In depth Model Discussion

[johnholmes]

I finally got round to reading that Pete Lavington Forecast Techniques Review. Ah the FRB(Forecasters Reference Book), such memories, I do still have my original issue, and its by the pc as I type!

He is right in that some of the empirical methods seem still to work, 40-50 years after they were introduced and years before computers were used for forecasting. Some are not so good. MOS or Model Output Statistics, were used initially in the models from about the mid 1980's I think and it was good fun trying to beat 'them' at whatever station you were at, be it for fog, formation or clearance, or minimum and maximum temperatures.

Yes a good read and it encapsulates the other data given by Evo on just how difficult some of the forecast parameters to get right, even just a few hours ahead.

I could try and print my original Applied Meteorology Course from Shinfield Park to show the immense complexity of some of the equations needed to solve weather forecast(I should say to try and solve) but I've no idea how to show Laplacian operators let alone double differential and Integral calculus. Well that's my excuse.

yes snow, and fog are the two most difficult parameters to forecast and I hope my 'will it snow' will take most of the work out of it for people on this site.

Thanks again Evo for all that work, we could make a forecaster of you yet?

John

Edited February 4, 2007 by johnholmes

[Nick F]

A rather complex situation developing this week, and one that the models notouriously have difficulty handling when trying to model the advancement of milder Atlantic air against cold air advection from developing cold surface highs over NE Europe.

Early on this coming week we see a strong jet streak up to 200 knots propagate East across the Atlantic out of NE Canda to the South of New Foundland, ahead if it, an upper trough (marked on the 300hPa charts below) shows signs of disruption as it tries to move East thanks to a developing upper high over iceland which forces the upper trough to flatten and elongate more NW to SE, and together with the left exit of the jet streak propagating East, allows a cut-off upper and surface low to drift East along the English channel ahead of the main primary Atlantic low pressure system over the NW Atlantic.

T+36 to T+54, upper trough begins to disrupt as jet begins to bifuricate (split) and an upper high builds over Iceland, forcing the upper trough to elongate more NW-SE:

t+36:

t+54:

Cyclonic circulation modelled over North Sea over next few days by today's runs is drving cold air advection further South than previous runs, and at the same time high pressure builds over Scandinavia as the low(s) to the East sinks South, a few wintry showers can be expected across the East coast/E Anglia coast Monday/Tuesday ...

t+45 850mb temps show UK under sub -5C air:

t+45-51, some wintry shows for the East, otherwise cold and dry with some hard overnight frosts in places, Weds and Thurs morning could see temps as low as -6 to -7C over inland favoured areas, maybe even lower.

During Wednesday, the upper trough and surface low pressure begin to propagate East over France undercutting building pressure to the North and East, at the same time a secondary low develops off NW Spain under the left exit of the jet streak :

Thursday, and into uncertain terroritry now, 12z GFS sees this low to continue to deepen as it heads ENE across N'ern France, ppn likely to fall as snow over Southern counties according to 12z with dew points of 0C or below and sub -5C 850mb temps, this driven on by winds touching gale force along South coast:

Some sleet and snow is progged to push into Scotland aswell.

UKMO doesn't deepen the low quite so much, but has low in similar postion over NW france at t+96:

UKMO t+96: http://www.wetterzentrale.de/pics/Rukm961.gif

GFS t+96: http://www.wetterzentrale.de/pics/Rtavn961.png

Beyond, another low looks to arrive from the WSW on a more N'erly track, with rain more likely south of M4, sleet and snow for Midlands North, but anything after Wednesday or Thursday is pure FI IMO.

Sorry for typos, being edited now!

[Nick F]

Now the fly in the ointment in the form of the 12z ECM output, it has the elongated upper trough (shown above from GFS run) much further North, so it turns milder more quickly across the South than GFS/UKMO - which had the cut-off low running East along the South of the UK with a cold Easterly flow still intact over UK.

