Model Output Discussion - Into Winter

[LRD]

MOGREPS - London

Carlisle

Lots of scatter next week

[ITSY]

I’ve got to say I’m a bit bemused by this talk of -7/-6 being necessary upper air temps for southerners. I’ve had inches of snow in Greater London and indeed Cambridge with -2/-3 uppers before. When you throw in late December sun, relatively low dew points and extremely low thickness values, temps of -3/-4 could readily suffice for surprise snowfalls in heavier bouts of precipitation. The reverse is also true of course and I’ve had rain in -4 uppers because the other factors have been out.

-4 (certainly not -7) is the general line we should look for as a max upper air temp but it’s quite possible, in given circs with the right other factors,  for snow to fall at higher temperatures than these even in southern areas without altitude. 

Edited December 13, 2023 by ITSY

[Severe Siberian icy blast]

3 hours ago, Kasim Awan said:

Uppers don't matter much in a setup driven by evaporative cooling events and low heights.

 

A stiff westerly would be too much for that no? 

 

[Kasim Awan]

8 minutes ago, ITSY said:

I’ve got to say I’m a bit bemused by this talk of -7/-6 being necessary upper air temps for southerners. I’ve had inches of snow in Greater London and indeed Cambridge with -2/-3 uppers before. When you throw in late December sun, relatively low dew points and extremely low thickness values, temps of -3/-4 could readily suffice for surprise snowfalls in heavier bouts of precipitation. The reverse is also true of course and I’ve had rain in -4 uppers because the other factors have been out.

-4 (certainly not -7) is the general line we should look for as a max upper air temp but it’s quite possible, in given circs with the right other factors,  for snow to fall at higher temperatures than these even in southern areas without altitude. 

0.5C is the general max with frontal snow and evap cooling events.

[I remember Atlantic 252]

8 minutes ago, Kasim Awan said:

0.5C is the general max with frontal snow and evap cooling events.

Higher in March to May

[Kasim Awan]

7 minutes ago, I remember Atlantic 252 said:

Higher in March to May

I'm referring to 850s

[MattStoke]

26 minutes ago, ITSY said:

I’ve got to say I’m a bit bemused by this talk of -7/-6 being necessary upper air temps for southerners. I’ve had inches of snow in Greater London and indeed Cambridge with -2/-3 uppers before. When you throw in late December sun, relatively low dew points and extremely low thickness values, temps of -3/-4 could readily suffice for surprise snowfalls in heavier bouts of precipitation. The reverse is also true of course and I’ve had rain in -4 uppers because the other factors have been out.

-4 (certainly not -7) is the general line we should look for as a max upper air temp but it’s quite possible, in given circs with the right other factors,  for snow to fall at higher temperatures than these even in southern areas without altitude. 

Yes it’s quite tiresome hearing how snow will be over mountains when I’ve seen snow from such setups countless times at very modest elevation.

[Cheshire Freeze]

I’ve had 2 inches of snow fall here in December on a strongish NW’ly with uppers barely scraping -5. That’s at 55m elevation. 2011 I think.

Edited December 13, 2023 by Cheshire Freeze

[damianslaw]

Models surprisingly in sync today, the theme continues to be high pressure to the south eventually sent west due to a movement of the PV east, but no signal for the high to build north. After a very mild period ahead, temps in the lead up to christmas back to average or a bit below further north.

I would not be surprised to see sudden short term developments in the run up to christmas, with runners and secondary low developments on the west flank of the trough. The unknown is how far south the trough will dig, further south and this would put a longer brake on the atlantic ridge toppling in and a return to milder westerlies and allow for a colder arctic maritime feed, not so far a dig and a shorter lived polar maritime blast.

A few days ago I thought we might see a Dec 01 redux, currently looking more a Dec 04 redux, but perhaps timings a bit earlier, in 04 we had a white christmas, this is not a prediction for a white christmas I add. In 2004 the atlantic ridge crashed in on the 27th. 

Once again the 13th year in a row with no substantive sustained cold in the immediate lead up to christmas, last year it fell apart on the 18th...  but there is some cold and wintry interest, in many recent years this has not been the case.  

 

Edited December 13, 2023 by damianslaw

[Kasim Awan]

6 minutes ago, Cheshire Freeze said:

I’ve had 2 inches of snow fall here in December on a strongish NW’ly with uppers barely scraping -5. That’s at 55m elevation. 2011 I think.

 

[Ali1977]

T132, compared to the 12z the PV has more of a NW/SE tilt to it, and more Height further north in Canada. To me that’s an upgrade, poss bringing the cold air in faster than the last run - slightly more potent.

we shall see in 10 mins

[Kasim Awan]

Gfs too far east for me - joining the GEM.

[Mcconnor8]

4 minutes ago, Ali1977 said:

T132, compared to the 12z the PV has more of a NW/SE tilt to it, and more Height further north in Canada. To me that’s an upgrade, poss bringing the cold air in faster than the last run - slightly more potent.

we shall see in 10 mins

Depends where the PV drops, the slower evolution on the 12z is what dropped the PV closer to us, would be weaker uppers if it heads further East.

[weathercold]

Wish I could share the enthusiasm here but unless you’re on high ground up north this is a total non event for most. Cold rain for most.

Uppers of -4 approx nowhere near enough for snow on low ground irrespective of thickness. There’s also zero high latitude blocking in place so any temporary chilly conditions will be swept away with Atlantic fronts v quickly.

Edited December 13, 2023 by weathercold

[TSNWK]

Huge difference at day 7 looks better synoptic wise.. but will it drop south now.

[Ali1977]

1 minute ago, Kasim Awan said:

Gfs too far east for me - joining the GEM.

Hopefully it’s just winding up around Labrador ready to drop a snow storm from the NW on Xmas Eve ️

[Kasim Awan]

GFS18Z looks good now.

[Allseasons-Si]

It's all about the scenario's on whether you get snowfall below elevation's:-

in a PM airflow it has to be below -5 uppers to get snow down to modest levels and the thickness dam below 528

a battleground scenario is slightly higher in terms of the uppers from the legassy of a cold pool in the vicinity where stagnent air has pooled to the surface so 0c uppers or lower would be required dependant on the dewpoints and wet bulb at the time

A continetal flow has dryer wet bulb and colder dew points due to less moisture track from the continent,this direction could be from the NE/E/SE.

 

[That ECM]

Just now, TSNWK said:

Huge difference at day 7 looks better synoptic wise.. but will it drop south now.

Hang on, we are having a detailed debate, again, on what we need for snow. Stop showing models in the model output discussion.

[TSNWK]

Going east.. preferred the 12z

[BartyHater]

Any plunge significantly muted on the 18GFS at 168hr. Back to more of a glancing blow, with the cold going east.

Lets see what the rest of the run says, but it’s a poor start.

 

 

 

[Kasim Awan]

This is a close run, not sure I like it.

[Ali1977]

Different flow, but if we get the hammer from the NW on this run around Xmas there is a huge amount of extremely cold air ready to come with it compared to the 12z 

[weathercold]

Just now, Kasim Awan said:

This is a close run, not sure I like it.

Uppers a non event for entire UK. Not sure what others are seeing here. 

[MJB]

Just now, Kasim Awan said:

This is a close run, not sure I like it.

But it was good a minute ago