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!
Originally Posted by: Retron