Interesting to see GEM's trough disruption in the western Atlantic appear on the GME 00z for Mon-Tue, given that it's higher resolution is arguably advantageous when handling such developments.

It's a good way to trap the Euro trough close to our shores going forward, providing a quicker route to a long-draw easterly, as GEM is all too happy to show us:

Shield the young one's eyes! 

Of the 'big' models, UKMO comes closest to achieving the trough disruption needed, while ECM and GFS show little interest on their respective op runs.

Seems like it would be winning the lottery, as the ECM/UKMO/GFS combination is rarely out of line at just 4 days range.

Longer term, signs that the Siberian High will become interested in our little island... but at the same time there is a push from the Atlantic.

Two main options are shown  by GFS and ECM;

The former has that push heading entirely NE with the Azores High reaching out to the Siberian High and drawing it toward Scandinavia, with the potential for an Atlantic undercut then illustrated. The Atlantic air hardly gets a look in until right at the end of the run.

The latter aims the Atlantic due east, bringing the Atlantic air back to our shores by day 10, leaving us looking for the Azores High to back west again and allow low heights back into NW Europe. With the Siberian High edging into Scandinavia, the potential for such a slide of low heights SE from the vicinity of Iceland is greatly improved... but there's no relying on the high behaving in this way.

I am given the strong impression that we will finally get to play for the 'Beast from the East' during the second week of February, but there's no telling to what extend we'll win.

I'd rather go down the GEM route though... I think my grin would sever the top of my head if the Atlantic trough did behave so favourably 

Originally Posted by: Stormchaser 

Dewpoints are certainly important.

It's an interesting subject generally wrt what actually falls with precipitation intensity, evaporative cooling and upper temperatures in the mix  

Originally Posted by: Sevendust 

I imagine that back at New Year, given the seemingly rather dire model output we were seeing for much of the time, the odds on not one but two cold spells occurring later in the month must have been rather long. Just goes to show how sometimes things can change quite quickly with the models.

Originally Posted by: David M Porter 

In general, based on a combination of studies at university and past experience:

Steady precipitation with a DP of zero can bring about an air temp. of zero close to the ground, so wet snow is then possible if the air at higher altitude is also at or below freezing. Settling can be an issue though, unless the precipitation is particularly heavy.

A DP of -1*C improves the likelihood of snow falling, as rain can be more intermittent yet still suffice. Settling chances are also improved.

Once the DP hits -2*C you're either in for some decent quality snow or some freezing rain, again dependent on the air temperatures at higher altitude.

BUT...

There are exceptions than can be caused by evaporative cooling or colder air being dragged down from higher altitude. It's thanks to a combination of these that I have known heavy, settling snow with a DP of 1*C (Portsmouth in January 2010).

- - - - - - -

Turning back to the models for a second, I just compared the 00z GFS, ECM and UKMO runs with their respective 12z runs from yesterday and there has been a notable elongation of that Canadian trough on day 4, with the mid-Atlantic ridge a little stronger.

Given such notable adjustments at such short range, something like GEM or GME's 00z runs appears within reach. We just need a bit of luck... the ridge is a tiny bit stronger on the GFS 06z as of +84 hours. Come on GFS... cause a major spell of cold ramping 

Originally Posted by: Stormchaser 

You can see why whatever falls today for a large part of England will be wet:

http://expert-images.weatheronline.co.uk/daten/proficharts/en/euro4/2015/01/29/basis00/ukuk/taup/15012912_2900.gif

However, a small window later this afternoon early evening:

http://expert-images.weatheronline.co.uk/daten/proficharts/en/euro4/2015/01/29/basis00/ukuk/taup/15012918_2900.gif

Originally Posted by: Whiteout 

In other words,it might stay cold or it might become milder.....lol!

Originally Posted by: colin46 

I imagine that back at New Year, given the seemingly rather dire model output we were seeing for much of the time, the odds on not one but two cold spells occurring later in the month must have been rather long. Just goes to show how sometimes things can change quite quickly with the models.

Originally Posted by: David M Porter 

The tricky thing about sliding lows heading SW of a trough rather than an area of high pressure is getting that cold undercut from the east to produce the widespread snowfall.

GFS just about manages it on the 06z as the LP slips away to the south.

Longer term is interesting, as the Euro trough is trapped in place without much escaping to the NE. This shift in the direction of GEM appears to be largely down to the ridge being a little stronger. Interesting that it occurs despite the Canadian trough still not disrupting effectively.

There's almost a new import of low heights to Scandinavia from Svalbard on day 8, but as usual GFS wants to take the energy NE instead. No worries - a cold UK high with large areas of lying snow and uppers as low as -10*C isn't something to complain about 

- - - - - - -

Quantum, I agree that wet bulb temperatures are useful when winds are light 

This parameter is becoming available on an increasingly wide range of sites, which is good.

Originally Posted by: Stormchaser 

GFS Op and the high res WRF both proving a notable snow event for EA overnight tonight as a front crosses from NW to SE. Euro4 takes it much further south and GEFS have the uppers too high and the front falls as rain. V tough one to fall at just + 19hrs away - interesting to see which way the pros call it and whether faxes, forecasts and warnings give anything away in coming hours. Wonder what the pros in here think...GEFS certainly trending cooler for the south this morning!

Originally Posted by: ITSY 

from fergie on NW ---------
 

It's all still very marginal next week with developments Mon-Tues. An awkward rain v snow situation depending on track, scope of deepening and phasing with diurnal cycle. ECMWF-DET remains at the extreme end of solutions (offering disruptive snow W Mids/parts of W Country/S-Central England) but with discernable move of UKMO-GM towards the *broad* scheme of things; i.e. trend to pull the low centre a bit closer to SW UK. EC 00Z ENS continue to offer a fair range of solutions, including deeper centres running further to E; equally many are much less developmental. For now, we continue with a low (30%) PROB of something unfolding akin to EC DET outcome. Rest assured it's a key point for our attention into medium range, with constant re-assessment at Ops Centre. We will keep you fully updated via BBC forecasts but I do urge suitably cautious acceptance of hyperbole or any attempts (for now) at deterministic forecasts re snow/areas affected/etc etc. We are a way off getting to that level of forecast confidence or detail.     

Originally Posted by: sizzle 

Here is your answer ITSY. The METO think it will affect Wales, Midlands and the South:

http://www.metoffice.gov.uk/public/weather/warnings/#?tab=warnings&map=Warnings&zoom=5&lon=-3.50&lat=55.50&fcTime=1422489600&regionName=uk

Originally Posted by: Easternpromise 

Hi Dave, agreed, as you can see I am looking at the parameters, more specifically the dp, that we have as well as seeing what is falling and it is fascinating. The dp's and WBT seem to correspond to what is falling this morning.

Down south it does look like we have the right conditions between 6pm and midnight for some snow to fall if there is precipiation around.

850's of -5, dp's of -2 on both Euro 4 and GFS and low WBT. Let's see what falls (if anything).

Originally Posted by: Whiteout 

20 January 2015 13:19:19

A lot of interest today in FI, atlantic getting up to go to the toilet, not properly waking up. 

By the time we get to saturday (when the cold spell truly ends) 240hr will be 120hr, I think it could be worse. 

Quantum wrote:

21 January 2015 

NAVGEM is fantastic too! Can anyone else feel that there might be something big here, or am I just mad? 

Quantum wrote: