45 replies to this topic

[Paul]

As per the post in the discussion thread, this new thread is intended for members who want to post a more considered view, to perhaps summarise the models, a particular run, look a specific angle on the output, or just something a bit more in depth (doesn't have to be technical) which can be viewed by members in a quieter thread.

Members are more than welcome to copy and paste any of their own posts from the discussion thread should they wish so as to get the best of both worlds for a more considered post - eg discussion and reaction in the main thread + the opportunity for people to view over a longer period in a less fast moving atmosphere in the other thread.

We're keen to keep this thread to more considered posts etc, so please use the main thread for general discussion and chat.
http://forum.netweat...entary-1st-feb/

Thanks

Paul

[phil n.warks.]

Ok i will start us off.

It does seem that overall the 12z outputs have taken a small step back from pushing the cold away over the weekend.
Looking at the fax charts compare T84hrs. from 00z output with this evenings T72hrs.-both for the same time.



We can see now on the T72hrs the approaching Atlantic fronts slowing and buckling suggesting the possibilty of rain mixing with the cold air already over us and giving snowfall somewhere over the Central or Eastern part of the UK.This is a changing situation and tomorrow may well move things further west or back east pushing the milder air through.One to watch and at quite short range.

If we look at the models at T96hrs. we can see how the cold air is still near or over the East of the UK,which opens possibilities of a return of the cold further west towards the UK.

http://www.wetterzen...ics/Recm961.gif
http://www.wetterzen...cs/Rtavn962.png
http://www.wetterzen...ics/Rukm961.gif
http://www.wetterzen...ics/Rgem962.gif

Forecasting surface developments where there`s such large temperature differences over and near the UK is proving very tricky- as the daily changes in the Operation runs are showing.

Edited by phil n.warks., 01 February 2012 - 21:07 .

[nick sussex]

The last week in the models has had more twists than an Agatha Christie novel with the Siberian high causing chaos in the models and this shows little sign of easing up as we head towards the weekend.

Certainly not a time for making concrete predictions about who gets snow and who ends up in the frustrating rain or dry zone!

The modelling of these battleground scenarios is the poorest in terms of detail, if you think about 100 miles on a global scale its nothing but this makes a big difference when you're dealing with the expected weekend set up.

It probably won't be until 24hrs before the event that we'll have a more narrowed down area at risk and even on the day this will still likely have some margin for error.

[The Enforcer]

To add to Phil's post, the preceding T96hrs Fax for Saturday here:

shows a slightly slower mild incursion initially, which the T84hrs Fax made more intense, but the slowing of the T72hrs is more evident when compared to both charts. This illustrates perfectly, the push and pull nature of each output that Phil was talking about.
Here are similar comparison charts for Sunday T120hrs and T96hrs respectively:

On this occasion I've spared everyone the crazy annotations, as the subtle changes in emphasis should speak for themselves. Also note the emergence of the front sneaking in from the east, enough time for that to move about a bit based on where it lies at 00hrs Sat T60hrs:

[West is Best]

Interesting cross-model 0z operational agreement in the medium term, with slight variatons having a big impact locally for the UK, specifically on the precise positioning of the high next week. There is no doubt that the developments for those against zonality have been vastly improved these past 18 hours, with the high holding the jet to our north and the current set up retaining some continental cold over the south-east of England under the high which is no longer a pure Azores one, but positioned close to the UK.

Of particular interest to me though is that despite the operational agreement there is still huge ensemble scatter in the short and medium terms, suggesting that there's little real resolution to this.




I still think the high will be pushed down by the jet but there's no guarantee that it won't go the other way. If so it would open all that cold sinking into southern France right up to the UK.

As a footnote it looks from these runs like a cold half-term for Alpine skiers.

[chionomaniac]

West is Best, on 02 February 2012 - 08:37 , said:

Interesting cross-model 0z operational agreement in the medium term, with slight variatons having a big impact locally for the UK, specifically on the precise positioning of the high next week. There is no doubt that the developments for those against zonality have been vastly improved these past 18 hours, with the high holding the jet to our north and the current set up retaining some continental cold over the south-east of England under the high which is no longer a pure Azores one, but positioned close to the UK.

Of particular interest to me though is that despite the operational agreement there is still huge ensemble scatter in the short and medium terms, suggesting that there's little real resolution to this.


Post1.png

I still think the high will be pushed down by the jet but there's no guarantee that it won't go the other way. If so it would open all that cold sinking into southern France right up to the UK.

