THE NORTH PACIFIC DURING THE PERIOD

Today their 6-14 day summation gave much the same comment as the one above and awarded 4/5. For the latter part 8-14 days they said this

DURING THE WEEK 2 PERIOD, THERE IS RELATIVELY GOOD AGREEMENT BETWEEN THE GFS

AND CANADIAN ENSEMBLE MEAN SOLUTIONS AND THEY INDICATE PROGRESSION AND A

DEAMPLIFICATION OF THE EXPECTED MEAN PATTERN DURING THE 6-10 DAY PERIOD. THE

MAIN CHANGE DURING THE PERIOD IS THE SLOW DEVELOPMENT OF A RIDGE ACROSS THE

NORTH PACIFIC. SPAGHETTI PLOTS FROM BOTH THE 00Z AND 06Z GFS ENSEMBLES INDICATE

RELATIVELY LOW SPREAD. GFS OPERATIONAL RUNS CONTINUE TO INDICATE CONSIDERABLE

DEVIATIONS IN THE LARGE-SCALE PATTERN. THEREFORE, THE GFS SUPERENSEMBLE WAS

FAVORED DURING THIS PERIOD IN THE OFFICIAL BLEND. AGAIN, THE GREATEST

DISCREPANCIES BETWEEN MODELS REMAINS THE MORE PROGRESSIVE NATURE OF THE GFS AS

COMPARED TO THE EUROPEAN AND CANADIAN SOLUTIONS. They marked the models 3/5.

So plenty of chops and changes there.

NOAA 500 mb anomaly chart for today for the period 6-12 November is below and shows a +ve area over Scandinavia and the eastern States with a –ve area over the Atlantic south of Greenland. This is pretty much in line with its ideas over the past few days as it has built the +ve area NNE of the UK especially. It also keeps the idea of a fairly marked 500mb trough in mid Atlantic.

ECMWF and GFS 500mb anomaly charts and comparing with the one above; its for 9 November so does allow reasonable comparison. They too suggest +ve in a broad band from east of Greenland over Scandinavia and Russia with a growing –ve area west of the UK.

Turning now to the routine outputs by GFS and ECMWF

Here I am starting a rather different way. I think that to look only at the north Atlantic and European area is not sufficient at this range, post T+168. Better to take a northern hemisphere view. First at T+240, ECMWF on the left and GFS on the right.

The ECMWF version did show, and so did UK Met a genuine high cell over the pole around T+144 but this has disappeared by T+240. GFS

data not out on NW yet for 240 GFS or 384; will add when I see them.

And then looking at how GFS takes it to T+384

Finally looking at the 30hPa over the pole and it shows a very slight dip below the average line so nothing of real import for several weeks down the line from that source.

GFS 240 and 384 not yet out will post when they are

too late off to bed 2314, stuck at T+120

Summary

Taking all the above into consideration then a brief comment would be more anticyclonic than disturbed and probably below normal temperatures rather than normal or above over the period as a whole if averaging the whole country.

The north is likely to see the effects of the high more than the south with that –ve area perhaps giving some rain at times now and then at first. Overall though I suspect it will be mostly dry, fairly light winds, no storms on the horizon, day and night temperatures will depend on cloud cover. As the winds look to have an easterly component then a west-east split may occur. Fog and frost away from the south and east early on but probably more generally for a time as a high cell tends to settle over/near the UK. No sign, to me at the moment, of any northerly return but it will depend on where the main high settles. I am unsure really about that but have gone for it being east rather than west of the meridian.

Validity checks on 7 and 14 November and the next update on Wednesday 5 November.

Edited October 29, 2008 by johnholmes

[johnholmes]

No 33 issued 5 November for 14-21 November

The temperature levels are as issued in No 32; roughly from 8C to about 11-12C-north to south

Looking through the various factors to take into account

ENSO etc

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

•Equatorial SST anomalies were near-average across much of the Pacific Ocean, with small areas of negative SST anomalies in parts of the east-central Pacific and off the coast of South America.

•The atmospheric circulation has been highly variable week-to-week due to an active MJO.

•Based on recent SST trends and model forecasts, ENSO-neutral conditions are expected to continue into early 2009

There has been no update from IRI since mid October in which their summation was

Most of the ocean-atmosphere indicators suggest that ENSO-neutral conditions are likely to continue not only during the Oct-Nov-Dec season in progress, but through the remainder of 2008 and early 2009

Other Tropical factors suggest that there is some, not a lot but some, indication of effects that may impact on the wave pattern in the northern parts of the northern hemisphere, effectively creating blocking patterns. Just where these may settle is still not clear. Indeed at the moment there is not much in my view to support any of this, other than on a temporary basis, in the period of this lrf.

NOAA inputs, text and charts

Text

They appear to have reasonably high confidence in the overall pattern the models, in general, are showing for North America, scoring 4/5 out to day 14(from last evening), so 18th November.

Their 500mb anomaly chart for the furthest ahead is below, out to 12-19 November

This appears a logical progression over about the past week with the 500mb trough tending to warm out and move east of the UK. –ve values showing north and north east of the UK with +ve in a broadish band from Iberia across the Atlantic into the north east of USA/Canada. This would seem to imply high pressure mostly south of this country and low pressure to the north at about that time.

The ECMWF and GFS version of the 500mb anomaly charts, out to T+240, 15 November, shows a flattish westerly 500mb flow on GFS with a more ridged version by ECMWF. Both show –ve anomalies for largish areas to the north of the UK; similar +ve anomalies from them both for northern Russia and the eastern and northern areas of North America. Its possible this is the start of a more meridional flow pattern?