Possible that there could be some brief leading edge snow across England and Wales Wednesday evening/night into early Thursday on ECM run - though without precipitation output from ECM it's hard to tell:

... but by 12z Thurs milder 850mb temps above -2C (wrong side of marginal for snow) are already progged over S'ern areas:

By t120-144, turning milder everywhere - with a strong SW'erly flow beneath a deep depression to the NW which is to the NW of Scotland by t+144:

[Evo]

Thanks for the kind words Richard & John, I hope to come back to the subject when this has all died down a bit.

Thanks again Evo for all that work, we could make a forecaster of you yet?

John

Steady on now

Back to the here and now, I must confess that what little I've seen today is certainly very interesting, though I feel on balance that the 12z GFS was probably an outlier.

Picking up on what Nick is saying above, I've put the other Meteociel 850 charts for T96 into one image for comparison.

From right to left, top to bottom, ECMWF, GEM, GFS, JMA. Meteociel doesn't have the Met O. 850s sadly.

There is broad agreement on balance. As is typical with the UK, the small changes between the models seem to be concerned with our shores. It looks to me that the other models really want to push the milder air further into the British Isles than the GFS.

Also I'm minded that the 12z GFS has a reputation for being the most progressive of the GFS runs and I wondering how this will relate to it's positioning of the shortwaves, or indeed their existence full stop.

So, if we look at the 12z z500s:

From right to left, top to bottom, Met O, GFS, ECMWF, GEM, NOGAPS, JMA.

Met O and the GFS have very similar uppers, with the GFS having more trough development. ECM and GEM are very similar with the GEM the one being more progressive and the NOGAPS and JMA seem very similar. All of the models seem consistent in terms of the location of polar vortices, the difference really being down to the jet I would guess? Location and intensity of the jet streak probably making the difference.

I suppose more questions than answers, but I'm definitely sitting on the conservative side of the fence at this stage in terms of predicting snow distribution for later in the week.

[johnholmes]

me too re the last sentence from Evo, which is why when I finish the next blog I shall say its impossible to call beyond Thursday and be realistic. I tend to feel that the models have perhaps had the furthest extension possible with this set up and will back track a shade over the next day or so.

Anyway, as the old saying goes, time will tell.

[tinybill]

unless the gfs it total rong and in the next few days it upgade it self acc to that the cold air is a lot more east than mostof the other models i will wait untill tomorrow night to see if thinks have changed any i hope not and the gfs is right and the other models are rong !!

[Stratos Ferric]

Some quality analysis in the above posts, well done to all.

The trend I currently detect is for HP to the E to build slightly. I did mention this last night as an ironic potential spoiler, holding the frontal activity to our W and S. The right call at present though is the one variously made, the detail will change yet, and on knife-edges like this the detail matters a lot. I suspect this won't be possible to call until Wednesday afternoon, assuming that we don't get a huge collapse before then.

I'd like to see much more of a ridge to our N before I get too excited about the genuine potential for heavy snow anywhere; this afternoon's runs were suggesting a classic NW-SE alignment of the main pressure axis, but if the HP development continues then we will just end up with a barrier rather than a battlefield; meanwhile the air to the west occludes at the front - not helpful.

[BrickFielder]

One thing you can be sure of is that the models will not have the details nailed done until very close to the event. The best chance of snow is for north east Scotland where showers coming from Scandinavia are likely to bring a little snow.

For other areas then we have to look to Wednesday and Thursday. Looking a little deeper into the possibilities then we can have a good guess about snow and at the least look at the relevant criteria so that closer to the time we know what to look out for.

Wednesday sees an upper level lobe of vorticity (300hPa) cross the southern half of the UK with a small low pressure system forecast to cross just to the south of the UK. Matching 850hPa temperatures and precipitation then things are not looking hopeful with temperatures at this level above -5C where precipitation is falling.

Surface temperatures look a little high for snow as well.

I don't think this feature looks like giving much in the way of snow.

Thursday during the early hours I think is a different prospect. Again you would have to say that the 850hPa temperatures below -5C and precipitation do not align very well and hence the reason that people are talking about a leading edge of snow.

The difference here though is the surface temperatures with quite a hard frost ahead of the front taking temperatures down.

Soil temperatures do not look to rise much above freezing for the midlands northwards.

The zeroDegreeIsotherm remains quite low for more northern areas.

