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A splash of autumn

Posted 24th September, 08:14

September has brought a lot of dry and warm weather to much of the UK. However, it looks as though there will be a more autumnal feel to things during the last days of the month.

Unsettled and cooler

The London GEFS 00z 16 day plot below shows forecast upper air temperatures and precipitation. In the short term upper air temperatures remain well above the 30 year average (thick black line), but by the 28th September they take a dip. That marks the start of a more unsettled period with disturbances pushing in from the Atlantic. 

The likelihood is that all parts of the UK will see outbreaks of rain and windier conditions at times through early October. Given the calm and warm September which much of the UK has been having it will feel noticeably different. 

GEFS 00z London 850hPa temperatures and precip, init 24th September 2021

Drier from October 5th?

However, there are signs of it turning drier again, at least in the south east, from around October 5th. On the lower half of the plot the number of rain spikes decreases once again.

The GEFS 00z plot below shows forecast 2m temperatures from all of the runs in the ensemble model. The time axis is stepped at 6 hour intervals giving the saw tooth pattern. Values are increasing during the days and dipping through the nights, but key is the general trend. 

Things begin very warm with most of the runs taking maximum values into the mid 20Cs. The downward trend starts from about the 28th September as the more unsettled period begins.

GEFS 00z London 2m temperatures, init 24th September 2021

By October 5th a big spread develops which means there are significantly warmer and cooler possible outcomes. In other words the forecast confidence falls as the lead time increases. Nonetheless, it looks as though temperatures may be on the up by then once again. At least it is something to watch in the coming days.

TL;DR

September has been notably warm and dry in large parts of the UK. A transition to more unsettled weather is expected by the early part of next week. Thereafter, all regions have lower temperatures combined with wetter and windier periods.

By October 5th the amount of uncertainty grows, but there are signs of it turning drier and possibly warmer again, at least in the south east. 

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