Updated 15th September
Covering the period 22nd September - 14th October
Frequently warmer than average, settled at first, becoming more changeable, especially in the west
Temperatures will be about 2C above the 1981-2010 average in most parts of the country during this forecast period, although Northern Ireland may be nearer 1C above, with an increasing emphasis on warmer-than-average nights as the period progresses, and frosts are expected to be largely absent.
Rainfall totals are expected to fall well short of the long-term average in eastern England and in south-eastern Scotland, but will be above average in the north and west of Scotland and near or slightly above average in other western parts of the UK.
Sunshine totals will generally mirror the rainfall anomalies, with above-average sunshine in eastern parts of Britain, close to average sunshine in Wales and western parts of England, and below-average sunshine in western Scotland and in Northern Ireland.
Monday Monday 22nd September - Sunday 28th September
Although pressure will gradually fall to the north-west of the British Isles during this period, high pressure will remain close by to the east of the country, often with a ridge connecting the Euro High and the Azores High, giving mostly quiet and dry conditions for most parts of the British Isles, especially the east and south. Between the 22nd and 25th, most of Britain will be dry with variable amounts of cloud and some sunny spells, with Wales and central and southern England generally sunniest. Some weak frontal systems will give spells of rain for the north and west of Scotland, and weakening rain belts may track southwards into southern Scotland, northern England and Northern Ireland but rainfall amounts in these areas are likely to be small.
As pressure continues to fall to the north-west of the country, the period 26th-28th September is more likely to be wet in most parts of Scotland, Northern Ireland and north-west England, with more active fronts spreading south-eastwards, but further south and east rainfall amounts are again likely to be small, with high pressure holding on close to the south-east of the country.
As a result this period will feature below-average rainfall, except in northern Scotland, where rainfall totals are likely to be close to average. With winds mostly blowing from a south-westerly direction, temperatures will generally be a degree or two above the average for the 1981-2010 reference period, with afternoon maximum temperatures of 20-22C commonplace in central and southern parts of England. Sunshine totals are likely to be below average in most parts of Scotland, especially northern Scotland, and near or slightly below average in Northern Ireland, but generally above average over England and Wales, with Wales and western parts of England likely to see the largest excesses relative to normal.
Monday 29th September - Sunday 5th October
Pressure will continue to fall to the north-west of the British Isles and this will result in a changeable pattern continuing to spread south-eastwards through the country, and the high pressure to the east is likely to retreat into south-eastern Europe. Pressure will end up below average to the west and above average to the east which means that warm southerly and south-westerly winds will below more frequently than usual, and so temperatures will often be a few degrees above the long-term average for the time of year, but the positive anomaly will often be larger by night than by day. A west-east split in rainfall anomalies is likely with many western parts of the country having significantly above average rainfall, while the east of both Scotland and England will have near or slightly below average rainfall. Sunshine totals will generally mirror the rainfall anomalies, with eastern parts of the country generally sunnier than average, but a shortage of sunshine is expected in the western half of the country.
Rest of the month
Monday 6th October - Tuesday 14th October
Most of the long-term signals suggest that this period will be warmer than average with frequent southerly and south-westerly winds, but there is some uncertainty over how dominant the low pressure to the west will be, relative to the high pressure that will often affect central parts of Europe. Eastern and central England and south-eastern Scotland are most likely to see significant fine spells of weather, with the possibility of one or two reminders of summer with southerly incursions bringing sunshine and temperatures of around 20C, while western parts of the country, particularly the west of Scotland, are most likely to be wet and windy for significant periods, close to the Icelandic Low.
Overall, temperatures are likely to remain a few degrees above the long-term normal for the time of year, with western areas likely to see a greater positive anomaly by night than by day, whereas eastern parts may see a greater emphasis on high daytime maximum temperatures. Eastern parts of the country are likely to be drier and sunnier than average, while the west is likely to be wetter than average, with sunshine totals close to or below average.