My opinion at the moment is that because this is happening overnight then the prospect of snow for the midlands northwards may be a little underestimated by the models.I doubt whether the snow will last long and should start to melt during the morning. Because details are likely to change with more accurate perameters being available to the models as we get closer then my opinion will change.

[BrickFielder]

A large shift in the stratospheric vortex is forecast over next week and I am starting to think it will re activate the atlantic conveyor belt of weather systems.Look at the situation now where the vortex is to the north of Russia.

See how it shifts towards the north of Canada.

This will have an affect on the weather patterns around the world.My best guess would be a signal for high pressure towards siberia and towards Greenland and northern Canada.

The end result would look a bit like this ecmwf chart which is probably not the best one for snow lovers.

A little further on and we start to see a possible signal for a mid atlantic block and perhaps synoptic conditions which could be favourable for snow, although with days getting longer by then the chance of snow is slim.

The AO index forecasts show it going negative yet the NAO is forecast to stay neutral so big changes upstream which are likely to make models a little unreliable beyond the short term.

[kold weather]

I agree Brick. The PV shifts again in the next 72-120hrs towards NW Canada and prehaps even further east eventually towards W.Greenland. what this will do is shift the jet streaks back eastwards. However the main problem for the next 7-10 days is that because ther eis still likely to be a weak PV however near Russia. This will means at least for a breif time we shift back to a more progressive global pattern which should also mean the jet roaring back.

Beyond that and a real chance of something far colder emerging for the last 10 days of Feb, tohugh I've not the time to say right now exactly how i think it'll emerge, but Bricks ideas are simila rto mine and also...think Feb 06 evolution from the 10th-25th.

[Stratos Ferric]

Still a very interesting prospect for Thursday for the N and NE. I'd also watch Saturday on the basis of potential evolutions across the N of England / Borders.

There is at present a small chance (20% or so) of the biggest widespread snow event across the north for a few years. The cold air never really leaves the north as the small secondary features track around, and certainly on high ground some decent falls are quite possible. That statement, of course, will prove to be the kiss of death.

Overall the set up is still far too dynamic to project with any certainty, and as suggested last night this is going to go right to the wire in terms of forecast detail. In the "favoured" zone the atmospherics are marginal away from high ground in the north, and precise location matters a lot. It takes very little for synoptics to shift 2-300 miles and that band covers everything from nothing to rain to sleet to snow right back to nothing.

...A little further on and we start to see a possible signal for a mid atlantic block and perhaps synoptic conditions which could be favourable for snow, although with days getting longer by then the chance of snow is slim.

The AO index forecasts show it going negative yet the NAO is forecast to stay neutral so big changes upstream which are likely to make models a little unreliable beyond the short term.

I agree that there is a continuing hint in this regard, but this winter the pattern has always been for pressure to our W never to bridge across to Greenland anywhere other than on the maps of FI. The same old spoiler that has haunted us all winter needs to be watched for: the secondary low.

[nick sussex]

Good news for those that would like to see the ukmo verify. Discussions over at NOAA have slated the gfs, gem and ecm operational runs, the gfs gets a double dose of slating which doesnt surprise me. Lets hope the ecm gets the early part wrong but the later part correct seeing as its come up with a tantalising longer term trend to drive the jet even further south and build pressure westwards from russia.

Heres the link and suggests the gfs has been much too progressive, the PV is expected to remain over canada and the pacific ridge is not likely to be going anywhere fast.

http://www.hpc.ncep.noaa.gov/discussions/pmdepd.html

[kold weather]

Hi Nick, I can't say I'm suprised about that, i suspected that the models would try and do something like this as early as the 25th of Jan:

Sometimes this is the case as this is the global flow generally anyway however the problem for the next 2-4 weeks arrises in the models because the the cut-off low to the SW/SE of the huge block will stick around for too long and exert to o much energy away from the block meaning the models will destroy the block too fast and then all the sudden the jet can change and go to a more zonal flow

SF, its very hard to call because as you say the front is alinged nearly W-E which means the snow-rain line is going to be mighty tight. Exactly how cold the air gets on Wednesday night is going to be of vital importance I suspect to exactly what will happen. what normally happens in these set-ups is the mild air will push a first front northwards, which usually always decays as it hits the colder air. Exactly how cold the air is at the lowest parts of the atmosphere could well determine how long the snow can last.

