Check out these 500 mb geopotential height charts off the GFS ensemble. More food for thought and something to wet the taste buds!
Chese charts again reiterate what has been said time and time again but when I look at these and see them starting to show the ideas I’ve had since September, I admit I am getting excited. This also reinforces my reasoning for why a positive AO was crucial in November for later down the road. Have that positive too early and we don’t see the same intensity of arctic air. Have it positive too late and we have a warm December and not having it positive at all and there’s no charge-up of cold air, therefore equates to a winter with nothing to right home about.
Remember, once the NAO goes negative, this will reach into the high octane arctic air and force it south and straight into deepening troughs and the GFS ensemble has the ridge positioned perfectly for the UK to be beneath one of those deep troughs.
The filling of Siberian air into western Canada means frigid times are on the way for the US too.
Boy, what a wild start to December we appear to have on the way. Very exciting!
Below is a step by step interpretation as to what I think will happen over the next 2 weeks if these models are correct.
24 hours
First of all, notice the ridge over eastern Siberia and the piece poking up through the Bering Strait and into the Arctic, this is already disrupting the polar vortex and is feeding arctic air into Western Canada where the snowcover is expanding day by day. The Lower 48 wants to watch this and this could significantly impact weather 10 days from now.
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96 hours
By 96 hours (above) there is two things which stick out, 1) the two ridges over eastern Siberia join forces and strengthen, raising heights and 2) the two troughs over the Atlantic also join to become one huge one. The activity over Siberia is interesting because as the two ridges join and push against a strong polar vortex, the entire system gets jolted. The huge ridge over eastern Canada gets nudged east and the two troughs 1) south of Greenland and the other over the UK, get joined.
While this is going on, pay attention to the connection of isobars between Siberia and North America. This looks to show a cross polar connection.
Siberia has been growing increasingly cold over the past 2 weeks and remember that North America is 3 TIMES smaller than Siberia and so the air is so much colder here than anything Canada can produce. So, it’s not hard to see why North America’s coldest outbreaks are caused when there is a cross polar flow, this means when there is a jet which draws Siberian air, drives it across the pole and forces air over Canada to grow colder.
144 hours
Now, notice the changes by 144 hours (next Friday). See the troughiness now gone from Greenland and the trough over the UK much weaker. The eastern Canada high is becoming weaker and more stretched out so this will make the atmosphere more conducive for this high to get shifted as the isobars flatten out. It’s like the atmosphere opening out the runway for the high is slide up towards Greenland.
Meanwhile, the eastern Siberia/N Pacific ridge continues to build north into the arctic, the connection between Siberia and Canada remains. Also notice how the low (where the isobars become a circle over the pole), this represents the core of the polar vortex, it has shifted from from the north pole to Greenland thanks to that ridge. This setup forces the arctic air into a favourable position for us here in the UK.
240 hours
By 240 hours (next Tuesday), the ridge is lost over eastern Canada and the trough significantly weakened over the UK. Notice how the model sees heights starting to rise south of Greenland and north of the UK? With the arctic air now in favourable positions and the jet stream significantly weakened, all it takes is for heights to build over Greenland and the arctic air is forced south as heights lower over both the Lower 48 and Northwest Europe.
Because of the cross polar flow, I think the cold that’s coming could shock many, not realising just how cold the air is that’s on the way.
312 hours
By 312 hours which is Friday Nov 30, check out the trough diving into the southeast US and Europe as heights build over Greenland. Also notice the Greenland and Siberian high joining. This would reverse the entire hemispheric pattern, forcing warmth north and all that arctic air south.
384 hours
While things are looking good for a wild start to December, there are a lot of uncertainties and I want to add that things can still alter.
Aspects such as whether we get a decent spell of snowfall as the leading edge comes down is important. I do think we see the first 5-7 days where snows shood be frequent and should, through time lay down a widespread blanket of snowcover. This will allow these moderation of arctic air once it gets here. Without the reflective snowcover, we won’t be as cold. Still cold but not as cold!
As for the worst of this episode, that remains unclear but I am swaying towards a period which enters week two of December, in which the core of cold swings into Scandinavia in the form of an arctic air and I believe this could drift over the UK sometime nearer the 10th. If this happens, expect brilliantly clear days but bitterly cold. Snow would help keep days bitter at between -1 and -6C widely with nights, beneath clear starry skies and light winds at or below -10C.
The duration of this pattern could last anywhere from 10 days to a whole month. As to the chances of a white Christmas, I am not sure yet. It will depend on whether the NAO/AO wants to flip back around or not during the mid part of December, if it holds in negative territory then I think we could have a very tough December which holds onto the cold and snow through Christmas.
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