rory o gorman

There is a slight warm sector in the low Hence the more widespread precip and higher DP's

Simply put it's the thickness between the 500-1000 hpa layer, It's a strong indicator on the depth of cold in an airmass, generally under 528 is needed as a minimum, but the lower it gets the higher the probability of precip being snow.

The all elusive 528 DAM line should be in over the north by about 00z. Precip should be of mostly of snow from there onward.

Just a quick undate , Looking over the latest data .There appears to be two main areas of interest in relation to organised snowfall. Firstly the running wave along southern counties should give some back edge snow as it clears given the depth of cold behind.The second feature is an embedded mid-level shortwave that moves across the country from north to south from above 4 to 8 pm . This should give good accumulations in many places. The only issue is how east the precip goes, that will be a nowcast as all models differ on placement. There is be also the standard sea effect snow showers running into the north and west most of the day.

Quite promising in the short term for the white stuff in many parts Thursday morning into the afternoon/evening 850's sub -8 country wide . Thickness <528 , DP's under zero. A number of subtle shortwaves moving down across the country. For a polar maritime airmass its as potent iv'e seen in years.

Bartlett /slug high develops again at 180hrs with a mobile westerly pattern resuming .

GFS trying to put the final nail in the coffin ? Shocker.. Although there are a wide range of solutions throughout the suite the op and control are very similar on how they model the greenland shortwave energy and 300mb flow. Of course a disengagement in the subtropical jet would lead to a much different outcome, but past experience would my give me little faith.

Because the GFS low res will always revert back to zonal when the pattern is as erratic as shown. Expect a wide scatter on the suite later with the lack of upper air sampling this run is better off binned, there is obviously feedback within the algorithm regarding hemisphere pattern change.

90% chance will end with a slug low and zonal . Searching for cold and reading what the models are actually depicting are different things..

We now have the room to develop a shortwave on wednesday .... Continues to trend west .

I don't really understand your point here? There is a clear shift in the ens pack to a broad number of outcomes. It doesn't mean that the chance of prolonged cold is finished , just that there are some plausible solutions which result in a mobile pattern being obtained. 24 hours ago it was as close to locked in as possible, in the timeframe with classic WAA to greenland and possible cross polar flow and rapidly deepening full-neg tilted trough pulling east . So this uncertainty is unfortunately classed as a downgrade.

Control backs the OP in both runs , but yes there are wild fluctuations some of which are backing prolonged cold . Taking some perspect almost all 21 members were depicting cold this time yesterday today is less than half . This is a downgrade as much as anyone may want to spin it. The rapid increase in the subtropical jet as a result of cut off heights is a credible solution that has been seen many times before. Just looking at this objectively and without bias.

Very bias post I'm afraid. The gfs is consistent now in it's output , with the ECM backing the evolution this morning.

You do realise they can change their output overnight , I have a number of friends that work in the NWS and they often follow the ECM raw output as it models the midwest and eastern board rather well.

The ukmo doesn't run out to 168 hrs you said all 3 models? , the gfs and ecm have no to toppler with almost full cross polar flow.

How does one topple a cut off greenland high ?

You realise that the 'low' is entirely different from the one depicted in the 00z . This low forms as a result of falling heights and a 700mb vort lobe embedded within, known to many here as a 'trigger low' . Not related to the AZ trough on the 00z in any way.

06z is far more in line with the ECM mean , Heights building back east and a much more pronounced gradient north of developing surface low.

Basically what I was gearing up to write , Looking at it objectively there is still quite a spread in the GFS ENS regarding the ridge placement ,The development of the AZ trough my infact over amplify and result in a blow torch instead. This is far from a slam dunk and a non bias forecaster would would still air on the side of caution .

I agree a very west based ridge here, You can see the Met office's evolution to a westerly pattern clearly here.

14/21 ENS members go for a greenland / mid atlantic block , the slight concern exists with a trend to a more westerly based height increase which hampers deep cold arriving .

Gfs finishes with a nor'easter , London becomes New York ... Again all arbitrary after 180 hours on specifics .Upstream is at 09/10 levels.

Yes , I would ignore the downstream modelling at this point as synoptically it's quite unlikely to be so slow given the amplification . This as a pattern is a very encouraging run for sustained cold.

Far better WAA to Greenland on this run. Trough appears to centred further west though resulting in slower timing of deep cold. I wouldn't overly read into this though, focus on upstream.

Given the SST'S current and the 500mb to 850mb temps, any 700 mb shortwaves ejecting south would be highly convective and provide quite a bit of snow even inland. Again the chance's of polar low development in the 12z output seems favorable . ECM is key , often seeing eastern seaboard amplification differently. Again we wait...