Feels like spring? Not for long, chillier next week and a Sudden Stratospheric Warming looking possible

It felt spring-like for some in the sunshine today, with temperatures widely in double figures, with some locations in eastern England reaching 14C this afternoon, thanks to high pressure and a mild westerly flow. Another mild day on Saturday, but from Sunday onwards, it looks to turn progressively colder with a return of widespread overnight frost and it will turn chillier by day. A cold front will clear south early Sunday introducing cooler conditions, with temperatures returning to single figures. Meanwhile, high pressure centred close to the south today looks to build north during the second half to the weekend. The high will be quite intense thanks to a plunge of dangerously cold air over eastern Canada and NE USA buckling the jet upstream and forcing warm air aloft downstream over NW Europe. Central pressure of the anticyclone building across the UK on Sunday could get pretty high for February and is forecast to reach 1047mb.

The high over the UK by the end of the weekend is then forecast to drift east towards the Baltic countries of eastern Europe early next week. This will pull in chillier continental air via a southeasterly flow from a cold central Europe through next week and with pressure high and skies generally clear - there will be a return of overnight frost.

Some tabloid newspapers have been hyping about the Beast from the East arriving next week, thanks to the American GFS model producing several operational runs forecasting deep cold air being brought to the UK on easterly winds from much further east, bringing low temperatures and also a threat of snow showers. This has been causing a stir on the weather forums too. The GFS, up to yesterday, forecast the high building north this weekend to drift northeast over Scandinavia rather than east towards eastern Europe like other models showed. However, this looked like an outlier, so unlikely, and yesterday GFS moved in line with the other models to show the high to the east next week, rather than northeast.

This was a tweet I made on Tuesday  when the GFS was showing a very cold easterly, but other models weren't:

It’s looking a mostly dry picture throughout next week, with high pressure in control, however, an Atlantic front may move in from the northwest later in the week, bringing a spell of rain and perhaps hill snow to northern and central areas for a time before fizzling out as it moves into high pressure.

Beyond next week, rather a lot of uncertainty, as would often be expected after 7 days, but high pressure may hang on across the south and east, while the north and west perhaps turns more unsettled, windier and milder for a time.

Looking further out to late February, for those who are hoping for much colder weather and perhaps snow before we say goodbye to winter, recent GFS runs and the last few ECMWF extended runs (EC weeklies) that update twice a week, have increasingly indicated another attempt at a Sudden Stratospheric Warming (SSW) between 14^th – 23^rd of February. We saw a minor warming recently, but the polar vortex will restrengthen again for a bit. But this second warming looks to be stronger and more sustained, with recent GFS operational runs and many ensembles going for a reversal (below 0 m/s) at 10 hPa 60N around mid-month. Last night’s 12z run had the zonal winds at -20 m/s on the 18^th February, today’s 06z has -10 m/s . Meanwhile, yesterday’s EC weeklies update, the 2^nd consecutive run, showed the distinct possibility of a SSW too around the 20^th February, earlier than the previous update.

Sudden Stratospheric Warmings can (2 out of 3 times they occur on average) trigger high latitude blocking that brings colder conditions to northern Europe, including the UK, one of the most notable recent examples was late February / early March 2018. The downward propagation of the wind reversal from the stratosphere to the troposphere (where our weather happens) can help these high latitude blocks to form and push cold arctic air to mid-latitudes of Europe and North America.

There is normally a lag effect of any SSW on our weather of a few weeks at least, sometime quicker, so if an SSW does occur mid-month, we may not see its effects until the end of February at the earliest. That’s if we see any impacts from it, which there is no guarantee. Not all SSWs lead to cold weather. The SSW on the 1^st January 2019 didn’t lead to cold weather, in fact it did the opposite. But before then, we may see the Madden Julian Oscillation (MJO), an enhanced wave of tropical convection, move over the western Pacific by mid-month. This can often amplify the upper flow in the mid-latitudes of the northern hemisphere in winter, by way of energy from the convection pushing poleward and altering the jet stream over the North Pacific and creating changes in pressure over eastern Asia which eventually lead to upper flow changes downstream over the North Atlantic which can promote high latitude blocking. This in turn leading to cold spells in northern Europe – with a lag of around 10 days after the MJO wave moves over the western Pacific – which could coincide with any downwelling from a SSW mid-month too.

So, this winter could see a sting in its tail in the form of much colder and wintry weather before spring arrives. If a SSW does occur and brings high latitude blocking, it could also delay the onset to spring-like weather in March too. Such as we saw in 2018 and 2013 following SSWs back then.