As you say though the real reason for such uncertainty at the moment is that these frontal zones can set-up in different places then progged. Main reason for this zis due to less data from the polar regions that are put into the models. This is even more imprtant in this type of set-up because a slightly stronger high will push the deeloping frontal boundary further south while a weaker one will bring it further north before it is forced to slow down as it nudges into the high. Its a very hard set-up for even the highest resolution models and our main problem is that at least in recent years its been quite a rare set-up which means we don't really have a solid clue as to exactly what the model slight biases may well be in each set-up so we are pretty much flying blind and we can't have any relaly accurate idea about which model would be right in the end.

Longer term and as I said the block does look like holding firm to our east however the PV does look likely to move a little further east, tohugh as Nick's report suggests maybe not as far east as expected. This is relaly important as well for our longer term prospects because with the block progged to be very close to the UK it really wouldn't take much of a shift to bring the cold air back into the UK early next week.

Saying that I don't think it'd be that cold even if we did drag back a easterly, but temps would struggle to get above average I suspect if thats the case.

Beyond that and of course we do have a lot of uncertainty about exactly where the PV's set-up and we had the same problem 10 days ago as wlel which suggests the global set-up may well be here for a while.

Not only that but it may be quickly worth just looking at what happens with the AO over past months and note that it stair-steps its way up and now note what its doing:

Now if we take this to its logical conclusion we can see that it probably will rise again towards nuetral and indeed thats wha tthe models are suggesting right now. Heights in the polar regions weaken, as you can see on the models right now as the current Greenland fades into the distance, which was the smain feature that caused the dip in the firs tplace.

However take it further then that and you can see that its quite probable that we will see another pretty sharp dip by the 15-20th, prehaps down to the same sort of values we saw in Feb 05. Of course this doesn't mean that we will get cold but it is worth noting that nearly every single very cold month has had a -ve AO.

The key question is where will the -ve AO affect the most, or in other words where will the blocking set-up. A very hard call to make however given the PV position its quite likely i feel to orginate from the east. For a while the jet looks like it may be to ostrong however IF the models past biases do occur here then don't be suprised if a weak scandi high pokes its head out next week like the eCM but maybe a touch further west.

Once the jet eases down (and given the fact that el nino has really weakened, it will...) then we will see the best chance this winter for cold and once again by chance it is late Feb!!!

I'll take a guess right now and say a mid-atlantic trough dives SE around the 15-18th (note, could be very zonal for a time, though intrestingly the jet does look quite supressed despite relaxtion of heights over the poles.) and mid-atlantic block forms which eventually sort of fuses with a powerful high to our east giving a cold, though probably not bitter set-up(think reecnt late Feb-Mar type cold...). Then IF the AO does stair-step again further -ve we have a real shot at a proper cold set-up and quite possibly a very cold last 10 days of Feb though the really cold air may never reach our shores and may just miss like Feb/Mar 05, but will be intresting to see if we evn get there first!!!

Edited February 5, 2007 by kold weather

[BrickFielder]

Just a quick note that the stratospheric vortex and polar vortex are not the same. and indications are that the polar vortex currently over northern Canada seem likely to move back towards southern greenland and Iceland spilling mild air over the UK. There is just a hint out in FI that the PV may move towards scandinavia late february before breaking up which could be interesting.

[Glacier Point]

A few thoughts on what has been and what may come to pass.....

Back on the 25th Jan the analysis was of a dramatic transition period with high latitude blocking becoming more of a feature with a split polar vortex and potential snow event. The blocking may not be as prolonged as anticipated but the AO looks pretty supportive of a big hemispheric change. As for the snow event, we'll just have to wait on that one a little.

