Some downgrades were inevitable following last night's 18Z which was about as good as it could get. I certainly didn't expect that level of excellence to be consistently maintained from run to run.

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

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ECM this morning shows what can happen if everything gets far enough west to start with - we really need to watch where that trough disrupts over the weekend!

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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

 

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ECM this morning shows what can happen if everything gets far enough west to start with - we really need to watch where that trough disrupts over the weekend!

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.

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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!

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Cheers Darren

A great informative morning from yourself 

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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!

 

A very clearly explained and informative post. Thanks Darren.  

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Cheers Darren

A great informative morning from yourself 

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

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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?

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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.

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

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. 

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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.

 

haha no interest before day 8 then insightful 

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 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.

 

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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.

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.

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haha no interest before day 8 then insightful 

Yes, clearly unless a cold spell can last until mid March it's not  worth bothering with... :)

 

Nice explanation Darren. Often we end up with a result halfway between ECM and GFS. That would be good for the majority. 

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Yes, clearly unless a cold spell can last until mid March it's not  worth bothering with... :)

I'm not saying that, just highlighting a really poor pattern that looks likely from the second week of the month onwards.

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I do hope any cold weather we get comes with a properly Easterly. That’s better for London, and as all my draughty windows have West it’s also better for central heating.

It’s an odd synoptic pattern. There is a lot of sub-10C air in various parts of the North Atlantic both west and East. Not really a battleground as such, more general coldness with variations on the cold theme.

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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.

Yes, I'm disappointed too that we will have to wait 8 days for the return of mild - thanks for calling that out.

 

Meanwhile the 6th looks quite wet for Edinburgh with a number of spikes:

http://www.meteociel.fr/cartes_obs/gens_display.php?ext=1&lat=55.9533&lon=-3.1883&ville=Londres

 

 

 

Shropshire your really poor week two has a Mean of -5c.

A new snow row record as well for London 153!

 

 

 

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