February 2021 was a wild, extreme and historic month with a one of the most abrupt mid-month flips on record. The UK among other European continents endured one of it’s greatest monthly temperature swings on record. From -23C to 18.4C within 2 weeks. That’s an impressive 41.4C.

Despite a record to near record warm 2nd half to February, the intensity of the cold during the opening 10 days allowed the month to remain colder than normal for the UK and Scandinavia.

The average to slightly below average overall winter can be directly attributed to the weak polar vortex followed by SSW which led to strong arctic and for a time, North Atlantic blocking which led to the reversal of winds from west to east driving the coldest, driest air of winter across the UK and Ireland.

Why Such An Abrupt Swing In Temperature?

The below graphic is a cross-section of the stratosphere and shows persistent warmth through much of the column from late December through mid February. This represented a significantly weak and displaced polar vortex.

The narrow concentration of deep reds from 1 to 10mb at the turn of the year was the occurrence of the Major Sudden Stratospheric Warming Event. Interestingly, while the AO turned negative as expected, the NAO didn’t and therefore the UK found itself in a constant battle between Atlantic and Arctic.

The reason for a lack of shutdown from the Atlantic is likely due to a delay in the warmth (within stratosphere) mixing down into the troposphere.

The tropospheric response didn’t truly arrive until the beginning of February. The usual response from an SSW from top-down is usually 10-14 days. That was the response time ahead of the Beast from the East in 2018. This time around it came 30-days after the event, the reason is quite likely down to a resistance caused by both the La Nina and warm phases of the MJO.

The above chart shows the deep reds reaching the bottom at the beginning of February just as the coldest air of winter arrived to many parts of the mid latitudes.

However, notice while the reds finally mix through the troposphere, at the same time, blues show up at the top of the stratosphere. This shows significant cooling and strengthening of the polar vortex and indicates a reversal was likely to follow. It did and the speed at which the reversal came, I didn’t expect. A strong mild pattern followed the cold of the opening 10 days of the month for the UK and a large swathe of Europe.

The response from this rapid regroup of the PV down into the troposphere was abrupt, flipping the entire 500mb height field within a WEEK, the below AO graphic shows this. The La Nina which likely slowed the SSW response may have also helped speed up the flip from cold back to warm!

So, we are now into March and we have some return to, well let’s just say, a little below ‘average’ for the time of year.

The AO chart above suggests a return towards neutral AO. This along with semi-favorable MJO is likely the reason behind a return to colder with blocking high pressure over the UK currently.

At the close of February there was quite the disagreement between GFS ensemble and CFSv2 weeklies. The GFS ensemble showed a nagging cool pattern holding through March 10-15 but CFSv2 had warmth. So far the GFS ensemble is winning but for how long given the AO is projected to surge strongly positive.

GFS ensemble

CFSv2 weeklies

Thanks to a steep thermal gradient over the US East Coast, the jet stream fires up this weekend and flips the UK pattern from the cool, settled now to stormier next week.

Current

Next week

SOURCE OF CHANGE – Big temp gradient over E North America

Jet stream strengthens over the Atlantic.

Reaches UK breaking down blocking high.

Following a chilly and relatively quiet open followed by a stormy mid-month, I believe MILDER air makes an attempt at pushing towards the UK from the Azores.

However, the North Atlantic is slightly below normal and at the coldest time of year, therefore despite west-southwest winds, the air which reaches the UK may in fact be cooler.

The GFS 10-15 is warm with high pressure centred over the North Sea. A warmer-than-normal 2nd half to the month looks increasingly likely with the +AO.

The CFSv2 agrees with high pressure dominating over the UK.

 

Tags: featured, monthly-outlook