Model output discussion - deep cold inbound?

[Blizzard]

21 minutes ago, Singularity said:

Talk of drifts has got me wondering, for the first time I can recall in fact, whether the models in any way factor in how wind affects snowfall.

It becomes relevant when the snow is dry and powdery - it can travel dozens of miles downflow in a brisk wind. I've experienced a moderate fall of powder snow beneath clear skies with the nearest clouds out near the horizon.

Incidentally, turbulent winds can also lift powder snow quite high into the air - but I doubt that can account for much snow transport in this country with all its obstacles sticking up out of the ground!

That's really interesting.  I grew up in Skegness & in the mid 80s (can't recall which year) I swear that the clouds were full of snow - yet very little was coming down (dry snow - settled in very little drifts on cold hard ground) - yet inland (maybe helped by the Lincolnshire Wolds?) they were getting many inches.  Eventually the winds calmed down & it did warm up slightly... we then got buckets of snow.  I've often wondered about that as was very frustrating at the time....

[kumquat]

13 minutes ago, jason6347 said:

Hi are these charts a good guide usually?

Only when they show snow for your area 

[Purga]

Significant extension of the cold spell already in the ENS 06z

Edited February 4, 2021 by Purga

[Typhoon John]

6 minutes ago, Nick F said:

Powder snow does seem to have amazing powers, not only is it prone to cause great drifts if its windy unlike snow with high water content that sticks to everything, it can find its way through the smallest cracks. Always remember the infamous 'wrong kind of snow' which I think was coined by the back then British Rail in the Feb 1991 severe cold and snowy spell. The dry powder snow infiltrated the mechanics of the electric train motors and caused issues with short-circuiting. I remember getting a train home to Kent from London on the onset of the Jan 1987 severe spell and remember the powder snow coming inside through the gaps between doors on the train, amazing qualities it has.

Anyway, going off topic. Easterly arctic continental flows are inherently dry at the surface, so snow falling from moister upper levels tends to have lower water content that the moister polar maritime air that brought snow back in January via the NW. 

06z GFS op for London quite close to the average of the GEFS spread  through most of the run, quite a few colder options in there, not really any sign of a 'warm up' until late in the run - which is too far out to have any guidance. Certainly good scope to stay cold for most all of next week.

Powder snow also has higher ratios.  1:10 may be typical but get dry powder and it can increase up to 1:40 and create huge snow depths in the right conditions.  Also a chance to see some stellar dendrites if it is cold enough!

[Paul]

We could probably try to make chart which uses a variety of params to estimate snow type and possible depths, as I do agree the raw snow depth output I'm seeing at the moment does look a bit iffy. Will have a think through it and try to come up with something which will either be fun to look at, or useful (or both, or neither!). 

[Rollo]

1 hour ago, nick sussex said:

I hope everyone enjoyed my Crunchy Snow and Ice Day committee !

I’ve added a new sub-committee !

CEC ( Cold Extension Committee ) .

First meeting this evening after the ECM and its ensembles come out .

Conclusions to be published later this evening .

What about WISIC, Will it snow in Carlisle as well!

[That ECM]

Direction of travel in 10 days? Who knows but not a bad mean imo.

Edited February 4, 2021 by That ECM

[markw2680]

So normally say when you have wind coming from the west with low pressure you get showers pushing right across apart certain areas when hills and mountains take them all up, so my point is with the winds basically the opposite so coming from the east then I’d imagine snow showers and longer spells of snow pushing pretty much the whole way across to the west, and there’s less in the way of hills swallowing it all up. Mmmm very interesting 

[Man With Beard]

Developments in the ECM clusters: there is now a cluster that keeps the jet further south out to D15; this contains the control run (which I believe the op was following until D10). However, the majority view remains to bring a milder flow over the UK from the middle of February

[Ice Day]

An improved set of GEFS for my location - cold is in by 72 and more cold members at 192, showing a trend to extend the cold.

And for our northern friends, these are for Manchester - even colder! 

The for those in the north east - sweet baby Jesus 

[sunnijim]

58 minutes ago, kold weather said:

GFS ensembles are really struggling to get the first LP too far north, and sliding it off to the east.

This is a mixed blessing, it keeps the country in colder air for longer BUT the upstream low I've got to admit looks beefy. I suspect regardless of what the 1st low does (IMO it may well slide over the SW and into the channel) the 2nd one will come through like a steam train, and if thats the case it may be a literally leading edge snow-rain event, especially the further south you are.