As a footnote it looks from these runs like a cold half-term for Alpine skiers.

I think that it is best not to concentrate on the GFS solely for the output. However if you do , WIB you will note that the ensemble scatter is far more favourable to that yesterday. In fact from memeory the GFS ensembles were all keen on a 850hPa warm up from Sunday. But just look at that now, the mean stays well below -5C with some members keeping below -10C.

I suspect that any snow that lies may be around for quite a time yet!

Edited by chionomaniac, 02 February 2012 - 09:05 .

[johnholmes]

Not a detailed post just a few comments having seen this kind of scenario many many times.Models are not much better than human forecasters at very short time scales so bear that in mind when talking/looking/wondering about snowfall amounts. I do think the front will pose less problems in one sense. The air ahead of it is going to be pretty cold (dewpoints and upper air) that the problem snow or rain, away from western areas, will be less of a problem than it usually is in the UK. In another sense and its how much activity it will retain as it moves east/SE? I suspect that it will become fairly weak by the time its into Se'ern areas.
Complicating matters is what I raised in the model thread late last evening. The deepening cold air over the N Sea and increasing moisture, partly due to the cold pool/surface low (shown on Fax charts), will give decent snow showers from about Lincs southwards. The problem of that is how far inland before they run out of energy/moisture. I think the SE of East Anglia and Kent may see the most activity but also some places further west from that zone. This of course occurring before the frontal situation extends into those areas.
For a forecaster its both highly interesting and also highly complex. I don't envy either the Met forecasters on TV nor our own Nick and colleagues on Net Weather!
snow lovers enjoy if you are the lucky ones-others enjoy the piccs they will undoubtedly post for us.

Edited by johnholmes, 02 February 2012 - 09:30 .

[johnholmes]

In the longer term then the 500mb anomaly charts show no signs of any Atlantic type dominated weather, NOAA last evening 6-10 and 8-14 pretty similar to the EC-GFS issue this morning, see below; and all 3 have been consistent for days now on the likely upper air pattern. Detail of course will change but the overall idea of cold not mild seems pretty secure into mid Feb at least.

http://www.meteo.psu...0z/hgtcomp.html

[THE EYE IN THE SKY]

A very short summary from me for the medium range follows on nicely from John H post.

http://cdn.nwstatic....bridgeshire.png

Very little sign of any mild weather as we head into mid Feb with the mean around -4C.

Only difference from our current pattern is the cold in mid feb may come from the N instead of the E.

[johnholmes]

as promised to try and help new folk confused by the multitude of posts about this weekend

 weekend of 3-5 feb-rain or snow.pdf   345.57K   794 downloads

[alijoe]

johnholmes, on 02 February 2012 - 17:10 , said:

as promised to try and help new folk confused by the multitude of posts about this weekend

weekend of 3-5 feb-rain or snow.pdf

Thank you John for the time spent on that, it was much appreciated.

[johnholmes]

johnholmes, on 02 February 2012 - 17:10 , said:

as promised to try and help new folk confused by the multitude of posts about this weekend

weekend of 3-5 feb-rain or snow.pdf

blimey seems popular
I'll do the same tomorrow afternoon after the 12z comes out and by then we can look at how the closer models, fine mesh type, NAE (Met Office=T+48) and just about the NMM(Net Wx=T+36) are dealing with things.

[The Enforcer]

Latest evolution of Fax Charts for the weekend from left to right:

Saturday:

Sunday:


Reason for edit - to add T72hrs chart for Sunday.

Edited by The Enforcer, 02 February 2012 - 19:21 .

[Harsh Climate]

This is where I am currently expecting the heaviest falls of snow saturday/sunday

In the red zone I'm expecting widely 7-15cm. (Due to the marginalty of this situation heaviest accumulations will be restricted to above 100m)

In the orange zone 5-10cm. (Down to sea Level)

In the Yellow zone 1-5cm. (Down to sea Level)


I expect that narrow red strip to get the heaviest accumulations due to an abundance of moisture pushing in from the north west and marginal uppers, this will only aid precipitation and I would expect to see some locally huge flakes associated with snowfall at marginal temperatures (could be some good hourly snow rates ).

The orange zone should also see some intense periods of snow, less in the north east at first but as the front backs west on sunday and grinds to a halt, this area should be set for a good period of snow. With long periods of moderate snow in the linconshire area saturday/sunday.

