So after 6 weeks of zonality, it seems we finally have a cold spell of some kind in the offing. How long will it last?

As always, keep it civilised and on topic. Off you go!

Well I'd be very excited if the weather evolved like the 18z GFS. Shedloads of precipitation pushing into sub zero surface temps and dewpoints. All happens at around the t+108 mark. 

About time this area had a decent snow fall ;-) 

I await tomorrow's runs with interest. 

18z is a decent run to be fair as Joe has said plenty of cold air and ppn around.

Some really harsh frost on the way

We are running with increased risk 2-5 cm snow having accumulated by Sunday afternoon approx N Kent/S London down WSW to Hants. Reasonable support now for this outcome between EC, UKMO-GM and E4.           

From IF on NW

Also

PS Ditto re more marginal snow risk tonight-tomorrow on northern edge of convective cluster moving in from SW. Really needs 6-8mm p/hr to generate snow but it's plausible. Mendips for example, then Hants-Kent tomorrow.

TALLY

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NAVGEM first model (I think) to import some deep cold into scandanavia with the possibility it being diverted to the UK. 

I don't like this, its going to do what it always does and go to the midlands. . Anyway, a moderately impressive snow event none the less.

Here are the before and afters.

So its a 2-10cm event for lower levels and 15-30cm event for higher ground which I suspect is about right. And guess where the little white area is that gets no snow at all...

I doubt even north yorkshire will get that much, in fact I'd bet the higher totals will be further SW than this, the lows always trend south, and the NE often misses out. This is why lake=win for me.

As always if you wish to enjoy these fantastic 1960s styled charts in all their psychedelic glory then click open in new tab.

That ties in with "Showers most likely occurring during Sunday, perhaps with some prolonged snow for a time." in today's Met Office SE England forecast. That said their raw output just shows heavy rain all afternoon here!

Last night's ECM control run incidentally collapsed the block as early as day 8 with mild gunk spreading swiftly in thereafter. It was a marked mild outlier by the end of the run however. The extended ensembles clearly show two clusters developing by the end, a colder cluster with the majority of members and a more mobile, much milder cluster.

In the last thread the question of why are the operational runs mild outliers sometimes was asked, the answer is simply that there's more uncertainty in the forecasts and from time to time they'll "go off on one". It can also be because the operational's higher resolution has picked up on a critical feature which the ensembles miss - and in that case assuming the operational run is consistent it's not unknown for rest of the ensembles to flip in due course.

We'll need a "top up" source of cold air if the block's to extend far into week 2 - but that's still a long way off yet!

We have several potential snow events in the next few days, then it looks like staying cold for several days more - no change there!

Beyond that it's still uncertain but the majority clustering in the ensembles retains a cold look to things. ECM then shows the Siberian High ridging westwards towards the end of the run, with a warm sector being squeezed out in the process over the far west of the UK.

It's still a very interesting outlook in my view.

I agree although to be honest in isolation the ECM op run is quite different to some of the other output. Generally though things are pretty similar to yesterday with rather chilly output for the coming week and the chance of snow for many.

Temps for next Saturday , not sure what has changed the outlook is still cold.

As for snow , in these sets ups it wont become clear until 24 hours before any event , it is always a case of how far N,E,S or W blah blah.

Isn't this slider what we have been keeping our eye on for the last few days, H Willets showed it in her forecast Wednesday night, it looks the same set up and to be honest its as far as you dare look 120 hours, I am struggling to see why there has been a climb down..............I had better check the 384 chart eh?

Yes but look how mild it is in a week Marcus 

http://www.wetterzentrale.de/pics/Rtavn17417.html

http://www.metoffice.gov.uk/public/weather/forecast/gcr01t9qp

Met Office has me down for Snow on Sunday, no doubt change but shows the difficulty of the whole evolving set up

http://expert-images.weatheronline.co.uk/daten/proficharts/en/euro4/2015/01/16/basis00/ukuk/prty/15011800_1600.gif

I guess this is the ppn on Sunday that could give us Snow IMBY

There's still a range of solutions long-term from average to very cold, so focus on the shorter term as several opportunities present themselves for snowfall - starting this morning actually. Sunday looks intriguing for England and it could go either way with the placement of the nose trough still not known. Then we have the slider from the NW later Tues, which leaves us either in an easterly or a westerly depending on which solution you believe.

The Fantasy Islanders will like the ECM which has some real eastern promise landing at +240, but this model-watcher has plenty to resolve in the next three days without worrying about that LOL.

Ask yourself some questions?

In this formula, variables are in a range: Very good or never different at all = 5 to hopelessly bad or hugely different = 1

How often do charts of one model at +48 verify with any accuracy  = V1
How often do charts at +180 of the same model verify? Answer = V2
Let's assume there is little difference between the model solutions at +48 (D1=5)
How different on average are the solutions between the main models at +180? Answer = D2

The 'confidence' factor of a solution at +48;  C1 = V1 X D1
The 'confidence' factor at +180; C2 = V2 X D2

What is the difference between D1 and D2?

JFF

Yes - That had me mystified

Looks cold to me for the next week and there are potential snow events, albeit occasionally marginal.....but that's normal for the UK 

Yes Marcus. I predict this feature will dominate discussion as it evolves. In short we have a developing low of mid-Atlantic origin (with associated moist air wrapped up) overriding colder, dryer air at the surface. A very marginal environment but the potential for a significant snowfall along and near the northern extent of the ppn band has been raised. It looks like the centre will pass well to the south but the N-aligned trailing trough (or nose trough) is the one to watch.
But, the question of course is - where does it go and how much snow?

Never take too much heed of maxima on media output maps. They are very generalised, are the maxima for cities and are often way out. I've had snow falling with a predicted maximum of +8 before.

An interesting feature indeed. The northern extent is yet to be determined but with colder uppers to the north tucking in afterwards you wouldn't rule out a rain to snow scenario pretty much anywhere under its influence.

GEFS at 144h shows Scandi HP 5.Sliding LP 2,Uk HP 3,Azores Hp 3,Atlantic flow 9.

ENS tilting away from Scandi HP development.Much will depend on exact track of Tuesdays slider LP.ECM OP has a longer lasting Scandi HP.