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September forecast

The month ahead


04/09/2017

Autumn begins

The TWO September forecast suggests a mixed month. Unsettled periods are expected with temperatures close to the average overall. During the second half of the month the chance of warmer and drier periods increases. September 2016 brought the hottest day of the year with 34.4C (92.8F) recorded at Gravesend, Kent. A repeat of that is not expected this year.

Autumnal mist

"Nebelostfriesland" by Matthias Süßen. Licensed under CC BY-SA 2.5 via Wikimedia Commons.

Mid-term prospects

During the first half the month mixed weather is expected. It could become very unsettled during the second week with all areas having wet and windy spells. There are indications of high pressure having more influence towards and beyond the middle of the month which suggests an increasing chance of more settled periods.

The chart below shows the forecast pressure patterns on Monday 18th September from each run in the GEFS ensemble computer model. The key things to note are:

1) A small majority of runs show high pressure having an influence, particularly in the south

2) A significant minority of runs keep a much more unsettled pattern in place

3) About half of the runs have a warm air mass over the UK but a minority of runs show cool conditions

GEFS06z 500hPa postage stamps

Fine spells most likely in the south

In general terms it looks like the most settled weather during the second half of the month can be expected in the south of the UK. There is a greater chance of the north remaining under an Atlantic flow with disturbances continuing to bring the risk of wet spells.



Summary

The September weather is looking varied and autumnal at times. The signal for much warmer and more settled conditions has faded in recent days but there are still indications of drier conditions developing at times during the second half of the month.

TWO monthly forecast

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