Sportsmans bet, the second Low won't come through 'like a steam train'

It will be modeled  later as a weaker feature and stall over Western parts with a snow to rain event for the SouthWest.

Why? Years of watching Blocks get the upper hand over a seemingly alarmist set of Atlantic driven options.

Also,once we have the cold block in place it will take some shifting..

A Low as modeled would certainly shift it,although I'm not buying it at all next week as shown.

 

[Cuban Zebra]

1 minute ago, Ice Day said:

An improved set of GEFS for my location - cold is in by 72 and more cold members at 192, showing a trend to extend the cold.

And for our northern friends, these are for Manchester - even colder! 

The for those in the north east - sweet baby Jesus 

Just when you think it can’t upgrade again!!!

[kold weather]

10 minutes ago, sunnijim said:

Sportsmans bet, the second Low won't come through 'like a steam train'

It will be modeled  later as a weaker feature and stall over Western parts with a snow to rain event for the SouthWest.

Why? Years of watching Blocks get the upper hand over a seemingly alarmist set of Atlantic driven options.

Also,once we have the cold block in place it will take some shifting..

A Low as modeled would certainly shift it,although I'm not buying it at all next week as shown.

I hope your right sunnijim.

Historically what Ive noticed is the first attack often does get deflected, but the 2nd or even 3rd normally is the one that forces everything through.

It takes an exceptional event for it to keep getting rebounded. 

Who knows, maybe this maybe one of those.

Even then, I suspect the south will see temperatures moderate upwards regardless, only so much of SE/S winds you can have before the surface temperatures respond upwards down there. That pattern would be a good one for the Midlands though and has often given some good events for those guys along with Wales, etc. Indeed thats what alot of the GFS/ECM ensembles show as well fwiw. Also worth noting the cold may well hang on in the north anyway.

This is a great ensemble mean though to be fair:

Edited February 4, 2021 by kold weather

[Stu_London]

3 minutes ago, Ice Day said:

An improved set of GEFS for my location - cold is in by 72 and more cold members at 192, showing a trend to extend the cold.

And for our northern friends, these are for Manchester - even colder! 

The for those in the north east - sweet baby Jesus 

Those north east ensembles are not far off the GFS para was showing a few evenings ago - perhaps it was onto something.

In the meantime, it does look like we will be in for a classic modelling case of the breakdown being shunted back continuously. If I had to guess, I would say most of the UK will now get a minimum of a full week solid cold spell, with the potential for more. 

Only concern is for the far south and south west as more marginal conditions are never that far away. 

[Battleground Snow]

These charts are exactly 30 years apart and you can definitely see some similarities between them.

 

[Day_9]

1 hour ago, Ice Day said:

Look how the 6z is losing the dartboard low in the Atlantic seen on the 0z at 150, and disrupting and stretching it more. This will help it slide more and keep the bitter air in.

  

I really think this is going to be the direction of travel and the breakdown maybe a fair way off.  We're on the cusp folks! 

All Atlantic lows become sliders as they get crushed by sheer strength of cold and nothing behind them as back up to drive em through 

[IanT]

13 minutes ago, Ice Day said:

An improved set of GEFS for my location - cold is in by 72 and more cold members at 192, showing a trend to extend the cold.

And for our northern friends, these are for Manchester - even colder! 

The for those in the north east - sweet baby Jesus 

Could you point me to the source of these please? I have been unable to find it through google etc.

[kold weather]

8 minutes ago, Stu_London said:

Those north east ensembles are not far off the GFS para was showing a few evenings ago - perhaps it was onto something.

In the meantime, it does look like we will be in for a classic modelling case of the breakdown being shunted back continuously. If I had to guess, I would say most of the UK will now get a minimum of a full week solid cold spell, with the potential for more. 

Only concern is for the far south and south west as more marginal conditions are never that far away. 

Yeah whats interesting is how the first low is already getting increasingly modelled weaker, though quite a few ensemble and ops are still starting to weaken the hold of the cold air a little over the south due to a switch around to a slightly kore southerly direction.

Won't make any difference away from the coasts mind you as the cold is pretty well established at the surface at least to start with.

