Model output discussion - deep cold inbound?

[Matt Jones]

Liking tonight's UKMO Sunday through to Tuesday 850's looking good. Not liking GFS runs of late though wanting to bring less cold uppers in from the south and been consistent with it's breakdown from the SW, it can't be ruled out but hope it's got this all wrong. 

[Paul]

Before we get too heavily into the 850 temperatures snow / not snow / snow types thing again. Just posting @snowking's guide back up for those who may not have read it:

Also, bear in mind that 850hpa temperatures aren't the only indicator to look at, in fact I'd say they're probably one of the last indicators to use. 850-1000 temperatures are a far better guide, with anything under 1304 (approx) giving a chance of snow, but the deeper into those blues you get, the higher the risk is. This is using the 03z UKV, so soon to be superceded by the 15z, but it gives a good indication that just about everywhere will be into an airmass that's cold enough for snow at that time (midnight Sunday)

The GFS does move marginally less cold air into the south early in the week. But a. it's a bit early for details on this sort of thing. And b. In the main it's still cold enough for snow to be the most likely thing to fall out of the sky for many. 

 

[Thundery wintry showers]

This is likely to be my last South West winter for a while, it's been a frustrating one so far but I'd say there's definitely potential for a South West snow event to arise around Monday-Wednesday next week, depending on how that frontal attack from the south-west behaves.  The GFS 06Z this morning had a favourable setup for snow across much of south Wales, central southern England and the West Country spanning a couple of days before milder air pushed north, and today's UKMO 12Z looks even more favourable for that, with widespread frontal snow probably spreading further east across southern England as well and the possibility of the low sliding eastwards and not pushing milder air up from the south.

On the other hand we have the GFS 12Z which would probably see the West Country miss out (except over Dartmoor/Exmoor and other high ground) due to the milder air pushing north too quickly, though further north (Wales, Midlands, Cent S + SE England) could get some significant transitionary snow out of that.

Still looking very likely that E Scotland + NE England and down to Lincolnshire will be in the firing line for plenty of snow showers off this easterly, maybe some longer outbreaks of snow from trailing fronts, with sleet/rain, if any, confined to extreme coastal fringes.  Convection is more uncertain for the SE but so far todaty's runs have more of that than last evening's with more of a NE rather than N surface flow. 

 

[kold weather]

1 minute ago, Jason H said:

That's very much a worse case scenario. All I can see is snow from that set up. 

Nah not even close to worst case, worst case was what GFS was doing a couple of days ago!

As I said this morning, about 20 runs had no settled snow in the SE on the ECM ensembles, so something like 30-40% of the runs. 

I don't think its what will happen tbh, but its not an insignificant possibility. Further west and north you go the lower the risk, and I think north of EA your fine regardless.

[Drifter]

GEM keeps deep cold lurking over scandinavia to the end of the run and the high pressure looks to be re-grouping.

I wonder if we might see the cold relax for a day or two and then re-assert itself as models often underestimate a cold block of air?  This could just be the starter.

[Jason H]

1 minute ago, kold weather said:

Nah not even close to worst case, worst case was what GFS was doing a couple of days ago!

As I said this morning, about 20 runs had no settled snow in the SE on the ECM ensembles, so something like 30-40% of the runs. 

I don't think its what will happen tbh, but its not an insignificant possibility. Further west and north you go the lower the risk, and I think north of EA your fine regardless.

Ok. I'll agree to disagree. I was looking at the operationals from the latest runs. 

[Tim Bland]

5 minutes ago, Neilsouth said:

Yes but equally it could force low pressure systems into France, with a number of channel low's

You may get 1 Chanel low but IF the jet becomes that strong and the Atlantic stays firing on all cylinders we are not going to get continuous channel lows. At least I can’t remember it ever happening ??  
Our best chance at longevity is that the cold over the Us is moderated, the lows are much shallower and further south...then we stand a chance

[nick sussex]

1 minute ago, kold weather said:

Nah not even close to worst case, worst case was what GFS was doing a couple of days ago!

As I said this morning, about 20 runs had no settled snow in the SE on the ECM ensembles, so something like 30-40% of the runs. 

I don't think its what will happen tbh, but its not an insignificant possibility. Further west and north you go the lower the risk, and I think north of EA your fine regardless.

I hope people appreciate your  honesty and reluctance to spin things . Because the forecast for southern areas is more complicated than further north . 

 

[Geordiesnow]

6 minutes ago, kold weather said:

All depends on the wind direction and plenty of other factors.