Looking at the AO, the trend is clear to see:

..... a downward correction fuelled by strong ridging into the pole across Alaska as the ridge there steadily built and greater flow went into the southern jet over the Pacific then transfered eastwards towards the Atlantic in association with a deep trough over the eastern and mid west USA. The combination of greater energy going into the southern jet and confluence of the jet over the western Atlantic was always going to cause the models huge problems and so this has come to pass.

This was all timed with the weakening of the QBO - as monitored throughout the second half of January - the average monthly value was 2.71. High altitude westerly winds are still flowing over the equator indicating that we are still in the westerly QBO regime although surely its days are numbered and I think we are looking for a change-over to an easterly QBO during late Feb or early March. This is interesting in itself as we have seen precious few late winter changeovers and this will almost certainly co-incide with the final breakdown of the polar vortex, potentially leading to some quite potent weather events as it does so.

Looking at the thermal anomalies over the polar region, we have seen many changes in the last 10 days indicative of both the shifting QBO and its impacts on height anomalies over the Pole (the AO).

Late Jan

Early Feb

Here we can see a building of warm air anomalies at around 100 hPa over Greenland and cold columns towards western Siberia, and, indications of some Stratospheric warming starting to occur at the very margins of the Stratosphere which may propogate downwards and across the Pole to Canada in 2-3 weeks time. The sharp shift in heights over the Pole is shown nicely here by the CPC....

The reanalysis of the polar thermal profile for weak westerly QBO Febs is supportive of this type of evolution - note the pattern of anomalies - warm at 100 hPa and cold at lower levels towards Scandinavia.

This is an ok sample size for this set-up and could be taken as a reasonable guide to the likely evolution in the next few weeks towards the end of Feb. At 500 hPa, the height anomalies suggest southern Greenland to have a strong +ve anomaly (blocking) and deep -ve anomalies towards eastern Europe.

Up until the 12z ECM for 05/02/07, the models have not shown any hint of big -ve AO manifested as blocking over Greenland. Then up pops the ECM showing at the t168-240 range some substantial blocking which fits nicely the predicted QBO reanalysis pattern as well as declining El Nino for Feb. This is not occuring out of the blue - the model has been working on average heights for the time of year over the last few runs and although it could be a one off, the trend is good. Other long range modelling (and some ECM) has been for height rises towards Scandinavia for the blocking but I think this is where we may take our eyes of the real deal looming on the horizon.

With the ridging stubbornly persisting over Alaska we continue to fuel height rises over the Pole and divert flow into the southern Pacific Jet. With height rises over Greenland associated with the weaking QBO (and I would contend the underlying pattern which has until now been masked by the westerly QBO / solar minima / stratospheric cooling trinity), we could conceivably complete the equation which we have only now fleetingly achieved. Remember, just a transitory -ve AO / -ve NAO / +ve PNA phase has managed to deliver cold to the UK. Now, the task is to lock these three into place and as I see it, there is at the very least some opportunity to fully achieve this.

But isn't it too late ? Maybe, but with the solar minima and collapse of the polar vortex timed for late Feb, there is I think every prospect for Winter to overrun quite considerably this year until mid-March.

GP

Edited February 5, 2007 by Glacier Point

[WINTRY WALES]

Wow,

What fantastic posts in here. Please feel free to delete my post from this thread, i just wanted too show my apreciation.

Keep up the good work all.

[Glacier Point]

Following up yesterday's outlook, some analysis of where the ECM blundered this week but continuing trend for something quite tasty in 10 days time...

If we compare Friday's evolution since it was first programmed, we can see that the ECM (like many models JMA excepted) failed to come to grips with the height rises over Iceland and the draw-down of cold air across its eastern flank. This series (in chronological order) depicts attempts to break the heigt rise down or shift it further east:

Just at present there is a hint in the broader modelling that this is continuing to happen with undercutting lows rather than the main depression and upper level trough making its way eastwards towards the UK. There is some risk that the models are continuing to make the mistake of the last week in underestimating these height rises to our north although I think on balance the call would be for a brief warm up post this weekend.

The 00z ECM however picks up my thoughts last night nicely:

What we see here is a distinct split flow developing across the NE Pacific [ALERT - models will struggle] with a lot of energy going southwards. This sets up a rather slack looking flow to our north with a gradual migration of +ve height anomalies from Scandinavia across Iceland to southern Greenland - bang in line with the expected fading QBO pattern.