The western part of the yellow zone will probably be too marginal for the highest totals of snow, the wet nature of it should restrict snow totals (But a shift west in the charts again could very well change this ; ).) And in the southern part of this zone I think the front should be weakening with mainly light perhaps moderate at times accumulations, so not expecting too much here.


Sorry about my map been a bit sketchy, was the only map I could find and had to use paint.

Attached Thumbnails

• 

Edited by Harsh Climate, 02 February 2012 - 20:10 .

[OldMetMan]

It’s amazing how much model output can change in but a few days!

There is fairly broad agreement on maintaining the current block but for the 12Z ECM that is trying to bring in a NW flow late in the period. There are some variations in the location and intensity but HP is maintained over or near the UK, with a light flow off the near Continent or a high cell right over us.

The main area of interest at the moment is the LP forecast to move S over the country during the weekend. Again, there are slight variations but the general predicted path is down the middle of the country with snow on its E side. Once it is past, then pressure is expected to rebuild to the N.

It is going to be extremely difficult to assess snowfall in this situation. The projected path would certainly provide several hours of snow over the E Midlands, NE England and central and S/SE England, but with a slight fetch off the Channel coastal areas may escape. It is worth bearing in mind that the forecast is for a shallow LP that is unlikely to develop much so snowfall amounts could vary enormously from place to place.

That apart, the GFS shows another similar development next Wednesday, with the LP in this case heading S over the W of the country. It is hard to say at this stage how well surface temps will support snowfall with this event, if it happens, although with a mostly slack pressure field over the country and a pool of stagnant, cold air, there could certainly be some.

In the longer term, the models show a general trend of retrogressing the HP W, eventually leading to HP in mid-Atlantic. Upper air developments are showing some significant changes upstream with increasing meridional flow. Over the US/Canada, there is now a developing upper low over the W, moving E and amplifying, with a fairly strong ridge over the E Pacific. The upper low to our NW is moving little with the upper high holding firm over us, with an upper low covering much of Europe. This pattern has the look of a more stable wave pattern so the longer-term model depiction of a mid-Atlantic HP is a distinct possibility.



The jet is also showing some interesting changes. The is a complex split flow over US/ Canada with one arm heading NW towards us, and a more southerly branch which looks as if it will start to propagate on a much lower latitude. This raises the possibility of the UK being attacked on 2 fronts, with the development of an arctic front with LP heading down from the NW, together with the polar front displaced well south bringing systems in from the SW. If the cold air largely persists over the UK in the coming days with a light E or SE flow, this could provide some entertaining situations, especially if the HP over mid Atlantic extends W towards Canada – that’s the sort of pattern associated with prolonged cold spells.



I suspect the GFS may be first to indicate whether the above looks likely to happen. Meanwhile, the weekend will be very interesting!

[West is Best]

Will post in the other thread but some interesting synoptics at the moment, or at least we hope so. I'm not going to attempt to forecast what will happen the next 48 hours from the 0z runs as I'm poor with boundary snow events. All I would say is that despite the cold pool the angle of attack is unusual for a big snow event. Normally the rotation of this front would suggest to me sleet and snow turning readily to rain, but it may stall and therefore may be widespread. I'm not overly optimistic though.

Further ahead, and the models diverge. Looking at the 0z runs we have UKMO, GEM, NGP all pretty Atlantic dominated. The best hope is the slight delay that UKMO suggests at T60-T72, although it too eventually anchors the high over the Azores allowing a return of the westerly flow and it goes pretty full on zonal at T120 and T144. As the UKMO is, for me, the model of choice this is a concern.

Both the ECM and the GFS operational are very different, with the high anchoring over the UK prior to the FI retrogression in the GFS starting out at T240. The GFS pressure chart ensembles, however, show scant support for this high pressure to be over southern England and it could be called a true outlier:




However, the fact that the ECM backs that scenario adds considerable credence and the 850hPa ensembles continue to show a lot of scatter suggesting uncertainty and divergence:



In the meantime I hope everyone who likes cold can enjoy what we have, and hopefully some of you will see some snow, without worrying too much about next week. In terms of sustained cold we need to see if the GFS Greenland High in FI is the sign of a trend, or just another one of its guessing game scenarios. I'm not sure the GFS timeframe past T192 is a lot better than a game of Jack Straws. However, if that is indeed a trend then the key to unlocking true cold potential would be in the door.