Turning to a look at the 500mb flow pattern with GFS and ECMWF for T+240 for the northern hemisphere, GFS on the right; ignoring the msl pattern and looking at the 500mb flow there is some similarity in the far east and south of the area between Alaska and Russia. However the flow over N America is not in sync nor over the Atlantic. This could be due to differing model methods of determining flow patterns with them both trying to come to terms with what events may show further south around the Tropics. To be honest I don't know. By T+384 the GFS hemisphere model, see below the two charts,

T+384 GFS

This shows almost a 4 major wave pattern. It has the upper high (and surface high) east of the UK. Whether this is realistic

(For 21 November) we simply have to wait and see. There is the POSSIBILITY, no more, of the ridge, both upper and surface, extending all the way into Greenland and the far north of North America.

Returning to the 'signals' for how true this is

AO and it shows very marked irregularities, either side of zero. This MAY be a sign of events further south impacting on it with uncertainty on what the final signal will be.

NAO the left hand one from 2 November and the right hand one from 4 November to show any changes; in a way it's a sort of less pronounced mirror image of the AO, varying either side of the zero line.

Summary of the above

To me there are conflicting signals from the various sources. For the time scale shown I believe –ve heights to the north and +ve heights to the south of the UK are more likely than the other way round. By the end of the period then, IF, the conflicting ideas settle towards the blocking pattern and meridional idea more solidly then we may see a change. I doubt it will occur before the end of the period.

Lrf Summary (for 14-21 November)

An unsettled start seems likely with lows tracking ENE near north Scotland and brief colder air behind them, probably gales in the north and west. Less unsettled the further south east one goes but even here breezy with rain at times. Frost seems not to be a major factor other than in any temporary northern ridge, likewise for snow other than Scottish (mainly) hill tops. Temperature levels tending to fluctuate with the lows but around normal with no wide departures from this.

Later on in the period, still a north west-south east split but less active and more anticyclonic seems likely so drier than the first part and with overnight mist/fog and possibly some frost for the southern half; still unsettled and quite windy at times for the more north western parts of the country.

It remains to be seen if the occasional pointers to a more blocked/meridional type of flow are more in evidence over the last few days.

On a separate note, the 30mb temperature curve has just taken an sudden upward surge. I'm not sure if this is a data problem or a genuine indication. In any case it will be late November before any effects, at the earliest, would be felt from this.

[johnholmes]

No 34 issued on Wednesday 12 November for the period 21-28 November

The average temperatures, north to south are around 7C to 9C but nearer 10C in the south west.

Taking the various possible influences on this period.

Events around the Equator and Tropics

The comment on 31 October from CPC gives an idea of what is expected to occur for most items

PROGNOSTIC DISCUSSION FOR MONTHLY OUTLOOK

NWS CLIMATE PREDICTION CENTER CAMP SPRINGS MD

300 PM EDT FRI OCT 31 2008

30-DAY OUTLOOK DISCUSSION FOR NOVEMBER 2008

ENSO-NEUTRAL CONDITIONS CONTINUED DURING OCTOBER 2008. SEA SURFACE

TEMPERATURES (SSTS) IN THE CENTRAL TROPICAL PACIFIC ARE NEAR LONG-TERM AVERAGE

VALUES. COOL SST ANOMALIES CHARACTERIZE THE NORTHERN HEMISPHERE SUB-TROPICS

AND THE NORTH PACIFIC JUST TO THE WEST OF NORTH AMERICA, WHILE WARM ANOMALIES

OCCUPIED A LARGE PORTION OF THE NORTH PACIFIC TO THE NORTH AND WEST OF HAWAII.

THIS LARGE-SCALE PATTERN OF SST ANOMALIES PROJECTS ONTO THE PACIFIC DECADAL

OSCILLATION (PDO), WHICH IS OFTEN CHARACTERIZED AS EXPRESSING THE

DECADE-TO-DECADE VARIABILITY OF ENSO. CURRENTLY, THE PDO IS IN ITS NEGATIVE

PHASE, WHICH IMPLIES A PREFERENCE FOR COLD PHASE ENSO EVENTS. UNFORTUNATELY,

REGIONAL SEASON-TO-SEASON IMPACTS OF THIS DECADAL OSCILLATION ARE DIFFICULT TO

SPECIFY, IN PART BECAUSE THE TIME SERIES OF OBSERVATIONAL DATA NEEDED TO DRIVE

RELIABLE STATISTICAL FORECASTS, AND COMPOSITES OF THOSE IMPACTS IS RELATIVELY

SHORT, AND BECAUSE THE AMPLITUDE OF THIS DECADAL OSCILLATION IS QUITE LOW IN

COMPARISON WITH OTHER SIGNALS ON SEASONAL TIME SCALES. RECENT FORECASTS FROM

THE CFS INDICATE ENSO-NEUTRAL CONDITIONS ARE LIKELY TO CONTINUE THROUGH THE

NEXT FEW MONTHS AT LEAST, BUT WITH SOME TENDENCY TOWARD WEAK LA NINA

CONDITIONS.