The GFS 12z has other factors that are more than good enough. For example a SE wind which allows for slightly lower dew points and a good feed of lower thicknesses into the region to compensate. 

00z ECM turned it to rain for the SE as the wind direction was too much ENE which meant the dew points and other factors also were too high, before it turned back to snow for most as the warm sector filled in and cooled down.

I think we will be ok down here, but I've gotta keep people aware of the downside, especially as this place becomes a cold hivemind ay times, then they all get surprised when things dont go as expected for some.

 

 

Very good post, when the GFS was coming out, I was getting nervous about the uppers because it could make a huge difference in terms of the weather we get. The margin for error for those in the south is small but even those further north  cant fully relax either.

Still a few question marks on just how quickly and how far south and west the coldest air gets.

[kold weather]

4 minutes ago, Jason H said:

Ok. I'll agree to disagree. I was looking at the operationals from the latest runs. 

This is the ECM operational from the 00z fwiw:

Thats probably a realistic worst case scenario at the moment to be honest!

It was something of an outlier with regards to the wind direction though which suggests its handling the low differently to the other models.

BTW Jason, the GEM is a heck of a gem for you guys, I used to live in Thurrock and this airflow would be leaving extremely excited.

Wouldn't rule out thundersnow in this sort o set-up btw! I'm an alumni of the Thames streamer!

Edited February 3, 2021 by kold weather

[Djdazzle]

Is the GFSP coming out this evening?

[Valleyboy]

2 minutes ago, Drifter said:

Who has said that?  Maybe Koldweather, but he's an extreme pessimist!

It’s too far out to know for sure.  Hants normally does well in these set ups.  I said earlier that spots that currently look favoured will change.  Chances are that the places not looking so good at the moment will end up getting plastered.

[Daniel*]

16 minutes ago, Tim Bland said:

If you look at the cold across the pond it’s no wonder the jet stream is going into over drive. The block is strong but with a jet like that it’s no match unfortunately.   Long cold spells usually occur when the jet / Atlantic is quiet and this is difficult with that much cold over northern USA. Still a chance that we can get an extra day or so out it and the first shot could slide and give snow , but I don’t think we can sustain our cold for long.
Still 5-6 days with widespread snow cover is significant! ️ 

As long as the jet stream is well to our south a powerful jet stream isn’t a deal breaker even could help getting some lows/moisture our way especially for western areas which don’t benefit as much from easterlies.

Edited February 3, 2021 by Daniel*

[kold weather]

7 minutes ago, nick sussex said:

I hope people appreciate your  honesty and reluctance to spin things . Because the forecast for southern areas is more complicated than further north . 

 

They are definitely more tricky for the south for sure.

I do also want to add, I think even if there is a warm sector, its not going to ruin the set-up, it will just give a small window of time where things may get a bit wet for some. It will almost certainly cool back down afterwards again and quickly turn back to snow showers.

If we can keep the LP weaker than what the ECM shows (989mbs) then we will probably be alright, its only if the LP deepens too much that it may introduce too much marginality. 

I suppose I'm playing the counterweight action on this thread atm, as I don't want to see people over ramp prematurely, you me and plenty of the other older folks on here have been there many times before!

Edited February 3, 2021 by kold weather

[Mike Poole]

Well, I think there has indeed been some narrowing of the uncertainty envelope in the 6 day outlook, here’s GFS, GEM and UKMO at T144:

Well wow!  UKMO is sensational!  And this one has maybe sniffed out a longer run of cold than the 5-7 days we might have been expecting from runs so far, pity it doesn’t go out to T384!!  

GEM is also excellent.

GFS is an improvement on the 6z, but is less to be trusted in these UK scenarios we know this, it tends to blow up lows rather than let them slide.  Yes, before anyone asks, it did well in some earlier setups this winter, but the area of interest then was off the east USA to Greenland, which is an area it does well, and better than the UKMO and ECM.  But for this, I don’t rate it.

[bluearmy]

6 minutes ago, Tim Bland said:

You may get 1 Chanel low but IF the jet becomes that strong and the Atlantic stays firing on all cylinders we are not going to get continuous channel lows. At least I can’t remember it ever happening ??  
Our best chance at longevity is that the cold over the Us is moderated, the lows are much shallower and further south...then we stand a chance

the usa cold temp forecast is at an earlier stage in the run as its upstream !  and being over a large continent i expect it will be right

i think the hope for nw Europe is that the neg AO is so large that it continues to drive the reverse flow against the northern arm in the atlantic and shears away at the trough, forcing it disrupt se 

exeter are stll referencing high slp to be more relevant than we are generally seeing on the ops to force the jet south

[kold weather]

GFS ensembles are getting increasingly snowy, to go along with the GEM and the GFS.

