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Dry with a question mark

Wetter mid term scenario possible

Posted 13th September, 11:12

Most of the UK will see some rain during the coming days with significant amounts in places. In the medium term things  look quite dry, but small changes in the pattern could lead to a wetter outcome. 

High pressure to the northeast

Later this week and into next week computer models are showing high pressure building to the northeast of the UK. That could well lead to Atlantic disturbances being held at bay to our west. A dry scenario for many, although there would still be more of a rain risk in the north west.

However, it's also possible that disturbances could become slow moving close to or over the UK. If that were to happen it would bring the potential for it to turn much wetter at times.

For example, the GFS model chart below for Thursday 23rd September shows heavy outbreaks of rain around this time in much of the UK. 

GFS t+10 days, init 13th September 2021

Ensembles look quite dry

Despite the potential for wet periods the GEFS ensemble is favouring a rather dry picture, particularly in the south east. Comparable charts for locations further northwest show a wetter picture.

On the London plot below the rain forecasts from all of the runs in the ensemble are shown on the lower half. Each spike represents a forecast of rain from one of the 33 runs plotted. They are made up from 30 perturbations, the control run, the GFS operational and the ensemble mean.

GEFS 16 days London, init 13th September 2021

The chart shows significant amounts of rain tomorrow (Tuesday 14th September) but then it looks mostly dry until September 21st. Beyond that there are some rain spikes, but it is not a particularly wet picture. 

The other thing to note is that upper air temperatures on the top half of the plot remain above the 30 year average throughout the next 16 days. With the monthly Central England Temperature (CET) provisionally running at a HUGE 3.7C above the 30 year average to the 11th, there is the potential for September to be a very warm month indeed.

However, if the wetter scenario materialises temperatures (at least daytime maximums) will take a hit. Even so I would be surprised if given the current statistics and the forecast upper air temperatures September didn't finish appreciably warmer than the norm.        

TL;DR

Computer ensemble models are suggesting there will be a good deal of dry weather in the medium term. However, there are indications of an alternative scenario with Atlantic disturbances becoming slow moving close to the UK. That could lead to a much wetter outcome. 

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