Cold and wintry risks may return next week, but signs of a mild end to winter

A cold and sunny but dry Saturday for most after a frost start will be a welcome respite from the recent constant grey skies and rain. Though rain returns from the west Sunday morning, preceded by snow for a time for some, more especially the Midlands north.

A look at weather apps for many southern locations (London below) Sunday onwards suggests average to mild temperatures over the next 7 days with a chance of rain everyday, while in the north it's looking colder, such as Edinburgh below.  However, a closer look at recent weather model runs and their ensembles suggests it may not be so clear-cut and that there could be some incursions of colder air spreading south across all parts, with wintry risks next week, more particularly in the north, but not exclusively.

The Northern Hemisphere is experiencing a period of greater atmospheric instability than usual with decreased confidence with forecasting weather patterns in the medium range for northern Europe, largely driven by an ongoing stratospheric warming, which has caused the Polar Vortex to be displaced to Eurasia. Couple this with tropical forcing working against impacts from the reshuffle of the polar vortex towards the northeast, with the Madden Julian Oscillation moving across the Indian Ocean tending to promote a +NAO (North Atlantic Oscillation) pattern of low pressure over Iceland and high pressure over the Azores/Portugal in the means - which brings milder weather to NW Europe.

Below diagram shows recent slowing of stratospheric winds and warming at 10 hPa 60N.

The impacts on the troposphere (where our weather happens) of the warming over the arctic in the stratosphere is not entirely clear, but appears that the warming may be leading to a weakening and more wavy jet stream developing next week across the North Atlantic and Europe, with small areas of high pressure developing near the UK - which means cold arctic air lurking close to the north and northeast of the UK may try to push south across NW Europe into the dips in the jet stream that develop. This looks to occur on Tuesday, after a return to milder conditions from the west on Sunday.

Jet stream moves further north compared to recently but also becoming more wavy next week, allowing cold air south at times.

However, the weather models diverge quite a bit on the outlook next week. After turning milder by Monday, most models agree on another brief blast of arctic air returning from the north on Tuesday, with chance of wintry showers in the north and east and a return of frost overnight into Wednesday. 

Cold air returns from the north early next week, possibly some wintry risk on Wednesday, even in the south

But from mid-week confidence quickly decreases over what to do with low pressure systems to the west moving towards the UK. It looks like a temporary blocking wedge of high  pressure / heights building near the UK will cause an area of low pressure moving in from the Atlantic mid-week to disrupt and move over central or southern areas of the UK - before moving southeast into the near continent. And with cold air in place - raising the prospect of some snow on the northern flank of this area of low pressure, though the track of this low pressure is uncertain, ECMWF furthest north across central areas, UKMO and GFS across far south. High pressure builds in across the UK to end the week.

At last! Drier for SW Europe to end the month

However, next weekend, it does look like there will be a change again to more usual winter fare of milder southwesterly flow with spells of rain off the Atlantic and high pressure extends from the Azores into Europe and low pressure dominates close to the northwest. A typical +NAO pattern, following a prolonged period of -NAO over recent weeks. Strong ensemble support for this. So a milder but still unsettled end to winter seems probable at the moment.

This build of high pressure into Europe to the south, strongest across SW Europe, will be good news for Spain and Portugal, which will finally be able dry out and enjoy some much needed sunshine.