High pressure has brought fine and warm weather to southern areas, but a change to more unsettled conditions on the way and it could turn a lot colder next week.
Wednesday saw the warmest day of the year so far, with a top temperature of 18.8C at Kew Gardens in west London. Hawarden in north Wales, which was the warmest place in the UK on Tuesday with 17.0C, was the warmest place in Wales yesterday, reaching 17.4C.
However, the ridge of high pressure that has been across southern Britain for much of this week will retreat south to allow more cyclonic conditions south across the UK as we head into Friday, through the weekend and into next week. This will allow frontal systems with bands of rain to move in across the UK from the Atlantic over the coming days, accompanied by stronger winds than we have seen of late, with a risk of gales in places. It will turn cooler too with this change to more to unsettled conditions, but by Friday next week it could turn noticeably colder and more like winter across all parts. But these flips from spring warmth to winter cold are fairly typical for March.
The change to more unsettled conditions is thanks to the jet stream, which has been to the north of the UK this week so far, shifting south and also becoming stronger – thanks to the large temperature contrasts the other side of the Atlantic over eastern U.S., which spawned Winter Storm Stella on Tuesday, driving a strong jet stream across the Atlantic toward the UK.
As mentioned, as well as turning unsettled from later today, it will turn cooler too, though this will be most notable across the north with temperatures still in the teens across the south over the weekend.
But next week, the weather models indicate noticeably colder air spreading across northern and western areas by mid-week, as colder Polar maritime (Pm) air wrapped up into an upper trough and surface area of low pressure dropping down from Greenland moves across the UK. This will mean showers spreading in across more western areas from Tuesday will turn increasingly wintry over higher ground – especially in the north.
The upper trough bringing in the colder air to northern and western areas by mid-week is forecast to continue to slide southeast across the UK during the second half of next week, while a ridge of high pressure builds north to the west of the UK and Ireland in response to the Jetstream buckling upstream over the North Atlantic. There is increasing uncertainty over the exact positions of surface lows moving southeast with the upper trough moving in the same direction, but although southern areas will start off mild early next week, there are signs that colder polar or even arctic air will eventually be pulled south and southeast across all parts by Friday, as winds turn northerly or northeasterly.
00z GFS operational run this morning was probably at the extreme end of the spectrum with a cold northerly it showed on Friday next week
00z GFS operational had a cold northerly by next Friday morning
Compared to same time on the 06z GFS operational, not such a cold flow
As the 00z GEFS ensemble suggests for Aberdeenshire, the operational (green line) was quite a bit colder than the mean (red line) next Friday (24th) suggesting it was a 'cold outlier', though not without support from a few other ensemble members. Still support for cold polar or arctic sourced air to spread south across the UK at the end of next week, with widespread overnight frosts returning.
The 06z operational run this morning, is more like the 00z ECMWF operational run, with upper trough not moving so far east and staying intact like the 00z GFS op, which brought the northerly, rather the trough disrupts over the UK into a cut-off low over Iberia and western France.
This then allows high pressure to build in over the top across the UK, as per 00z GEFS ensembles suggest with sea-level pressure on the rise later next week and into the following weekend. A chilly northeasterly wind setting in across the UK later next week/next weekend as this happens. But becoming drier too.
00z GEFS pressure shows pressure rising by late next week
So, despite the spring warmth of recent days, perhaps not prudent to have the shorts out ready and put away those winter woollies just yet ...
