Following a warm, wet November, December was a mixed, somewhat bookend month with cool spell to start and finish and mild in between. It remained very wet and followed a very wet autumn.

The polar vortex was strong to near record strong during November hence the often warm, wet and windy month. Through December, the polar vortex entered a weakening trend and with slight displacement off the pole towards Canada, Greenland and Europe, the month remained unsettled but turned colder with snow increasing throughout the mid latitudes.

December 2020 will go down as a warmer than average month despite the cold start and finish.

What a difference a year can make.

By the end of December 2019, the stratospheric and tropospheric polar vortex reached it’s strongest and coldest in 40 years. This literally wrote off winter and westerly winds cranked to super speeds up to 372 mph within the stratosphere and fuelled a 260 mph jet stream at 40,000ft over the Pacific and Atlantic!

Temperatures at the core of the vortex dipped close to -100C over Reykjavik.

Mean zonal winds blew at record strength.

The produced a record positive AO

6-13 January 2020 off CFSv2 weeklies

This year!

Significant weakening and deceleration of the zonal westerlies during the 2nd half of December supported a negative AO, allowing a mid latitude pattern colder than at any point last winter. Now, we have a major sudden stratospheric warming underway as seen at 10hpa from off Eurasia.

Just look at the flip in 500mb pattern through the 8-14 th of January off the CFSv2 compared to near the same period last year.

Mean zonal winds are near record weak!

AO is tanking negative compared to off the scale positive at the same point last winter!

The polar vortex is expected to split… Exactly where the split vortexes position themselves will be key.

One culprit to this SSWE? A Siberian high/Aleutian low index running +3 standard deviations above normal.

A surface high pressure system soaring to 1094hpa above Mongolia last week may have set a new world record for sea level pressure if verified.

I must emphasis that despite the current blocking and cold(ish) pattern which ended December and has started January ISN’T associated with SSWE now underway. This is because of a weak vortex and tropical forcings which have pumped heights over the Arctic and to a lesser degree, the Atlantic.

The test comes over the next week or so when the SSWE peaks and we see whether a split occurs. If so, where the two pieces wind up, will determine the pattern for probably the 2nd half of January.

At this moment, the models have a piece anchoring over eastern North America and the other over Europe.

We need some strong blocking to position over the North Atlantic and extend up over Greenland in order to shut off the Atlantic and flip the winds easterly.

Another important aspect is cold itself. In order to maximize the cold potential, we need WIDESPREAD snow AHEAD of a surge of cold off Europe.

As we enter the period between 3-6th January with high pressure strengthening north of the UK and low pressure to the south, winds turn easterly but Europe isn’t particularly cold though cooling all the same. To get a true ‘beast’, we need to see Siberian source air retrograde westwards across Europe and then sweep into the UK via easterlies.

IF a piece of the vortex manages to settle over Scandinavia, then a beast from the east scenario is very much a possibility but as you’ll be noticing from this writing, it’s a complex setup which needs to all come together perfectly.

Here is the January 500mb height anomaly for January.

850mb (500ft) temps are below normal over the UK and above over Iceland and southeast Europe, indicating a cold, blocky January for W Europe including the UK and Ireland.

So, the verdict for January 2020? I’m going with my overall winter thoughts. I alluded to the potential of a sudden strat warming event occuring late December/early January with it’s full effects felt mid January on. We stated a possibility of a ‘pull back of the cold’ briefly between New Year and the 10th, the chill arguably has weakened slightly as the flow is coming off a relatively mild Europe.

The SSWE SHOULD turn what’s been already ‘chilly or seasonal’, extreme from 15th of January through to the end of the month with frequent snowfalls throughout the UK and much of Ireland as well as the low countries, France and Spain followed by an arctic outbreak which rivals a similar intensity to December 2010…

It’s interesting to see how the ECMWF grows the snow cover across Europe as well as the UK towards mid-month

WATCH THIS SPACE!!

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