From a forecasting perspective we cannot place any faith in this emerging trend just yet given the uncertainty in the shorter term but this I believe is the evolution to be looking for, and which could potentially arrive sooner if the blocking to our north is continued to be underplayed.

GP

[kold weather]

Hi GP, its a very intresting trend and I note the jet stream on the 0z ECM by 240hrs is taking those LP's SE, it would only be a matter of time before a large block can form to our north which is just starting to develop.

If we can get a decent block to hold then the upcoming pattern in late Feb could well turn out very snowy in nature. This week's set-up proves it doesn't have to be a monster block either because the presence of a -ve AO has helped to shove that jet further south and given the jet is still fairly strong then its very intresting, as its been the jet stream we've missed in the cold set-ups because without it things do tend to go stale quite easily.

Also intrestimngly i think your right aobut heights to our north, the models constantly underdid the +ve phase of the AO and I had an idea it may do the same in the -ve phase so I wouldn't be at all suprised if heights are higher then expected.

[SNOW-MAN2006]

Here is my indepth discussion of the 12z run

Technical_Model_Analysis.ppt

Hope you enjoy

SM06

[BrickFielder]

Looking at Tomorrows prospects for snow.

Very complex situation and I still feel we will not really know untill the mid day soundings are in from Ireland what will happen. GFS low level vorticity charts clearly show the front moving across the southern half of the UK tomorow morning.

This is a little later than was forecast earlier in the week and there are suggestions that there is a fair amount of cloud ahead of it. I am not sure whether this will raise surface temperatures a little bit more than GFS is currently suggesting or whether a decaying front ahead of the main one currently giving light rain over southern Ireland will raise temperatures.

Buoy data from the area in question is suggesting surface temperatures around 9C and this needs to drop 7C before we see any snow. To complicate maters we have an upper trough crossing the area with cold air coming in at midlevels bringing with it an area of vorticity.

This is likely to change our frontal system into something a little more dynamic with a low pressure developing over the UK as suggested by the Thursday 12Z fax chart.

WV Satellite imagery suggests that things are little more dynamic as well. Notice how there is quite a lot of water vapour suggesting plenty of precipitation but notice the dark stripe behind the front and how it seems to bubble up at the back of the front.

My feeling at this point is that there will be significant snow for north wales and the midlands northwards. Surface temperatures below the front reduce the chance of snow further south as does the timing of the front being a little late. Finally I am going to put up 2 forecast skewT's to show how the atmosphere changes over the day and how things get closer to being unstable.

[BrickFielder]

Excellent Post by GP as usual and I thought I would have another look at long term prospects with a view to Stratospheric air.

The above chart shows the temperature at 100hPa and the contours are reflective of pressure at the surface for 3 hours time. What we can see is the influence that the stratosphere has on the weather systems below. Warmer air just above the tropopause tends to give lower pressure below it ,which is caused by an adjustment in the height of the tropopause by the air above. Cooler air just above the tropopause tends to give a better chance of High pressure at the surface. On the chart however we can see one sort of departure away from this rule where we have a high pressure ridge over the east coast of the US and relatively warm air above the tropopause. This is not that surprising when we consider that low and high pressure systems usually form a couplet and the strong area of warm just above the tropopause toward the north Pacific and China is forcing the Pattern.

The above diagram show the GFS forecast for temperatures at 100hPa (just above teh tropopause and although at first glance there is not much change there are some subtle ones. The cold air moves towards greenland which I think should suggest a signal for higher pressure in that region. The warm air out towards the north Pacific becomes less dominate and there is a suggestion that the UK becomes sandwiched between two small warm areas perhaps suggesting high pressure ridging twoards the UK.

I do think there is a reasonably strong signal for height rises over greenland perhaps ridging out towards Iceland and the north of the UK. There would also appear to be a signal for low pressure systems to trundle into the continent crossing Spain which would put the southern arm of the jetstream in accendance. I don't really see height rises towards scandinavia in the short term untill cold builds up over siberia and ridges out.