FINALLY, AS OF OCTOBER 31, 2008, THERE IS A STRONG TROPICAL INTRA-SEASONAL

OSCILLATION, ALSO CALLED A MADDEN-JULIAN OSCILLATION (MJO), IN THE WESTERN

TROPICAL PACIFIC. THIS IS THE MAIN SOURCE OF WITHIN-SEASON VARIABILITY IN THE

GLOBAL TROPICS. MJO PROPAGATES EASTWARD FROM THE TROPICAL INDIAN OCEAN,

REQUIRING ABOUT 30-60 DAYS TO CIRCLE THE GLOBE. AS IT MOVES, IT IS LED BY

ENHANCED LOW-LEVEL EASTERLY WINDS AND TRAILED BY ENHANCED LOW-LEVEL WESTERLY

WINDS. THESE ALTERNATING WIND ANOMALIES CAN PRODUCE ENSO-LIKE OCEANIC AND

ATMOSPHERIC RESPONSES, INCLUDING ALTERNATING COLD/WARM CENTRAL PACIFIC SST

ANOMALIES, AND WESTWARD SUPPRESSION/EASTWARD EXTENSION OF THE EAST ASIAN JET,

RESPECTIVELY. THESE RESPONSES CAN, IN TURN, PRODUCE IMPACTS IN THE LOCATION

AND FREQUENCY OF STORMINESS OVER THE CONTINENTAL U.S. PREDICTING THESE

IMPACTS UP TO WEEKS IN ADVANCE IS AT, OR BEYOND, THE FRONTIER OF OUR PREDICTION

CAPABILITIES, AND IS FROUGHT WITH MUCH UNCERTAINTY. THE NOVEMBER UPDATE

FORECAST PAYS RESPECTS TO THESE POSSIBILITIES, BUT MAKES NO ATTEMPT TO PREDICT

ACTUAL INDIVIDUAL MJO-RELATED IMPACTS.

As always some of this and some of that! The IRI ENSO update is the same as issued last lrf, from 18 October and the latest NOAA on 10 November gave this assessment.

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

•Equatorial SST anomalies were near-average across much of the Pacific Ocean, with small areas of negative SST anomalies in parts of the east-central Pacific and off the coast of South America.

•The atmospheric circulation has been highly variable week-to-week due to an active MJO, which weakened during the past week.

•Based on recent SST trends and model forecasts, ENSO-neutral conditions are expected to continue into early 2009.

AO situation

Below is the latest one on 12 November; it shows, and has done over the past 7 days, first a rise, even a twin one, before starting to fall later.

NAO,also on the 12th see below; it has been variable in rise/fall over the past 7 days, and the one below shows more fall than previously, possibly because of a similar trend by the AO prediction.

Remember, like all the other factors these are predictions not fact.

NOAA forecaster comments

They continue, out to day 14=26 November to rate the models at 4/5 with the major pattern, trough over the east with ridge over western areas then a trough into the far north west.

their latest 500mb anomaly chart is for 20-26 November continues the close similarity with their charts over the past week, see below

A marked ridge-trough set up across the whole chart with a 500mb flow north of west for the UK, high pressure west of and the +ve centre shows somewhere around 55N. Remember this is not necessarily where the surface high may be its just showing where the maximum +ve anomaly is at 500mb.

The equivalent anomaly chart for ECMWF and GFS is below, for 22 November. They both show much the same kind of –ve and +ve anomalies as the NOAA version.

Looking at the variability of ECMWF and GFS trends over the past week.

The major change, with both models, in the T+240 time frame, starting last weekend, is the prediction, at times, of a cold northerly or north westerly. This comes about as a result; see NOAA above, of a marked wavelength change over north America. see the GFS USA for 12z today for T+72 (Saturday 15 December). If this verifies, again NOAA are confident it will, then it has a downstream effect over the Atlantic and Europe. Typically it takes 3-5 days for this effect to occur. The run is shown from T+24 out to T+216. That is an awful long way with all kinds of things capable of happening to stop the T+216 looking anything like it is shown. However the meteorological reason for its evolution is sound.

T=24, T+96, T+168, T+240

What the atmosphere is doing, or trying to do in this situation, is a bit like having a long rope on the floor and giving it a tug, the movement causes a snake like movement all the way down the rope. The atmosphere is similar. At T+216 it is almost, not quite, into a 4 trough pattern round the hemisphere. If this occurs then that pattern, wherever it settles tends to last for several days, even longer is possible. Short wave features run through the major trough-ridge system but the main waves remain in about the same position. Sometimes even retrogressing, moving west, not east as is the usual pattern.

Its far, very far from certain if this will happen. But the building blocks are falling into place it would seem. A neutral ENSO, AO and NAO trying to go negative and the GFS and (note both are similar) ECMWF suggesting the above sort of set up over the coming days.

NOAA seem pretty confident of events over their area of main concern. If that is correct then from about T+168 we should see the beginnings of this wave change in our area.

Summary

I've, unusually, spent a lot of time showing a GFS chart sequence but it illustrates what the various factors are suggesting for the period towards the end of November. Whether it comes to fruition we have to wait and see. At least we can watch how its shaping up at various times ahead. Don't look for specifics; positions of highs and lows yet, watch to see if the major amplification occurs over America and how that impinges downwind.

Forecast Summary

There are two comments against a cold northerly first, one is synoptic, and the second is within the lrf realm.

The synoptic caution is the position of the surface high prior to this possible northerly. When a high is positioned north of 40N, in the Atlantic, then the empirical rule strongly suggests that the cold plunge will occur about 30 degrees of longitude east of the initial position of the high.

From the lrf viewpoint, events around the Equatorial belt are not totally conducive for the type of weather pattern needed; at least some of it is not.

Having gone through all these precautionary notes on to the lrf.

There will be a cold spell in the period under discussion, 90% certain. What is not certain is the type. Will it be a short northerly quickly followed by a surface high giving typical late autumn conditions for several days? Or, after the first burst will, the high start to retrogress as a result of the upper pattern.

I would suggest the following; yes a cold spell, the real start from around 19-20 November and the major trigger in the UK area being a low from eastern USA running across southern Greenland into the Iceland area, then as a north westerly jet develops turning south east down the North Sea during 21-23 November.. Due to the position of the surface high, although deepish cold air will briefly affect areas away from the south west, the major cold plunge will go into the continent. Surface pressure then rising as the high tends to become more dominant. Out to 28 November staying on the cold side for most areas and becoming dry for most areas away from exposed coasts and hills, and any showers becoming mostly rain or hail. Fog and frost probable dependent on cloud cover. Before the end of the period it is likely that another low will run round the northern periphery of the high. Just how this will turn out is simply too far ahead to give sensible guidance.

So briefly, becoming cold for all by the weekend of 21-23November, with snow for some, especially the north. Staying on the cold side after this with the weather type rather uncertain.

