Quite a remarkable 18Z run. Keep in mind -2C at T850 is usually sufficient for snow in warm/occluded front setups. So with temperatures often not getting above -7C at T850hpa this is comfortably cold enough for snow. It means we could see this trend milder by several degrees and still see snow.

-13C isotherm. The last time we've seen that must have been march 2013. 

Quite a big upgrade in the wintry and snow for next week too: Am I the only one seeingt this? A day or so back that HP to the east and cold block was going to lose the battle!?

Tuesday /Wed's next week shows max along the south coast of +2/3c with snow/wintry showers almost anywhere!

Strange how the GFS had 2 or three runs where it sort of got this right earlier last weekend and then dropped the idea spectacularly until yesterday.  if it had had 'the courage of its convictions' this forum would have done very well over the last several days.

.........Wife: 'where are you going with that thermometer and measuring stick.

Me:  'I couldn't open the window so I must go out to see if this verifies'

Some really cold ensemble members looking into FI with charts showing quite a mixture - ranging from bitterly cold easterly to Arctic air to cold, or average to chilly - Only a few milder runs BUT NOT exceptionally mild. Let's see what the 00z run has to offer though because we have seen hopes raised and dashed before dozens of times!

A potentially good set up for a prolonged and severe cold and wintry spell:

SNOW DAY? - Esp for the SW if this one comes about! 

Looks like GFS has sifted everything a couple of hundred miles east this morning. The mild sector is mild enough to bring rain into East Anglia by 144 on this run, but the extra "warmth" means much more in the way of snow for the majority of England, albeit increasingly slushy and wet snow in the SE. The further east the warm sector gets the warmer the air within is likely to be, as it's being "squeezed" less (for want of a better term - the more it's squeezed, the colder it'll be.)

Last night's amazing 18z run was the result of everything being west enough early on to allow the deep cold to the east to gain a foothold over the UK - as ever, tiny differences on a global scale would result in quite different conditions on the ground.

This morning's MetO run also seems to have moved things east. Hopefully just a wobble, as it's not a trend I'd want to see continue. As it is, down here the GFS now brings snow, followed by slushy wet snow, followed by rain (138>168). Yesterday's 18z, with everything further west, instead had 12cm of snow, with it snowing pretty much constantly due to showers blown in from the sea.

Essentially a continuation of the theme from yesterday, generally cold but generally not cold enough when there is PPN around. 

Arpege continuing to suggest a snow risk this Saturday. 

https://www.theweatheroutlook.com/twodata/arpege.aspx?run=na&charthour=60&chartname=ukpreciptype&chartregion=uk-region&charttag=Precip%20type

ECM 00z looks excellent, but it blotted its copybook the last time it was going for an easterly. I am putting more faith in UKMO this time round.

Okay, so going by all available evidence it looks like a cold spell across the UK is more likely than not to happen.

Remember before the cold spell how we were looking at warm air advection to get things started? That looks like it actually worked (it'll be the third one that kicks it into action, so well done to the models last week for picking that up - note that this was a case of being picked up from beyond 10 days out, so those extended charts do sometimes have a use!)

So, the next step is how to a) get snow and b) keep it going as long as possible.

A) is quite simple - while some of us lucky folks exposed to onshore winds can expect some convective stuff, for most of the UK it'll be fronts that are needed. Not just that: a warm front followed by a warm sector and cold front won't really satisfy (who wants to see snow, then rain, then perhaps more snow?). No, we need trough disruption (or sliders as some call them).

As a refresher, trough disruption occurs when a low bumps into a block and - importantly - has a strong jet behind it. This effectively compresses everything, forces the fronts to occlude and - providing it's cold enough - provides a reliable source of snow (and a fair bit of it).

The first trough disruption is now almost certain to happen on Saturday, as shown by the Fax chart:

http://old.wetterzentrale.de/pics/brack1a.gif

In this case, there's a tongue of milder air wrapped around the front, so it'd be marginal at best for snow (unless you have elevation).

This low as it moves away south allows a cold feed of air in from the east, and it's this which really starts the cold spell off.

Now on to 😎...

The next trough comes along on Tuesday. This morning's GFS shows the disruption happening to the east, meaning the UK is left in a chilly northerly feed, with the fronts bringing snow across much of the UK, turning to rain as it exits the UK. ECM, meanwhile disrupts the trough over Ireland. Rain there, followed by snow, with cold air remaining in situ over most of the UK (prolonging convective stuff in the east, but denying the almost UK-wide snow that GFS offers).

Given the two, I'd opt for the ECM scenario every time, even though it's not as up front with its snow. Why? Well, once cold is in place you can always get troughs, convection etc to bring snow (and it would be snow, because the cold air is in place). With everything taking place further east, you lose the gauranteed cold, then gamble that what's left will be cold enough for snow. This morning's GFS op shows well how marginal it can be, resulting in the usual old thing of some places getting a dumping (best winter ever!) and others just seeing sleety rain (what's all the fuss about?)

Furthermore, with disruption to the east it weakens the block overall, and as GFS shows it then makes it easier for another low to move in with milder air.

ECM, meanwhile, disrupts that trough to the west of most of us. That then leads to a third low disrupting around day 9, and it's that which would bring snow and probably herald the end of the cold spell. In those extra days, meanwhile, there would be much cold weather around and an ongoing risk of troughs/convective snow (which won't be picked up until 48-36 hours out).

Anyway, to sum up: action for a lucky few this weekend, but the main course is early next week. Whether we have time for a dessert (ECM style) depends on where those troughs disrupt. Plenty to keep an eye on, anyway!

Cheers Darren

A great informative morning from yourself 

A very clearly explained and informative post. Thanks Darren.  

Yes, Darren has nailed it. Give me the more prolonged ECM 00z solution any winter, over the shorter lived and marginal GFS 00z.

Great analysis, Darren, and I agree with your view of trough disruption. But as an old pessimist I wonder whether the air temps are low enough for more than sleet on low ground?

Clearly the ECM has the pattern further West , but with no proper blocking and the Azores High so close, a return to milder Atlantic weather will be inevitable by day 8 or so.

The ensemble shows otherwise, with large precipitation spikes coinciding with a "rise" in 850s to about -7C.

and that's the more progressive 0Z GFS, which would still bring snow to much of the country. 

haha no interest before day 8 then insightful 

The ECM this morning is actually progressive compared to its mean:

http://www.wetterzentrale.de/en/topkarten.php?map=1&model=ecm&var=2&time=240&run=0&lid=AVG&h=0&tr=24&mv=0

(No, I don't like mean charts generally, but they can be handy when judging an event such as encroachment of an Atlantic low.)

As you'll know, however, models are always keen on bringing back the Atlantic as they longer term have a bias towards climatology. This was last seen very well in 2010 (hence the charts I posted yesterday), as post-day 10 the ensembles invariably trended towards normal.

The ECM ensembles today should make pleasant viewing for cold fans. Frankly if we can score a few days with the -10C 850 isotherm bathing the UK then we'll have done very well by recent standards.

Are you ramping Ian? No milder Atlantic weather for at least 8 days? Probably a good call though, and we all know from this winter of unreliable model watching how futile it is to look at day 8 and beyond.

Still lots of interest this morning and Darren’s post details this morning’s position nicely.