Increasingly confident that there is going to be a metric shed load of snow around. Regardless of perhaps some marginality briefly on Monday for the SE, it is increasingly becoming more confident of a decent snow event coming up.

 

[Cold Winter Night]

5 minutes ago, Tim Bland said:

You may get 1 Chanel low but IF the jet becomes that strong and the Atlantic stays firing on all cylinders we are not going to get continuous channel lows. At least I can’t remember it ever happening ??  
Our best chance at longevity is that the cold over the Us is moderated, the lows are much shallower and further south...then we stand a chance

This is why it's good that models are toying with height increases to our North (NW and NE), like GFS 0z and ICON 12z.
After 144h there is still a lot to be resolved. I see @bluearmy mentioned this too now.

With Scandi/Iceland heights in place, a strong jet will either stay to the south and indeed continue to send lows along that line, or the jet might go NE and fuel a Scandi High through WAA.

Especially in February there can be a cold America and a cold Europe at the same time.

[TSNWK]

14 minutes ago, kold weather said:

This is the ECM operational from the 00z fwiw:

Thats probably a realistic worst case scenario at the moment to be honest!

It was something of an outlier with regards to the wind direction though which suggests its handling the low differently to the other models.

BTW Jason, the GEM is a heck of a gem for you guys, I used to live in Thurrock and this airflow would be leaving extremely excited.

Wouldn't rule out thundersnow in this sort o set-up btw! I'm an alumni of the Thames streamer!

Hello @kold weather that graphic really helps illustrate the outcome on where the low on Monday   sits has - nice and simple colours, may I please ask for you to share the same for 12z's line against the above - be great to see if any difference - more blue, less green Obs! 

Edited February 3, 2021 by TSNWK

[Kentspur]

7 minutes ago, kold weather said:

GFS ensembles are getting increasingly snowy, to go along with the GEM and the GFS.

Increasingly confident that there is going to be a metric shed load of snow around. Regardless of perhaps some marginality briefly on Monday for the SE, it is increasingly becoming more confident of a decent snow event coming up.

 

Yep best snow spikes of the season so far for Dartford, Kent

[Badgers01]

19 minutes ago, nick sussex said:

I hope people appreciate your  honesty and reluctance to spin things . Because the forecast for southern areas is more complicated than further north . 

 

I think people do and it’s a good counterbalance to the the hyperbolic run of the century posts we have had today ! Snow yes anything really historic not so sure ! If I was up Yorkshire way though would be pretty confident of some more decent winter - down here in the south not so confident but in with a chance ! 

[Man With Beard]

Based on all the models I've seen in the past 24 hours, this is my interpretation, well it was a picture or 1000 words  

[MJB]

35 minutes ago, AdrianHull said:

Yes you did. But I think people want  snow from the continent with low uppers. Not a wet 10cm snow /slush 

Certainly wasn't slush and will hang around longer as its much much colder , so much fixation on -10 850's

[chris55]

It looks like we have good model consensus on the initial easterly flow making it to the entire UK. Those in the extreme south/south west will always be close to the boundary, but overall, I think we can say an easterly flow is very likely with some very cold air. 

Looking back to 2010, and these are some of the coldest charts we have seen since then. The  Synopotic/broadscale pattern is one I can not ever remember seeing at such short range!

Once the flow gets established we have lots of options of where things could go, none even close to a normal UK winter! Going forward, after the initial easterly,  we have a few main options,

1 The Atlantic tries to push in but gets forced south east....those in the south west quarter of the UK get a blizzard.

2 The easterly flow sustains, those in the east continue to get buried with snow showers and streamers, those in the west get some snow and overall we all stay cold.

(1 or 2 most likely)

3 The Atlantic lows makes inroads and pushes the cold north, the boundary moves north and the blizzards are pushed north.

4 The Atlantic wins, mild for all.

 

Whatever happens its going to be one hell of a fun ride!!

[sheikhy]

Just now, Man Without Beard said:

Based on all the models I've seen in the past 24 hours, this is my interpretation, well it was a picture or 1000 words  

I think you better move that line that is on the border of east anglia further west and the one over the north midlands further south