The longer range, just looking at the Startosphere wind and temperature profiles shows nothing major, although the 30mb temperature chart has stayed above the average line for some time now, including a largish but short blip.

There will be no update next week, Wednesday 19 November, as I'm scheduled to be away until the following Sunday. Next update then will be Wednesday 26 November for 5-12 December.

Also I'll restart the JH countdown to Christmas with that issue.

Edited November 13, 2008 by johnholmes
Fixed attachment image problem

[Roger J Smith]

John, you may recall we were chatting about this period in mid-November back a couple of months ago, I find that the predicted far-northern low pressure has verified fairly well in this period. The event in question is timed for today (13 Nov) and it does seem to be a little to the east of the UK in terms of timing. This may mean that any retrogression in the period ahead, 5-10 days, could really open the floodgates for arctic air moving south, it will be a thing to watch on the next few days' output to see if the UK can share in this, or if it's mainly heading for Scandinavia. Either way, it is good to see some potential for snow in Scandinavia and some colder air over the Baltic region, because I think a big barrier to cold weather in the "modern" set-up is this remarkably warm Baltic regime with so little ice cover, it makes things very difficult for any potential cold spells to deliver much to the UK.

[johnholmes]

Updating lrf No 34 to cover the cold shot/spell whatever

On the GFS routine outputs every 6 hours we have now had 16/21 runs that agree on a cold shot. Short for sure but then, reference where the surface high was progged to be just prior to it starting, its what should be expected.

Remember north of 40N then the main cold plunge will go about 30 degrees east of the high centre PRIOR to the start. All 3 models have it anywhere, depending which T+ you choose and which model, but somewhere between about 5West and 25W. Thus it should be expected for the main cold plunge to go at somewhere between 25 East and 5East. If we look at where its predicted to be by the same models, at various times as successive cold plunges occur, its between 10East and 30East. The, probably main, cold plunge 21/22 Friday/Saturday (a week away!) shows it around 10-15East.

Even then 850mb temperatures over most of the country are predicted to be below

-5C for over 24 hours with many parts of central and eastern areas around -10C for much of that time. So a short but pretty potent northerly, albeit with the major shower activity, wintry of course, for the Scottish mountains, but not exclusively. Beyond that then I think we need to turn away from what GFS/ECMWF show and look way to our west.

The short range NOAA comment today was, at last, its been there for over 24 hours on their progs, commenting about the change in wave length now predicted to occur over their area through the period T+96 onwards. Its initial effect will be to sharpen the major trough-ridge pattern downwind of them, hence why, as some of you put it, GFS has ‘upgraded’, mostly for that cold shot 21-22. The sharpening upper system causing the flow to be more northerly rather than north westerly.

The longer term NOAA forecast chat seems very happy, scoring a very unusual 5/5 for their view of how the models behave out to day 10(24 November) and indeed still score 4/5 for the further 4 days (28 November). They do comment on the short wave features, as you would expect with the shortening wave pattern. Quite how this will affect our area by that period is a pretty impossible task to try and do, with any accuracy. The best one could say is that there is unlikely to be any real warming of the air, it will continue to be mostly from a point north of west, temporarily south of west as short wave troughs (surface lows) track round the main high. This is predicted to go nowhere fast on any longer term outlook, be it NOAA 500mb anomaly charts, GFS-ECMWF ones or indeed looking at both the AO and NAO which are, currently, not predicted to show any noticeable rise.

So to sum up

A cold snap, possibly several, over the coming days, starting from Monday/Tuesday for the far north east of the UK and then more generally from Thursday/Friday into Saturday/Sunday. Then one of the short wave features (on present timing) running round the high with probably another return to parts of the country of the cold through Monday into Tuesday (24/25th).

Beyond that my general suggestion of colder rather than milder continuing to predominate into early December.

I'll try and do another update before I go away next Tuesday.

[mcweather]

Thanks for the update John. Fascinating and informative as ever. A cold feel to weather genearally I think once we get the next few days out of the way.

[phil nw.]

Thanks for the update John.This colder NW-SE flow does look set for the last part of November and should bring plenty of interest in chart viewing in the coming days.

[johnholmes]

I'll do the final update on this lrf later this evening, then no lrf until No 35 which should be issued Wednesday 26 November for the period 5-12 December.

I'll also give my first thoughts on Christmas at the end; then from the following week try and update this as GFS then ECMWF get into the actual time frame, with Met O UK finally arriving at T+144.

Edited November 17, 2008 by johnholmes

[johnholmes]

Extending the lrf No 34 out to early December

Summary for the weekend, although its far from lrf timescales.

The original idea from GFS at in excess of T+240 of a marked cold burst for the period 20-21 November looks like being verified. Quite impressive in my view.

Brought about, in spite of nothing solid from the equatorial regions, but simply by the events falling into place at the right moment. Chance, who knows! Both the AO and NAO predictions for the period 20-21 November were not solidly in favour of the event either. The development occurred, in my view, thanks to events over the Pacific with a strong jet with the right orientation developing the right wave length over America which in turn created the right set up for a similar event over the north Atlantic/Euro area. This in spite of, or perhaps (?) because of, the Azores high extending into more northern parts of the Atlantic. Its been sat in about the same position for about a week and shows little signs of moving far in the next 7-14 days. This would take us into the end of November/early December.

So what of the rest of November?

Looking at all the data needed to try and assess what may happen, Equatorial, ENSO, AO, NAO, NOAA 6-14 day forecast comments, 500 mb anomaly charts etc I would suggest its never going to be mild in that period, other than a temporary hike in temperatures as a low moves west to east around the major 500mb ridge with its surface high beneath. The positions of lows and highs at the surface will move east/west/north/south but not enough to allow a zonal Atlantic in. Due to the overall major wave pattern being unlikely to change. Short wave features moving through but then the main long wave pattern re emerging in about the same position. Thus for much of the period into early December the present flow, upper and surface, of north of west will continue. There remains a probability of a further deep cold incursion(s) but from this far out its impossible to say when. So snow for the higher ground at times and just now and then for some lower areas, frost also likely. The other probability is for fog as at some time; the centre of the high is likely to be near enough to the UK to create that set up.

