So far December is playing out as expected, predominantly mild with a strong westerly or zonal flow pattern throughout the mid-latitudes, UK especially! This westerly has accompanied frequent spells of flooding rain. Cold weather has been minimal. Combination of one of the strongest El Nino episodes on record (likely driven enhanced rainfall via a stronger sub-tropical jet) coupled with a very cold polar stratosphere is likely the likely factors in driving this late November/early December hemispheric pattern.

Since the jet stream has slowed in the past 5 days, the upper air pattern has become somewhat more amplified or buckled with warm pools lifting north and cold pools descending south. Take Saturday and Sunday for example, temps climbed to near 20, locally 25C above normal over the Eastern US with New Jersey registering an insane 25C (77F). A mid May not mid December reading.

At the same time the ridge built north over Eastern North America, the trough dug south and deepened over Greenland in response to the major northward warm surge over North America. Summit Camp, central Greenland experienced a 30C drop within 2 days. As of this writing (late last night), the thermometer plunged to -60C or -76F. Coldest of the season for here.

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This flick of the skipping rope has a ripple effect throughout the mid-latitude atmosphere and is leading to milder air surging north this week over the UK again thanks to a northbound warm front.

So, very warm eastern North America means very cold Greenland and very cold Greenland means warming for Western Europe.

 

Note the blue over Scotland. That was our chilly weekend but warm to the south.

Check this out by Thursday!

With lows tracking north and to the west of Ireland, we’re mild this week but late week shows a speeding up of the jet and therefore stronger lows and a chance of backside cold this weekend into next week.

When Could This Pattern Change?

Before even at January, we must take a look at the pattern over the pole and particularly, the stratosphere.

Up till now it’s been strong and firmly positioned over the pole. This vertically stacked cold mass as you well know has driven our westerly pattern and most likely the strong El Nino has contributed to the unusual December warmth and increased rainfall with the Met Office stating that 6 months of rain has fallen in 6 weeks in Cumbria and Snowdonia.

For us to see colder conditions in January, we must see a significant weakening, displacement if not split of the polar vortex. Even at that, if there’s not a strong enough transfer top-down from stratosphere to troposphere, you don’t get the cooling response into the lower atmosphere

What is interesting is the consistency in longer range modelling building heights from north between December and January with a peak in February.

What’s worth pointing out is the likely major buildup of arctic air beneath that powerful PV. If that gets disturbed with either a displacement, split or full scale collapse, it’s quite possible the cold that drops south may be quite significant by winter standards.

Canadian continues to relentlessly show a negative under positive height field for January, more so in February, classic of El Nino winters.

The jet has slowed in recent days allowing some colder air to settle temporarily over parts of Europe but through this week, it’s expected to re-strengthen again. Looking at the longer term, in order for us to begin seeing these height rises the longer range models have been showing for weeks now, we must see a more permanent slow down of the westerlies and this is directly linked to the polar stratosphere.

While it’s been strengthening throughout December, there are at least MODEL indication of disruption and displacement late December as surrounding warmth begins to increase. A displacement towards Asia and warming on our side of the pole would suggest much weaker westerlies, greater amplification and increased chance of arctic/Siberian air entering our pattern as heights rise to the north and drop to the south.

This initial 50mb strat temperature profile would support westerly dominance and trough extending from Greenland to France.

This below at 168-hour indicates a slowing westerly and buildup of heights from Canada into Greenland and North Atlantic.

By 210-hours we see a stretched vortex with outside warming increasing and having greater influence.

This is at least some indication that winter ain’t done. Remember that many of our worst winters didn’t commence till January, the 46-47 winter didn’t kick in till late January and I’m sure many wrote that off by mid-January.

See this morning’s video for the discussion.

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