The 2nd low looks alot stronger to me, though agian as @sunnijimsaid above it may well end up being the case that even the 2nd low gets rebounded and shears away to the east. I personally think the 2nd one may be the one, at least for the south (and you can kinda see that on the ensemble graphs above for the south, not so much for the north though), but even then the surface temperatures will lag 24-36hrs behind any rise of the 850hpa profile.

[Tim Bland]

Probably more relevant than T850s at this time of year is the Dew point. As long as uppers are well below zero ;  the If DP is at or below 0c you can usually expect snow. The 06z ensembles mean expect this to be 0c or below up until 14th so that would be 7 days of snow opportunities Ps I expect this could be improved on further in future suites ...

[Man With Beard]

21 minutes ago, sunnijim said:

Sportsmans bet, the second Low won't come through 'like a steam train'

It will be modeled  later as a weaker feature and stall over Western parts with a snow to rain event for the SouthWest.

Why? Years of watching Blocks get the upper hand over a seemingly alarmist set of Atlantic driven options.

Also,once we have the cold block in place it will take some shifting..

A Low as modeled would certainly shift it,although I'm not buying it at all next week as shown.

 

Might be better not to have a clean break between the first and second low - too much of a break and heights will get in ahead of the second low, driving up SWly air ahead of it.

[ptow]

Quick Q - looking at the 850's on the GFS for say Monday - I can see that they are not as low in the south as the North however the 2m temps are just as low in the south ie about -1 or -2 at 3pm. Temps you wouldnt normally expect with -8/9 uppers?

[Stu_London]

3 minutes ago, kold weather said:

Yeah whats interesting is how the first low is already getting increasingly modelled weaker, though quite a few ensemble and ops are still starting to weaken the hold of the cold air a little over the south due to a switch around to a slightly kore southerly direction.

Won't make any difference away from the coasts mind you as the cold is pretty well established at the surface at least to start with.

The 2nd low looks alot stronger to me, though agian as @sunnijimsaid above it may well end up being the case that even the 2nd low gets rebounded and shears away to the east. I personally think the 2nd one may be the one, at least for the south (and you can kinda see that on the ensemble graphs above for the south, not so much for the north though), but even then the surface temperatures will lag 24-36hrs behind any rise of the 850hpa profile.

For anyone that didn't see the 18z Para from Saturday (I think) - it was showing brutal uppers of between -12C and minus -18 piling into the uk throughout the run from this weekend right up to +384h and would gone on for sometime thereafter, if anything getting progressively colder. There were a few days in the south where milder air won out for a time, but by the end of the run, the whole of the UK was back in the freezer. Nothing in the current modelling to suggest anything like that will happen, but the trend seems to be in that direction.

[kold weather]

2 minutes ago, Man Without Beard said:

Might be better not to have a clean break between the first and second low - too much of a break and heights will get in ahead of the second low, driving up SWly air ahead of it.

Thats exactly what I was getting up, it means the air is modified in front of the 2nd front and there is little resistance so the next front comes through with just leading edge snow.

Of course all that relies on the 2nd low being strong to push through in the 1st place, and obviously the further north you go the more leeway you've got to gamble with.

Us lot down here have little room for error in those set-ups.

[Battleground Snow]

2 minutes ago, ptow said:

Quick Q - looking at the 850's on the GFS for say Monday - I can see that they are not as low in the south as the North however the 2m temps are just as low in the south ie about -1 or -2 at 3pm. Temps you wouldnt normally expect with -8/9 uppers?

I assume it's because of the short sea track from the near continent helps the 2m temps, where it may mix out more over the longer sea track further north.

[Timmytour]

1 hour ago, Singularity said:

Talk of drifts has got me wondering, for the first time I can recall in fact, whether the models in any way factor in how wind affects snowfall.

It becomes relevant when the snow is dry and powdery - it can travel dozens of miles downflow in a brisk wind. I've experienced a moderate fall of powder snow beneath clear skies with the nearest clouds out near the horizon.

Incidentally, turbulent winds can also lift powder snow quite high into the air - but I doubt that can account for much snow transport in this country with all its obstacles sticking up out of the ground!

I remember a similar thing in March 2013...A part of the A10 near Buntingford was blocked by snow and I couldn't ever have imagined we would have had that much snow it would be the case.  Over the next two weeks I'd drive regularly past the areas which only slowly declined.  I think it was gap that allowed the snow to blow in from the fields which we slightly uphill of the road.