No update before Wednesday 26 November

Edited November 17, 2008 by johnholmes

[bluearmy]

the deep recesses of my memory seem to recall the ens showing a temp dip around the 20/11 as far out as T350 JH. not a bad effort then but i imagine there were just as many runs showing warmth which didnt make any impression on me.

[davehsug]

Oh dear John, you used the horrible word Hike instead of rise, my most hated of recent US imports. et tu brute? :lol:

Dave

[johnholmes]

No 35 issued 26 November for 5-12 December

Also a first peep at what Christmas Day might bring

Average temperatures north to south for early December are http://www.metoffice.gov.uk/climate/uk/ave...2000/index.html

6 in the N Isles to 7-8C for most of N Ireland and Scotland with 8-9C for England and Wales but 10C for the south west. Remember these are the average afternoon highs.

As usual going through the links that can affect the predicted weather 2-3 weeks ahead.

Stratospheric influences will be added at the end of the summary as they are really for further out than this predict.

Equatorial/Tropical signals, to me seem a bit mixed in brief

ENSO update mid November

Currently, the models see very little possibility of developing either El Niño or La Niña conditions, although the model probabilities for La Niña are slightly higher. The tiny sub-set of the models that do see the possibility of developing La Niña conditions are primarily the dynamical models, rather than the statistical models. All things considered, during the Sep-Oct-Nov-Dec season there is a 90% probability for ENSO-neutral conditions.

AO for 12 November and the one for Tuesday on the right; obviously quite a change in the prediction for this over the past 12 days. The two for today (26 November) both show a marked rise at the end of the period (around 15-17 December).

NAO with the same two charts as for the AO; again showing a change

Both of the above forecasts show a likely situation favouring continued blocking over the area of concern to the UK.

NOAA forecast discussion and 500mb anomaly charts

The chart for 2-5 December issued on Tuesday (25 November) and the next chart for 4-10 December issued on Wednesday.

There is quite a difference by the second chart with the major block to the west of the UK almost having disappeared and a 500mb flow suggested almost south of west for the first time in many days. A +ve anomaly shows over most parts of the polar regions from north of Russia across northern Greenland into a new large +ve area just south of eastern Russia towards southern Alaska.

NOAA on Tuesday and Wednesday were very happy with the overall pattern at upper levels and again commenting on short wave problems. They marked at 4/5 through the whole 14 days for the overall model outputs. They also commented this evening that the overall pattern seems likely to shift slightly eastwards by day 14 (10 December).

ECMWF and GFS 500mb anomaly charts

The latest is the one issued Wednesday on the left. On the right is the chart issued valid for 25 November and its pretty close to what we actually have, that is with a 4 major wave pattern.

The one for today for 10 days hence is showing some signs of a change, more especially the GFS version.

General GFS and ECMWF outputs to T+240 (10 days ahead=6 December) and comments on the output by GFS out to T+384 hours (=13 December)

These as many have commented on in the NW model thread have been fairly volatile. Just how much credence can be put on the overall pattern is difficult to judge.

Summary of the above

The overall evidence to me suggests that there is no real sign, in the period of this forecast, of a mobile Atlantic. This may be something to wonder about for the next lrf but not this in my estimation. A blocked pattern is about an 85% certainty it seems to me. What is not all that clear is just how the block will manifest itself. Changes are around in more than one area mentioned above. I am not experienced enough to be able to tell how they may pan out towards the end of the period. I have never shirked from saying I'm not sure when that is the case.

Forecast summary

Cold not mild is the overall message. A mix of anticyclonic and cyclonic type from a polar origin with the majority being cyclonic it appears to me. There is currently no real solid sign of a major polar outbreak in the period but one cannot be ruled out. Little sign of major winds it seems. Likewise I'm not sure that fog will be a major problem for inland areas either. Frost will occur with a bit above average frequency for this time of the year. Snow is certain at times for hilly areas, chiefly with an aspect between north west and north east for wind direction. Some snow is probable at lower levels as lows move near or even across some of the UK. Its far too distant to give any more detail.

Finally a comment on the Stratosphere signal. Latterly it is taking a bit of a tumble below the average value. I'm a newcomer to this but to me it suggests that, as I mention above, that a possible change of pattern may be indicated from about mid December onwards. One would imagine the Atlantic coming to life at least for a time as the blocking gives way and a shorter wave length becomes established.

But more of this period in the next lrf, due Wednesday 3 December for the period 12-19 December.

Lastly a comment about Christmas:-

I see no sign of snow for Christmas Day on any chart or model I can look at. How long the disturbed spell I'm suggesting from about 15/16 December may last I have no idea, that is if it occurs. My punt at the moment in terms of percentages would be for.

Tropical Maritime air of some kind, possibly with high pressure close by 50%.

Returning Polar Maritime again with high pressure close by about 30%.

Polar Maritime air with a cyclonic tendency 20%

its rather long and I'll add the charts asap

Edited November 26, 2008 by johnholmes

[johnholmes]

the one for issue this evening is almost ready but, thanks to the events unfolding over the next few hours I doubt I'll get chance to finish it and post with the charts, so it will probably be some time tomorrow for those who like to read it.

[johnholmes]

Well at last, various things, including forecasting the weather short term, conspired to make this late!

No 36 issued Wednesday 3 December for the period 12-19 December

The average temps are as per previous issue and link

Looking through the usual sets of data

ENSO and other Tropical data

This update from NOAA encapsulates much of this into one simple to understand idea!

