Updated 1st September
Covering the period 10th September - 23rd September
A dry sunny month, warm spells in the first two-thirds, one or two cold snaps likely after 20 September.
September is expected to be a slightly warmer than average month, with a significant positive temperature anomaly during the first two-thirds of the month, but the last third of the month is expected to be somewhat cooler, especially in northern Britain. Relative to the 1981-2010 reference period, most places will be 0.5 to 1.0C above average.
It will be a dry month in all parts of the UK, except in isolated locations where rainfall totals are swollen by localised thundery downpours around the 10th-15th, with south-eastern Scotland and north-eastern England likely to report the largest rainfall deficits, with less than 50% of the average. In most other regions of the UK a rainfall shortage of 20 to 40% is expected.
Sunshine totals are expected to be above average in almost all locations, with western Scotland and north-west England likely to be sunniest relative to the long-term normal. Eastern parts of both Scotland and England, and southern England and south Wales, are more likely to have just slightly above average sunshine.
Monday Monday 8th September - Sunday 15th September
High pressure will be centred to the north-east of the British Isles during the early part of this period, bringing a sluggish east to north-easterly airflow to central and southern parts of the country. It is unlikely that there will be much cloud cover contained within the anticyclone, so most parts of the country will be dry and sunny, with warm daytime temperatures (temperatures between 20 and 24C will be typical), but coastal parts of eastern Scotland and north-east England will tend to be exceptions, with highs of 16 to 18C more typical. Low cloud may also be an issue near to North Sea coasts. Overnight temperatures will generally be near average.
During the second half of the period, pressure will generally fall around Iceland and the high pressure will weaken and drift further to the east, which will allow some showery and possibly thundery outbreaks of rain to develop, particularly in western parts of the country, but the high pressure will continue to exert a strong influence on Britain's weather and so plenty of dry sunny weather is expected as well, especially for eastern and central parts of the country, although again, eastern Scotland and north-east England will be prone to low cloud in counties bordering the North Sea.
Overall, this period will be somewhat drier and sunnier than average in most parts of the country, with some eastern counties seeing no rain at all, and western and northern Scotland having particularly large sunshine excesses relative to normal, but in Northern Ireland, Wales and south-west England rainfall may not be far short of the long-term normal. Coastal parts of eastern Scotland and north-east England may report close to or slightly below average sunshine due to issues with low cloud from the North Sea. Temperatures will generally be above the seasonal average by day, but close to average by night.
Monday 15th September - Sunday 21st September
High pressure will continue to erode from the north-west during this period with a strengthening Icelandic Low spreading eastwards, but high pressure is likely to stay close to eastern and southern Britain, especially early and late in the period. Thus, the weather is expected to turn more changeable, particularly over Scotland and Northern Ireland and Cumbria, and especially around the middle part of this forecast period, with weather systems moving in from the North Atlantic at intervals, but there will be long dry bright periods in between the rain belts in central, southern and eastern England, where rainfall amounts are expected to be small due to the proximity of the high pressure.
Temperatures will be above the seasonal average early in the period with high pressure hanging onto eastern and south-eastern Britain and pulling up warm south to south-westerly winds, but they are likely to return close to the seasonal average during the second half of the forecast period. Rainfall totals will generally be close to average in the north and west of Scotland and in Northern Ireland but are expected to be below average elsewhere. Sunshine totals will generally be close to normal in western Britain and in central and western Scotland, but most of central and eastern England and eastern Scotland will have above average sunshine.
Rest of the month
Monday 22nd September - Tuesday 30th September
The weather will turn quite changeable during the last week of September but not excessively so- I expect high pressure to often be close by to the south. There is a significant chance of one or two northerly outbreaks bringing cold arctic air south during this period with highest pressure transferring to the west and south-west of the country, and the Icelandic Low often transferring eastwards towards Scandinavia. Sunshine totals are likely to be close to the seasonal average during this period, while rainfall is likely to be near or slightly below average in most parts of the country. Temperatures will also be close to or slightly below average, with Scotland most likely to be below average and southern and western England and Wales most likely to be near average.