Updated 24th August
Covering the period 31st August - 22nd September
A wet second week of September, otherwise often anticyclonic
Temperatures will be about 1C above the 1981-2010 average in most parts of the country, but in Northern Ireland they are likely to be very close to normal.
Rainfall totals are likely to be rather below average, except in southern England, where the wet second week of September is likely to raise rainfall totals for the period close to the long-term average. Northern England and southern Scotland are expected to see the largest shortage of rainfall.
Sunshine totals will be close to or slightly below normal in Northern Ireland, northern Scotland and southern England, but most parts of central and northern England and north Wales will be sunnier than average.
Monday 31st August - Sunday 6th September
High pressure will extend north-eastwards from the Azores during this week, with an anticyclone moving slowly north-eastwards, mostly covering the south of the British Isles early in the week and then mostly covering eastern parts of the country.
The 31st August will be a showery day with north-westerly winds, although most southern and western parts of the country are likely to stay mainly dry. High pressure is then expected to ridge towards southern Britain but with frontal systems heading around the northern periphery of the high, so between the 1st and 3rd September there will be a few belts of cloud and light rain heading south-eastwards across the country, but the rain will tend to peter out before the fronts reach south-east England. Parts of eastern and southern England may see no rain at all. Cloud amounts will be variable and in between the fronts there will be some sunshine. Temperatures will be close to the long-term average.
The high pressure will continue to head eastwards between the 4th and 6th and this will bring continental air to the British Isles. This will result in generally sunnier conditions for most of the country, and temperatures will rise above the seasonal norm, although not exceptionally so. Some wet and possibly thundery outbreaks may make inroads into the west and south by the 6th, as the winds are likely to turn south to south-easterly with pressure falling to the west of the British Isles.
Temperatures will be around 2C above the 1981-2010 normal during this period for a large majority of the country. Rainfall totals are likely to fall well short of the normal, except possibly in south-western Britain where the week may end wet, causing rainfall totals to rise close to average. Sunshine totals will generally be above normal, mostly due to sunny conditions mid to late week as the high pressure moves east pulling in continental air.
Monday 7th September - Sunday 13th September
Low pressure will spread from the west during this period. It is expected to begin warmer than average, but quite wet, with the possibility of some thunderstorms. Between the 10th and 13th, temperatures are expected to fall close to or slightly below normal, with winds swinging around to a mostly westerly direction as the low pressure progresses north-eastwards. It will be breezy at times and rain belts are expected to cross the country at frequent intervals.
Temperatures for this period will be slightly above the long-term average due to the warmth of the beginning of the period, except in Northern Ireland, where they will be slightly below average. It will generally be cloudier and wetter than average although eastern parts of England may manage close to average sunshine.
Rest of the month
Monday 14th September - Tuesday 22nd September
After the preceding unsettled week, high pressure is expected to be more frequent than usual during this period, often ridging up from the Azores towards Scandinavia. As a result, it is expected to be drier and slightly warmer than average during this period, although not exceptionally warm, with the air rotating around the high pressure rather than coming up from a long way south. As a result rainfall totals will drop below average again, with western parts of the country likely to have particularly large rainfall deficits relative to normal. Sunshine amounts are hard to pin down at this range since cloud can be more stubborn in anticyclonic conditions in early autumn than during late spring and summer, but it is likely to be sunnier than average.