30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2008

SEA SURFACE TEMPERATURES (SSTS) IN THE CENTRAL EQUATORIAL PACIFIC OCEAN ARE

NEAR LONG-TERM AVERAGE VALUES. SST ANOMALIES IN THE NINO 3.4 REGION OF THE

PACIFIC (170W TO 120W AND 5N TO 5S) HAVE BECOME SLIGHTLY MORE NEGITIVE IN THE

THIRD WEEK OF NOVEMBER AND NOW ARE ABOUT .4 DEGREES C BELOW NORMAL. THIS IS

CONSISTENT WITH EXPECTATIONS FROM SST FORECASTS FOR THE NINO 3.4 REGION EARLIER

IN THE MONTH. ATMOSPHERIC CONDITIONS IN THE TROPICAL PACIFIC OCEAN ARE

CONSISTENT WITH ENSO-NEUTRAL CONDITIONS, WITH WIND AND CONVECTIVE PATTERNS IN

THE FEW WEEKS REFLECTIVE OF A WEAK MADDEN-JULIAN OSCILLATION (MJO). THE

MAJORITY OF SST PREDICTION MODELS PREDICT NINO 3.4 SSTS TO REMAIN SLIGHTLY

BELOW NORMAL THROUGH DECEMBER, BUT TO REMAIN WITHIN .5 C OF NORMAL, INDICATING

ENSO NEUTRAL CONDITIONS WILL CONTINUE THROUGH THE END OF THE YEAR. MJO

ACTIVITY IS EXPECTED TO REMAIN WEAK SO NO SIGNIFICANT PREDICTABLE IMPACTS ARE

EXPECTED AT THIS TIME FROM EITHER ENSO OR THE MJO.

AO

The two below are to show how the output can vary over several days

Although not as much as sometimes seen. Notice how the predicted fall in values actually occurred leading into early December then the also predicted rise.

NAO

Again two to compare, dates clearly visible

It too showed a predicted fall then a rise which also occurred.

Both AO and NAO are predicted to rise but rather uncertainly with some suggestions of falls as well

NOAA comments

Their latest, Wednesday evening had this to say, once again with 4/5 for the 8-14 day period, the same as the 6-10 period. Both talked of the retrogression

8-14 DAY OUTLOOK FOR DEC 11 - 17 2008

FOR THE WEEK 2 PERIOD THE GFS AND CANADIAN ENSEMBLE MEAN SOLUTIONS INDICATE A CONTINUED RETROGRESSION OF THE LONGWAVE TROUGH TOWARDS THE CENTRAL CONUS. THE OFFICIAL BLENDED HEIGHT CHART SHOWS A TROUGH EXTENDING FROM HUDSON BAY SOUTHWEST INTO THE SOUTHERN ROCKIES, A PERSISTENT RIDGE ACROSS THE NORTH ATLANTIC, AND A MODERATELY STRONG RIDGE ACROSS THE EAST PACIFIC.

NOAA 500mb anomaly charts

The latest issued Wednesday for 11-17 December is fairly similar to the 6-10 day version and others before it. The main change is that the largest +ve anomaly is further west, around 30 west; several days ago it was predicted to be almost on to the UK from the west south west. The flow shows just north of west for 500mb. Again this has been showing for the past week.

ECMWF and GFS 500mb anomaly charts

Again I'll show two for several days apart

I made the following comment for the first hand one, valid for 7 December

ECMWF shows 4 major troughs with 5+ for GFS, they are rather more cyclonic than earlier charts suggested.

The one below is valid for 12 December; it has both indicating a largish +ve anomaly to the west/south west of the UK with a deepish trough west of it and a 500mb flow indicated for the UK as n of west, especially with ECMWF.

This has been a continuing feature of these charts for a good many days-ECMWF more meridional than GFS. It has also led to a number of comments in the NOAA forecast area. Note also the large +ve area and enclosed upper high north of Alaska. I'm not sure what that may suggest or where it may move!

Overall output from GFS and ECMWF

This has been variable to say the least and led to much comment in our model thread. ECMWF being a more stable model and, as the comment above, more inclined to show meridionality than GFS. Frankly some of the GFS outputs, as close as T+144, have been hard to understand, given the data from other sources over long wave patterns around the northern hemisphere.

So I am placing little weighting to this area, especially regarding GFS.

Summary of the above

As per my last remark for GFS and then that the data does appear to correlate but I'm not totally convinced, just a feeling I have.

Forecast summary 12-19 December

I have found this one of the most difficult to do since I started in late January this year!

Taking up my last comment in the summary above, whilst most links seem to suggest no major change in the pattern we have had for some time now, something to me does not quite fit. I'm not sure if I have some doubt due to the odd behaviour of GFS recently or if its me not understanding some drivers sufficiently.

I believe that high pressure may become more dominant for a time than recently, at least for more southern areas. Most of the links show the upper high to be south west of the UK. Unsettled further north and here less cold than recently with some windy spells possible as lows run around the northern periphery of the high with a more easterly movement for these lows. Still wintry at times over the hills. Under any clear skies then fog and frost could be a problem, this chiefly for the south.

Beyond the first part of this outlook I am quite unsure, in spite of my Christmas comment, just how it will play out. Some drivers suggest more northerly outbreaks but some, at least to me, seem to suggest a more mobile westerly flow.

Overall I would say temperatures will be mostly below average for all parts, sometimes into the cold category with the wind flow mainly from north of west.

Overall confidence is lower than usual.

Stratosphere comments

Below is the latest issue and shows a continuing yo-yo up and down from the long term average. I doubt that much can be inferred from it, other than, so far, there is little to indicate a marked and longish cooling phase which would then teleconnect to a more mobile Atlantic. So far this does not show, say 21-30 days down the line=early January?

