Another cold spell another thread. To sum up the week ahead.

Monday: Some snow in the northern isles and northern scotland
Tuesday: Band of rain pushing south with risk of snow on its northen edge very briefly into the evening for parts of the north midlands possibly. Snow showers for the far north.
Wednesday: Snow showers for parts of Scotland, risk of a polar low for the extreme NW?
Thursday: Risk of snow for large parts of England north of the midlands. Many areas will see the snow turning to rain, some places likely will not.
Friday: ?????
Saturday: ????
Sunday: ????

---

Looks primed for at least a spell of heavy snow Thursday especially eastern Pennine upslopes with an easterly wind vector, pretty much guaranteed above 200m Peak District Northwards up towards Durham.
Lots of uncertainty regarding timing, where the heaviest precipitation will be and what happens to the front after it comes into Northern England. Those factors will determine  level of disruption, whether low levels are affected and whether it turns to rain.    

---

Forecast for here

Monday:No
Tuesday:No
Wednesday:No
Thursday:No
Friday:No
Saturday:No
Sunday:No

Repeat 30 times starting beginning of November to end of February

---

I'm expecting a rain-snow-rain event here, with at best a brief period of having a cm or two of slush on the ground (more likely no lying snow at all)

 

---

Pro tip: You can see evap cooling 'bands' in the T850 charts.

Look for pools of cold air (say -4C or -6C contours) seemingly appear out of nowhere just north of the front (i.e where the T850 gradient is), so essentially from south to north you end up with warm, cold, slightly less cold.

Anyway another thing to watch for is that big big T850 gradient; big gradients mean coherent fronts and they are more likely to be snow makers. Combine the warm, cold, slightly less cold pattern with a big gradient and you have a best case scenario (provided you are in the cold part). One of the most annoying things is being in the 'slightly less cold' bit and ending up with light rain rather than heavy snow; unfortunately that can happen if it ends up being marginal.
 

---

Pro tip: You can see evap cooling 'bands' in the T850 charts.

Look for pools of cold air (say -4C or -6C contours) seemingly appear out of nowhere just north of the front (i.e where the T850 gradient is), so essentially from south to north you end up with warm, cold, slightly less cold.


 

I would have assumed evaporative cooling at lower levels would lead to a warming at the 850 level. (Increased water vapour)

---

So this is not even close to being resolved. At 72h the front is (most southerly to most northerly)

UKMO: Mid wales to SE England
GEM: Mid wales to Suffolk
GFS: North wales to Nofolk
ARPEGE: North wales coast to Nofolk coast (slightly north of GFS)
ICON: Merseyside to Lincolnshire

And this is only the position of the front on Thursday. Don't even get me started about intensification and secondary lows e.c.t.
 

---

Forecast for here

Monday:No
Tuesday:No
Wednesday:No
Thursday:No
Friday:No
Saturday:No
Sunday:No

Repeat 30 times starting beginning of November to end of February

🤣

Pretty much sums it up for us Kent Folks, although we did manage a fleeting wintry spell a few weeks ago but almost missed it by blinking.

Fair to say, I won't be losing much sleep over this one. 

---

I would have assumed evaporative cooling at lower levels would lead to a warming at the 850 level. (Increased water vapour)

For sure it does moisten the atmosphere, but the ascent and condensational heating happens at an angle if that makes sense, since you get ascent over the surface warm front, and where the precip is the ascent tends to be weaker or you might even get some descent.
 

---

So this is not even close to being resolved. At 72h the front is (most southerly to most northerly)

UKMO: Mid wales to SE England
GEM: Mid wales to Suffolk
GFS: North wales to Nofolk
ARPEGE: North wales coast to Nofolk coast (slightly north of GFS)
ICON: Merseyside to Lincolnshire

And this is only the position of the front on Thursday. Don't even get me started about intensification and secondary lows e.c.t.
 

It's rare to get a decent snowfall here in Lincolnshire. A friend used to refer to us as "in the horseshoe round The Wash" where you missed out on interesting winters. Everything seems to be happening round us, only not quite here!
To be fair, over the past 40 years we have had a few good snowfalls. I recall one where we had an impressive crop of "snow rollers".

Two apposite local weather sayings:
'February fill dyke, be it black (heavy rain) or be it white (snow)'.
'A February Spring is worth nothing'.

