February 2019 - a winter heatwave

In the last ten days of February much of Europe was under the grip of an extraordinary winter heatwave, which brought unprecidented high temperatures and smashed the UK winter temperature record two days in a row. In the following analysis, I look at this heatwave from a British perspective. 

Temperature observations at 15:00 UTC yesterday (Tuesday). Credit: wxcharts.

Previously, the highest temperature recorded in February (and winter) had been 19.7˚C, at Greenwich Observatory on 13th February 1998. This record was first broken on Monday, when 20.6 °C was recorded at Trawsgoed, Ceredigion; the first time over 20˚C had ever been recorded in the winter. It didn't stand for long though - multiple stations in London and in north and west Wales exceeded 20˚C on the 26th February, with Kew Gardens topping out at 21.2˚C (or 70˚F in old money).
Temperature observations from the last few days at Kew Gardens. The diurnal variation was 21.4˚C yesterday, starting off with a slight air frost in the morning! Credit: weathercast. 

The dewpoints are rather misleading, however, as soundings from Nottingham and Herstmonceux yesterday afternoon reveal a very shallow moist layer close to the surface, while the dewpoints rapidly drop away in the lowest 1km of the atmosphere. Therefore, daytime mixing in the boundary layer rapidly dried out the ascending thermals, resulting in cloudless, blue skies over most of the country. With maximum solar radiation and light winds, this allowed temperatures to rise as high as they could at this time of the year and in this airmass.
Skew-t sounding for Nottingham yesterday afternoon, revealing the rapid fall-off in moisture in the lowest layers of the atmosphere. Credit: University of Wyoming.

Another frankly unbelievable feature of this heatwave (visible on the North Norfolk, Lincolnshire and Yorkshire coasts on the 15:00 temperature observations, top), was the generation of sea-breezes on some coastlines, maintaining much cooler temperatures along parts of the North Sea coast than inland. This is something I've never seen before in the winter.
10m mean wind simulation from the ultra-high resolution Arome model for yesterday afternoon - which generates a sea breeze along the northern East Anglia coastline (which I've roughly outlined). Credit: wxcharts.

​So what's been going on? Well, the weather has been dominated by anticyclonic conditions since before mid-month. Unlike this time last year, these have been 'warm' high pressure systems (again, crazy to have to say this in the winter), associated with persistent, above-average temperature anomalies. It could be argued that the recipe for this began during the first half of February, when cold anomalies were driven out of Scandinavia by persistent and mild Atlantic airflows. Coupled with milder-than-average temperatures which were already in place across most of mainland Europe, there was not any opportunity to advect cold airmasses towards UK shores. The axis of these highs was located more towards western and north-western Europe, meaning that any cold anomalies were limited to south-eastern Europe and the Balkans (where there was snow and strong winds in the days leading up to the heatwave). The mild airmass in place across the rest of Europe, coupled with strengthening sunshine as we moved into the latter half of February, helped to maintain and enhance the exceptionally mild anomalies across north-west Europe.
Met Office surface pressure analysis (bottom) for 06Z on the 25th February (when temperatures peaked at over 20˚C), typifying the location of high pressure during the second half of the month. A lack of cold air across Scandinavia, and advecting into central and western Europe around the high, has allowed the heat to build in north-west Europe. Compare this with a similar pressure pattern from the 26th January 2017 (top) - this time, south to south-easterly winds gave maximum temperatures widely below zero, and even an ice day in parts of London (due to much colder air in place across continental Europe). In the winter, no wind direction coming from the continent is necessarily mild/ warm. 
Credit: Met Office/ wetter3.

In the last 10 days of February, the wind direction was frequently from the S, SW or SSE, with surface airmass source regions most frequently from the sub-tropical Atlantic and occasionally Iberia/ North Africa. This set the foundation for the record-breaking temperatures towards the end of February. A back-trajectory analysis (below) for air parcels arriving in London on the 25th reveals a source region in the sub-tropical Atlantic. But incredibly for a wintertime heatwave, the air has been slowly descending around the top of the high, looping back around Scandinavia and central Europe, before reaching the UK. The fact that the air parcels have been arriving from what should be a (cold) source region is, on the face of it, strange. But a closer analysis of the parcel trajectories illustrates how they have been descending on their entire journey (96 hours prior to reaching London in this case), starting in the mid to upper-troposphere over the sub-tropical Atlantic. Thus, they were never part of a potentially cold surface airmass over Scandinavia/ eastern Europe (unlike this time last year). The descending motion has helped to dry out the troposphere, helping to prevent boundary-layer (and mid-tropospheric) cloud formation and leading to predominantly sunny skies.
​NOAA HYSPLIT back-trajectory analysis for the preceding 96 hours for air parcels arriving at 300m above London on the morning of the 25th. Credit: NOAA/ ARL.
VIIRS visible satellite image from 12:55 UTC 26.02.2019. Credit: NERC Satellite Receiving Station, Dundee University, Scotland. http://www.sat.dundee.ac.uk/

The above image shows the result of persistent high pressure over north-west Europe and a descending and drying airmass. It was very dry during the final two thirds of February in the UK, and the dry ground, coupled with a dry airmass and cloudless skies, as well as mild anomalies already in place across most of Europe, resulted in this unprecidented winter heatwave for north-west Europe. 

The previous UK February record wasn't only just beaten, it was decimated, smashed out of the park - and by MULTIPLE stations on MULTIPLE days. There was a huge moorland fire at Marsden in West Yorkshire, and another at Arthur's Seat in Edinburgh; the mountain snowpacks have been completely decimated and we've already reached 70˚F. I mean come on, this is February - it's supposed to be winter.

This is just another reminder of the rapidly changing UK and global climate, and continues to paint an alarming picture for the future.