Christmas another look 3 weeks down the line

Returning Polar Maritime, possibly with surface high pressure quite close 50%

Polar Maritime airflow and cyclonic rather than anticyclonic; a touch higher than last week at 25%

Tropical Maritime with high pressure close by 25%.

I would punt for, in spite of mentioning high pressure close by, for a possibly rather windy and unsettled period sometime over the Christmas period!

Next Update due Wednesday 10 December for 19-26 December, so over the Christmas period

Usual verification for the start and end of the period IF I remember

Edited December 4, 2008 by johnholmes

[SNOW-MAN2006]

Thanks John

SNOW-MAN2006

[BLAST FROM THE PAST]

Thanks John

I think the signals for a westerly flow mid month are correct and I also think the signal you have for a very windy episode over Christmas is there too. I updated my LRF the other day as I have picked up a signal for stormy period 24th.

BFTP

[johnholmes]

No 37 issued 10 December for the period 19-26 December (over the Christmas period)

Average UK temperatures(maximum) are from 6C in the N Isles to about 7C for much of Scotland N Ireland and 8C for most of Wales and England away from the south west(9-10C in places here).

Taking each link in turn

ENSO and Tropics

Nothing since their update on 20 November which suggested ENSO neutral throughout the northern winter and well into 2009.

Below is the NOA/AENSO update on 6 December

•ENSO-neutral conditions are present in the equatorial Pacific Ocean. However, atmospheric and oceanic features resemble the cold phase of ENSO, although the official indicator falls short of the La Niña threshold.

•During the last couple weeks, equatorial SST anomalies have cooled across portions of the central and eastern Pacific Ocean.

•The atmospheric circulation resembles weak La Niña conditions.

•Based on a majority of the model forecasts, ENSO-neutral conditions are likely to continue into early 2009

AO

Below shows the differences in time on two; the second one suggests less yo-yo effect but still there to some extent and still with a fair number at or above zero

NAO

And the same here

The NAO first part of the predict was good, it remains to be seen if the second half is as accurate; some spread but more at or above zero.

The two predicts take us just about after Christmas.

Do they match the Stratospheric temperature changes? I'll cover that later.

NOAA comments and 500mb anomaly charts

There is little to really note here, much of the time they seem more than satisfied with the long wave pattern but have differences over how the short wave episodes are dealt with by the main models.

The 500mb pattern anomalies continue to show a very strong almost westerly flow across much of the states and right on into Europe. –ve anomalies showing north of the Uk and +ve, in our area, to the south or south west. +ve areas are starting to show in fairly broad swathes off north Russia/Scandinavia and the Alaska area. The one below is a fairly good final sequence in what the charts have been showing over the past 7 days.

ECMWF and GFS 500mb anomaly charts

The chart for 20 December is also a sensible carry on from the last 7 days or so. Both models shows a broad westerly flow in the run up to Christmas week with large –ve anomalies to the west and north west of the country. +ve anomalies increasing in the area north of Russia and Scandinavia along with the far north east of China/Russia towards Alaska.

General ECMWF and GFS charts out to their time limits

As usual there are considerable differences between them before T+240 and they both have altered their outputs over the past week or so. The two charts below, the 12z today illustrate their differing ideas at T+240; GFS left/ECMWF right.

in fact rather closer today than much of the past week it seems

Summary

To me all pointers suggest a broadish westerly upper flow over much of the period. A large trough from west north west to east south east seems the most likely 500mb flow. Upper troughs running through this will distort this pattern at times. Most of the time, most of the country, seems likely to be on the cold side of the flow except with any deep low tracking along in that flow. Again, to me, the signals from most connections suggests that a change in pattern will be occurring over the Christmas period and beyond. What will show at first is the development of a ridge east or ESE of the UK. This will be prior to wave length changes well upstream.

Forecast

In the run up to Christmas an unsettled period, even stormy at times seems likely, especially the north. Temperatures swinging from above to below normal as lows run, for the most part, just north of Scotland. I am unsure but at some time, perhaps, one such low is more likely to run across more central areas. Behind each low colder arctic air will move south with the usual expectations at this time of winter for frost and hill snow, sometimes even to lower levels. This will depend to some extent on what time of day the fronts move in. Winds may be gale force at times, possibly severe in the more northern and western areas. In the far south less unsettled but with some disturbed spells as well. By about Christmas somewhat more anticyclonic type weather may (note the word) tend to develop over southern areas with fog and frost at night.

For a peek further ahead then perhaps the 30mb normal temperature curve and the differences from normal may begin to hold a clue. I suspect that GP may well urge caution here, but along with other Tropical events being predicted I think that late December is when a fairly major pattern change will occur. I am unable this far out, with only limited experience of this type of forecasting, to predict whether it will be another northerly or perhaps as a result of the major Siberian/Russian high retrogressing west across the north of us, perhaps linking with a tendency for the Greenland ridge to become significant as the forecast wave length change occurs.

perhaps the next update on 17 December will make things a bit clearer.

verification is due on 19 and 26 December

Edited December 10, 2008 by johnholmes

[yamkin]

I predict Xmas will have unstable East or North East winds bringing snow showers in from the North sea and this is when Kent and the Thames Estuary will be especially prone to falls.

[BLAST FROM THE PAST]

John Holmes

Fabulous summary and who would want to argue with that, thank you

BFTP

[johnholmes]

No 38 issued 17 December for 26 December to 2 January 2009

The average temperatures are as those shown on No 37

As usual taking each item in turn

Equatorial and Tropics

Both the usual areas seem to be rather more favourable today and seem to be following a reasonable looking profile. Below is the outlook issued for North America on 30 November which is useful background.

