Updated 26th August
Covering the period 10th September - 23rd September
Some brief fine interludes in the first half, especially for eastern areas, but generally a changeable and wetter than average period.
Temperatures are expected to come out very close to the 1981-2010 average. In eastern parts of Scotland and England they will be slightly above average (by 0.5 to 1.0C) but in Northern Ireland, Wales and south-west England they may come out fractionally below average (by 0 to 0.5C) with other regions generally 0 to 0.5C above.
Rainfall totals will be significantly above average in Northern Ireland, Wales and south-west England, and generally slightly above average in other parts of the British Isles.
Sunshine totals are expected to be close to the long-term average in eastern parts of England and in eastern and northern Scotland, but generally below average elsewhere, especially in Northern Ireland.
Monday Monday 1st September - Sunday 7th September
High pressure is expected to be prominent to the east of the British Isles during this week, which means that southerly winds will be more frequent than usual, although there is some uncertainty over whether the high will be centred mainly over central Europe or Scandinavia. However, there will be a persistent east-Atlantic trough which will bring slow-moving fronts and wet weather to Northern Ireland and western Scotland, and two or three of these fronts are likely to make eastward progress across the rest of the British Isles but these will weaken considerably as they move eastwards.
Unsettled conditions with rain belts moving eastwards across the country are most likely early and late in the period while the middle part of the period is most likely to be sunny and warm over large areas of the country with eastern and central England and eastern Scotland most likely to be favoured for warm sunshine, due to highest pressure lying to the east of the country. The near-Continent will not be especially hot, though, due to some chilly northerly winds bringing Arctic air southwards through central and eastern Europe, and so the southerly winds over Britain are unlikely to bring any particularly exceptional temperatures- it will most likely be slightly warmer than average but not exceptionally so.
As a result, this period will be slightly warmer than average (about 1.0C above the 1981-2010 average). The positive anomaly will be greater by day than by night over most of England and eastern Scotland, but in Wales, western Scotland and Northern Ireland (especially Ireland) the positive anomaly will be greater by night. Most central and eastern parts of England, together with eastern Scotland, are expected to come out drier and sunnier than average. Conversely, in Northern Ireland, the period is expected to be significantly wetter and cloudier than average with slow-moving fronts resulting in numerous grey wet days. In western parts of England and Scotland and in Wales, sunshine and rainfall totals are likely to be close to normal.
Monday 8th September - Sunday 14th September
There are many signals pointing towards a cyclonic spell of weather, but early in the period we are likely to see high pressure continue to ridge north-eastwards from the Azores into central and northern Europe and this will produce a spell of dry, sunny and warm weather for most of England, Wales and south-eastern Scotland, but further north and west the weather is more likely to remain changeable with a significant influence from the Icelandic Low. After around the 10th/11th, pressure is expected to fall to the north of the British Isles and this will result in a changeable cyclonic/westerly type establishing over the British Isles and this will also leave us prone to influences from deep lows containing remnants of tropical storms. Thus, as we head towards midmonth some wet and windy weather is likely.
Temperatures will again be slightly above the seasonal average in eastern Britain but are more likely to be close to average in the west. Sunshine and rainfall totals are likely to end up close to normal in most parts of the country with a dry sunny start to the period offset by a wetter and cloudier end, though some eastern parts of the country are more likely to see above-average sunshine.
Rest of the month
Monday 15th September - Tuesday 23rd September
There is considerable uncertainty over this period as the longer-range signals are not particularly strong, but cyclonic conditions are more likely than anticyclonic conditions, with low pressure systems likely to track further south than usual. Thus, this period is likely to come out wetter than average in most parts of the UK, with the most likely exception being northern Scotland, which may be drier than average, especially in the north-west. Sunshine amounts are expected to be close to or below average, with below average sunshine most probable, except in north-west Scotland where it will most likely be sunnier than average. With lows tracking further south than usual, this may expose the British Isles to cool northerly and north-westerly incursions, and thus temperatures are likely to be slightly below the seasonal norm during this period.