A turnaround in Sunday's weather at 5 days.

[Paul from Dawlish]

Up until yesterday, rain and a breakdown of the dry weather was being confidently forecast for much of the UK. Now the high pressure is confidently set to persist, with a change of position to introduce warmer air. 

At 5 days, that's quite some turnaround. Anyone explain why?

[Graham Easterling]

Only the MetO had been so pessimistic, and the wording on the SW forecast has rather left their options open, saying any breakdown in the dry conditions is uncertain. It's been looking good on the GFS based sites for some days

Very, very dry here now. We've had plenty of sunshine since 22nd of August, only yesterday was predominantly cloudy. Cloudy this morning but cloud becoming well broken now. Back to 20C (& climbing) as it is even up on the Lizard plateau at Culdrose. Looking at the satellite imagery we have only just scraped into the sunshine.

The great thing is, there is now some surf it the forecast!! (But not quite yet) Completely flat again today, and the hoards have gone home.

13:50 Sennen  F3 or so offshore

Graham

Penzance

[Freddie]

On Thursday, 2 September 2021 at 13:21:07 UTC+1 Paul from Dawlish wrote:

Up until yesterday, rain and a breakdown of the dry weather was being confidently forecast for much of the UK. Now the high pressure is confidently set to persist, with a change of position to introduce warmer air. 

At 5 days, that's quite some turnaround. Anyone explain why?

In a nutshell, a change to the handling of ex-Ida has forced changes to the forecast configuration of the jet in mid and west Atlantic.  The changes mean a slower progression of features, meaning the front that was due to move in from the Atlantic on Saturday is now held much further west.

Until yesterday, models were split on the speed of progress of the front, with UKMO, Arpege, Canadian and Icon being faster, while GFS and ECMWF favoured the front not reaching the UK.  On Tuesday IIRC it was only the GFS that stalled the front, all other models had it progressing into the UK.

The difference of handling of ex-Ida are related to different forecast shapes of an upper trough moving into western North America from the Pacific on Wednesday morning.  The downstream impacts from the difference in shape had forecast repercussions right across the USA and the Atlantic.  Now the models are in agreement with the handling of the Pacific trough (probably due to better sampling of the upper air in its vicinity now it is over the North American continent) they are coming into line further downstream later in the forecast.

There is still scope for errors in the forecast as the warm air associated with ex-Ida is what is changing the shape of the mid-Atlantic jet, which is what will govern the speed (or lack of) of features moving in from the west over the UK at the weekend.

-- 

Freddie

Alcaston

Shropshire

148m AMSL

http://www.hosiene.co.uk/weather/

Stats for the month so far: https://www.hosiene.co.uk/weather/statistics/latest.xlsx

[Len W]

Freddie gives a good explanation of the dynamics but the bottom line is that the models have always struggled to predict the break down of a block and return to westerlies 5 days ahead.

This is despite a huge increase in computer resources and supoosedly better parametrisations of the physics over the last couple of decades.

Len

Wembury