300 PM EST SUN NOV 30 2008

30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2008

SEA SURFACE TEMPERATURES (SSTS) IN THE CENTRAL EQUATORIAL PACIFIC OCEAN ARE NEAR LONG-TERM AVERAGE VALUES. SST ANOMALIES IN THE NINO 3.4 REGION OF THE PACIFIC (170W TO 120W AND 5N TO 5S) HAVE BECOME SLIGHTLY MORE NEGITIVE IN THE THIRD WEEK OF NOVEMBER AND NOW ARE ABOUT .4 DEGREES C BELOW NORMAL. THIS IS CONSISTENT WITH EXPECTATIONS FROM SST FORECASTS FOR THE NINO 3.4 REGION EARLIER IN THE MONTH. ATMOSPHERIC CONDITIONS IN THE TROPICAL PACIFIC OCEAN ARE

CONSISTENT WITH ENSO-NEUTRAL CONDITIONS, WITH WIND AND CONVECTIVE PATTERNS IN THE FEW WEEKS REFLECTIVE OF A WEAK MADDEN-JULIAN OSCILLATION (MJO). THE MAJORITY OF SST PREDICTION MODELS PREDICT NINO 3.4 SSTS TO REMAIN SLIGHTLY BELOW NORMAL THROUGH DECEMBER, BUT TO REMAIN WITHIN .5 C OF NORMAL, INDICATING ENSO NEUTRAL CONDITIONS WILL CONTINUE THROUGH THE END OF THE YEAR. MJO ACTIVITY IS EXPECTED TO REMAIN WEAK SO NO SIGNIFICANT PREDICTABLE IMPACTS ARE

EXPECTED AT THIS TIME FROM EITHER ENSO OR THE MJO.

The latest from NOAA on ENSO ideas is below, issued on 15 December

'•Atmospheric and oceanic features resemble the cold phase of ENSO, although the official indicator falls short of the La Niña threshold.

•Recently, negative equatorial SST anomalies have strengthened across portions of the central and eastern Pacific Ocean.

•Based on recent trends in the observations and some model forecasts, ENSO-neutral or La Niña conditions are equally likely through early 2009.

Looking at slightly nearer to home type outputs and the changes shown over the past 7 days for both AO and NAO

AO

There has been a marked drop in its predicted value.

and now for the

NAO

that also shows quite a change from about a week ago.

NB

Remember these are predictions and they are sometimes not correct!

Turning to comments from NOAA about the models

As usual GFS is never too popular, at least its operational runs.

This is taken from their comments this evening, 17 December : they gave 4/5 overall marking on how the models are dealing with events in their area out to day 14 (31 December)

NOAA 500mb anomaly charts, taking an early one and this evening on the right

The early one out to 26 December shows The largish +ve area mainly east of the UK with a 500mb flow showing south of west from the major trough over north America.

The one on the right, issued this evening, and out to December 31; two major +ve areas and a largish upper low between the two. The 500mb flow over the uk is shown as south of west with an upper ridge over the country.

It seems a reasonably sound evolution from the earlier one.

Now to the output from ECMWF and GFS for a similar output

The one on the left out to 21 December shows the flattish Atlantic 500mb flow on both outputs with some sign of +ve areas developing wsw of the UK and well east along with similar west of north America.

The later one, out to 27 December shows the evolution it has been developing for the past week, +ve over/close to the UK and developing to the north east with a developing –ve area and upper trough over USA then an upper ridge with a large +ve area west of the USA. They are pretty similar really to the NOAA output. Note that this is not always the case.

On to the operational outputs from GFS and ECMWF

Flip flop is a term I've seen used on the model thread by those who watch every run. Its not been quite that bad, in my view, looking for patterns rather than specifics. Yes they have changed but that has been well out in their time frames, be it 240 or T+384. There has been a trend to build a high in from a southern portion of the chart and for that to appear to move north east and look to link up with rising pressure values further east and north.

There is much euphoria on the form at the moment as it seems to have started to make a meal of that idea, the cold flow from a point north of east I mean!

The 30mb temperature comments I'll leave until after the basic summary and forecast as its effects are unlikely to have influence, at least the latter part, on the time scale we are dealing with. Just to comment that it has shown very little tendency to stay below the average line for very long. Its spent more time above than below over the past month.

SUMMARY

To me, ignoring the furore the recent GFS and ECMWF runs are creating, the overall patterns shown by other data continues to show a similar outcome to the last two LRF's. The actual date of onset probably still before the New Year.

FORECAST for 26 December- 2 January

On or about this start date the building blocks look like being in place for a fairly major development of high pressure, first over the Uk, then extending north east and linking with pressure rises to our north east to start with. As the period progresses the idea of this, after low pressure, perhaps more than one, has skirted around northern Norway, then the high seems likely to retrogress westwards with a fairly major polar outbreak. This probably happening just before the New Year. All areas will be in the 'cold' category, some in the north and east perhaps 'very cold', as compared to the values quoted at the start of this lrf.

Detail is not for an lrf but snow and frost will occur for many areas in this period; dry at first but becoming less anticyclonic and therefore rather more unsettled as time goes on. No major wind problems expected anywhere in the UK.

No update next week, the next one will be, possibly 31 January, for 9-16 January 2009

Validations are due on 26 December (will have to be 27th, and 2 January for this lrf)

30mb temperature comments:- This is a pretty new area to me but from my limited understanding of it the departures from normal seem unlikely to encourage a mobile and mild Atlantic flow well out into January, other than for short spells such as now. This is possibly from the dip below normal leading into the end of November.

Edited December 18, 2008 by johnholmes

[TEITS]

Excellent forecast John.

[Guest zebra danio]

Excellent forecast John.

I agree - very well set out and consistent from previous forecasts. Credit to John and those assisting him with this

[johnholmes]

assisting him !!

I'm a 'team' of one mate, but thanks anyway

some of you may be less kind if 'nowt happens before the New Year!

[Day10]

Totally engrossed while having my coffee break, many thanks for the post John (and only you!!)