---

We have a yellow Met Office warning for snow on Thursday. I’m not holding my breath though. 

---

Evap cooling is doing alot of work, high res models actually show snow falling in positive T850s now which is genuinely quite unusual. Because of this we also have to consider the risk of ice pellets and freezing rain.

Edit: here is a WRF cross section that goes from left to right, SW to NE.
https://www.meteociel.fr/modeles/coupewrf/coupegfs_486_502_521_331_63_0_1000_250.png

IMAGE. Members enable at bottom of page

Note the warm nose w ith T850s as high as +2.5C and surface temps below 0C.
Forget the -5C line for this event basically.

EDIT2: This is also the scenario that should produce very high quality snow if it comes off. Small snow crystals fall through the warmer layer and get sticky producing huge conglomerate flakes which then firm up as they fall through the cold layer below!
 

---

We have a yellow Met Office warning for snow on Thursday. I’m not holding my breath though. 

Cold front is 100 miles or so further south than forecast! Good start!
 

---

Evap cooling is doing alot of work, high res models actually show snow falling in positive T850s now which is genuinely quite unusual. Because of this we also have to consider the risk of ice pellets and freezing rain.

Edit: here is a WRF cross section that goes from left to right, SW to NE.
https://www.meteociel.fr/modeles/coupewrf/coupegfs_486_502_521_331_63_0_1000_250.png

IMAGE. Members enable at bottom of page

Note the warm nose w ith T850s as high as +2.5C and surface temps below 0C.
Forget the -5C line for this event basically.

EDIT2: This is also the scenario that should produce very high quality snow if it comes off. Small snow crystals fall through the warmer layer and get sticky producing huge conglomerate flakes which then firm up as they fall through the cold layer below!
 

Maybe I will hold my breath, as you put it like that.  I was looking at temperatures and thinking they’re marginal for snow. But wasn’t considering evaporative cooling.  👍

---

I see that the (real) far north of the UK has a warning out for up 10-15cm of snow in places today too.

---

Here's an even more extreme cross section.
https://www.meteociel.fr/modeles/coupewrf/coupegfs_393_527_564_379_72_0_1000_250.png

IMAGE. Members enable at bottom of page

We need to have a very serious conversation about freezing rain. Some places see literal hours on end of freezing rain. Real risk of real ice accumulation here. 
 

---

https://modeles3.meteociel.fr/modeles/alaro/run/alaro-16-60-0.png?06-04

Remarkable, Alaro has snow at T850s of 2C.  

 

---

https://modeles3.meteociel.fr/modeles/alaro/run/alaro-16-60-0.png?06-04

Remarkable, Alaro has snow at T850s of 2C.  

 

I've seen another one of the leading models do something similar (850s were closer to 3 or 4 IIRC) in the past. I flagged it up with the agency concerned but never really got to the bottom of it.

---

I've seen another one of the leading models do something similar (850s were closer to 3 or 4 IIRC) in the past. I flagged it up with the agency concerned but never really got to the bottom of it.

It is theoretically possible of course. If the temp maxes out at 850hpa and the freezing layer is say only 30mb thick, then you'd get snow from that. And while 3-4C is very very extreme, I think it does occasionally happen in the US (strong warm front coming into cold air combined with some orographic jiggery pokery).

For this event, I don't think snow will be falling at those T850s but the surface warm front decoupling from the upper front combined with decreasing wind/evap cooling could produce +ve T850 snow, in 2010 I saw snow at +1C so it does happen even here.

The bigger worry is that when you see snow falling at positive T850s that also probably means a significant freezing rain band. I'm suprised the metoffice haven't mentioned it.

EDIT: I should add that all the high res models that are in range (except the UKV) are producing positive T850 snow, so it isn't just one model. Unfortunately the met does seem to have a bias towards their own model, if the UKV was showing this then there would definitely be a freezing rain mention.
 

---

 Looking at the GFS ensembles, it is far from settled how far the warm air gets. 
 https://www.meteociel.fr/cartes_obs/gens_display.php?ext=1&x=258&y=16 

Haven't seen the ECM ensembles but on the mean chart the 0c Isotherm is straddling my location on Saturday 12am, ECM Op has it much further North so likely quite a bit of scatter there too. 

---