[Harsh Climate]

Snowy Easterly, on 03 February 2012 - 03:12 , said:

The BBC weather site differs to your summary

yes latest models have pushed everything much further south east, remember my forecast was based on yesterdays 12z runs though. But I'm quite suprised as I really thought everything would be pushed further west .

[phil n.warks.]

With the next 48hrs. or so well covered in other threads i thought i would look beyond the weekend`s forecasted snow event and into the start of next week to see where we might be heading.

Tthe 00z runs suggest a rise of pressure across the UK as the block reforms,although the GEM seems less keen on this.
Yesterdays 12z `s runs all favoured some shortwave development further west around Tuesday which gave promises of undercutting and splitting the block with a return of an Easterly pattern.However GEM seems the only model still bullish about the undercutting this morning.We can see by around T120hrs. where the models suggest the energy may go.





We can see that GEM really digs that shortwave further west which would induce an Easterly quicker,the other runs seem to favour a more balanced split leaving the UK in a mid-latitude high.
GEM would retain the cold surface conditions for many with the SE still on the edge of the deeper cold from the nearby continent.
There would be a gradual warming from the north west on the other runs you would have to say.

Later output on the ECM,GFS and GEM show some form of Scandinavian High and weak Easterly pattern which suggests a return of some colder air off the continent.


So any slight warm up after the snow looks quite shortlived and the SE quarter of the UK looks as if it will remain quite cold for the next week at least and this cold spreading west again to other parts if the Easterly verifies as modelled.

Edited by phil n.warks., 03 February 2012 - 12:38 .

[damianslaw]

My words are on the ECM model. This continues to suggest retrogression of heights NW towards greenland later next week and formation of scandi trough. It is a theme it seems to want to stick with and is in line with its ensembles. Is it picking up the amplification forecasted to hit NE USA just before mid month that tad a little too early? or is it bang on the money.. who knows. GFS suggests similiar evolution but way out in the fantasy land.

The background signals all point to northern blocking as we reach the middle of the month, and the for the second half of the month to be dominated by a north westerly to northeasterly airstream thanks to stubborn heights to our NW - the perfect synoptic for sustained long term cold and a much more conducive synoptical situation for snow - indeed a similiar pattern to what we saw in late nov 2010 early dec 2010 and also mid dec 09 - early jan 10.

Those who believe the second half of feb is too late to deliver bitter cold and snow shouldn't, such a synoptic would deliver widespread very sharp frost, quite easily the coldest conditions of the winter for the north, and much more widespread longlasting severe spells of snow compared to what we are going to get tomorrow. Indeed tomorrow looks like just a pre-starter.

[weathermaster]

My thoughts on the weather models today,

GFS

First of all it must be said well done to the GFS last night most models tried to show a shortwave for mid next week but the GFS wasn't going for it now today we see most models moving over and agreeing with it. That's the thing right now we don't know who's right or who's wrong. So the GFS is showing really nice charts tonight in the long range with high pressure moving over Greenland giving us cold weather lasting well into mid February.

JMA

Still tries to bring a shortwave low in for Wednesday however doesn't really do the job to well. After this we do see it sending low pressure under the blocking letting high pressure build up in the Atlantic.

ECM

Shows the Russian high moving in over the UK for most of next week leaving the UK in settled weather and slightly warmer. After this once again the ECM is keen on building high pressure up in the Atlantic to give us a Northerly by next weekend. Its being showing signs of this for a few days now a good trend showing even the ensemble run shows high pressure building up in the Atlantic this is good we need to see the Atlantic weather ease off like this to give us our best chances for a good long spell of cold weather.

GEM

Bringing out some nice charts tonight as well it still seems very keen on sending low pressure down South and allowing high pressure build up over Greenland a somewhat similar trend to what the GFS and ECM show.

NOGAPS

Not showing great charts tonight with the Atlantic sending to much energy over our North not giving us good chances for high pressure to move up there.

UKMO

Not showing a shortwave tonight and that Azores high seems keen on moving up over the UK during next week so not excellent charts from it tonight thankfully it seems to have very little support on this.

Overall this weekend is looking very cold with Eastern England seeing the best chances of snowfall which is likely to be heavy at times. Currently we are unsure on how far in the cold will make it so look at the high res models like NAE and NMM for 24 to 48 hour ranges as they will give you a better idea. As for the longer term we are still unsure on things but there does seem to be a good trend in the GFS, ECM and GEM for a long lasting period of cold weather starting sometime next week this needs to be watched. For now those who are lucky enough down South enjoy the snow because its a rare thing this winter.