Warm sunny start for most, then turning progressively less settled
Updated 18th September 2017 - Covering the period 25th September - 17th October
Mean temperatures during this period are expected to be 1.0-1.5C above the 1981-2010 average in most parts of the country, with above-average temperatures dominant early in the period but then falling closer to average. In Northern Ireland, south Wales and south-west England they will be nearer 0.5C above.
Rainfall totals for the period will be below normal in most parts of Scotland, particularly the north and east, but will end up above normal in Northern Ireland, and much of Wales and southern and western England, and near normal elsewhere.
Sunshine totals will tend to be not far from normal overall, but with a tendency for below-average sunshine in south Wales and south-west England, and above-average sunshine in northern Scotland and most eastern parts of Britain.
Monday 25th September to Sunday 1st October
This week will see persistent high pressure over Scandinavia and low pressure over Iceland, with Britain in a generally southerly flow sandwiched in between. Early in the period it will be dry, sunny and warm over much of England and Scotland, with temperatures generally in the low to mid 20s Celsius across the south and east of England, with highs of around 25C likely in the south-east. Some large diurnal ranges can be expected, with minimum temperatures often falling below 10C under clear skies and with light winds. It will be cloudier over west Wales and around the Irish Sea and slow-moving fronts will move into Northern Ireland from the west at times, bringing spells of mainly light to moderate rain.
It will turn more unsettled and generally cloudier in the second half of the week with frontal systems making further eastward progress at times and eastern areas will also be more prone to banks of stratocumulus heading in from the North Sea. The Scandinavian blocking high will hold strong and so the rain belts will tend to stay away from the north and east of Scotland, but it will become wetter over much of England and Wales. It will generally be cloudy in the second half of the week but will tend to be sunnier than average in many parts of central and northern Scotland, away from coasts bordering the North Sea. Temperatures will remain above the seasonal norm, but with a reduced diurnal range, cooler days and warmer nights.
Mean temperatures during this period will be about 2C above the 1981-2010 long-term average over much of the country, but nearer 1C above in Northern Ireland, Wales and south-west England.
It will be wetter than average in Northern Ireland and with close to average rainfall over much of Wales and central, western and southern England, but it will be drier than average over much of Scotland and north-eastern England.
Sunshine totals will drop after a generally sunny start to the week, but for the week as a whole, most of the country will be sunnier than average. In Northern Ireland, however, sunshine will tend to be below normal.
Monday 2nd October to Sunday 8th October
This week will remain warm thanks to southerly winds continuing to blow more frequently than usual, while turning more changeable as pressure continues to fall from the west. The Scandinavian blocking high is expected to stick around for much of the week, finally retreating eastwards towards Siberia by the end of the week. With a blocking high to the east, it is most likely to be drier than average in the east, but the Atlantic weather systems will tend to bring above average rainfall to much of Wales, Northern Ireland and western England. In western Scotland, rainfall is most likely to be close to average.
Temperatures will tend to be 1 to 2C above the long-term normal over much of the country, tending to fall close to normal near the end of the week. Sunshine totals will generally be below normal, but in Northern Ireland they are more likely to be near or a little above, due to some brighter showery periods developing in between the rain belts.
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Monday 9th October to Tuesday 17th October
This period is expected to be cyclonic/westerly, with the trough moving slowly eastwards, so it will be an unsettled period, with rain belts moving eastwards at frequent intervals, and temperatures will tend to be very close to the seasonal norm. It will generally be wetter than average, but rainfall is more likely to be close to normal in Scotland, as the jet stream is expected to be further south than usual, taking the main focus of rain-bearing weather systems further south. Sunshine totals are most likely to be below average in south-western Britain, but above average in the north and east of Scotland and in north-east England.