Edited February 7, 2007 by BrickFielder

[Glacier Point]

Thanks Brick,

The warm 100 hPa is showing up nicely within a continuing stratospheric warming event going on over the western side of the polar region which could further accelerate the volatility and progression of vortex decay in the next few weeks.

Also worth considering are the changes in SSTA over the last month towards an Atlantic Tripole which could sustain / add to the development of -ve NAO conditions during March:

http://weather.unisys.com/surface/sst_anom.html

GP

[Glacier Point]

I think we are in a period of standing still until the models come to grips with the changes in the NE Pacific.

With the GFS chucking out all sorts of solutions, the ECM provides the only meaningful tool in the longer range outlook analysis. This is picking out the redevelopment of a trough in the NE Pacific and consequent ridge into the western US (+PNA) quite in contrast to the GFS which is at times hell bent on developing a -ve PNA pattern:

http://www.cpc.noaa.gov/products/precip/CW...a/pna.sprd2.gif

The other aspect of the ECM which, frustratingly is always at t240, is the prospect of a ridge developing from the NE Atlantic towards Iceland and Southern Greenland:

This is logical given that a low pressure scenario for the central Atlantic looks the most probable aspect of the modelling right now - and indeed what we know about weak El Nino.

With two convergences of the jet prorammed over the t168-240 period, we must have little confidence in any solution over the Atlantic but I would certainly buy into a low pressure out to the west pattern - but crucially with an increasing trend towards a -ve NAO (remember that one?). This interests me most because I am keen to test my theory that I put forward in my winter (busted) forecast of a -ve NAO winter given the relaxation of the QBO and diminshing influence of the earlier stratospheric cooling. To achieve this, we must shift the Scandinavia block temporarily to allow the jet to undercut enough to allow the polar vortex over NE Canada to displace enough south once more.

I note also some discussion on the AO on the other thread and this is one I definately agree on - trend down, maximum neutral if that. Add this to possible movement with the NAO and the last third of Feb promises much.

GP

[johnholmes]

short term attempt at forecasting Friday 3pm

This is the surface chart at 1500 with the low centre clearly visible.

It appears to be tracking ene or a shade north of ene. It will I think lose its actual low appearance and look more like a trough in the isobars as it exits somwewhere over E Anglia.

This may explain, at least in part, some of the differing opinions on what the radar track of ppn seems to be showing. It will tend to 'spiral' out from the low, so both ne and northward movement will appear.

chart below

I'll put some more comments about the upper air and how that may affect the nexy few hours. in a mo.

The two ske-t's of interest are Watnall(Nottingham) for areas north of about the M4 corridor, and Camborne for areas south and south west.

The Watnall ascent show thick cloud all the way up to about 30000ft. It has a zero C isotherm at around 1000ft. Moderate ppn could quite easily lower this to near msl, which is what is happening inb some Midland counties.

One would imagine that many with elevations of 500-750ft asl will get snow, assuming they are in the path of the ppn(I will try and do that shortly).

The winds are from a general swl'y direction of about 25kt.

The Camborne ascent shows the zero C level at about 5000ft with not much cloud other than patchy layers.

Its upper winds are mainly northerly up to 10000ft.

trying to work out who/where the main ppn may go I would suggest this.

Currently the radar shows the heaviest ppn, roughly bounded by this area

Nw Wales-s Manchester area-E Midlands-Bristol.

The movement is overall about in direction TOWARDS 030, note I say roughly, thus I would think the follwoing areas will get at least modd ppn at some time over the next hour or few hours, depending on where in that area you are now or in this forecast area.

Forecast area to move to ROUGHLY Whitby -Liverpool area=northern main extent, prob a bit further north and southward extent to be

Bristol to Wash.

hope this helps. Like I said anyone with the elevations I have mentioned in the path I ve suggested will get snow.

John

Edited February 9, 2007 by johnholmes

[Andy Bown]

Apologies for perhaps hijacking this thread but John's chart (above) shows me something I haven't seen before ;

Snow is falling along and to the South of the 534 dam line. I have always read/heard that 528 and below were the 'ideal' dams!!