Warm, dry and sunny for most

This September is expected to have plenty of more "summery" weather about than August did, with high pressure often close by, and a mean temperature comparable to August's despite the advancing season.

The first week of September is expected to continue predominantly dry, warm and sunny until the 5th across the country, with top temperatures reaching 20-23C widely around midweek. The high pressure will retreat north-eastwards on the 6th and 7th allowing a brisk south-easterly airflow in. This will allow fronts and rain into south-western Britain and Ireland, and the rain will be heavy and locally thundery. Meanwhile there will be some low cloud moving in off the North Sea to eastern districts, particularly on the 7th. Intervening areas will stay sunny and dry but feeling cooler in the wind.

I expect low pressure to slide eastwards across southern Britain during the second week. Much of England and Wales will see the frontal rain clear through, leaving a rather warm phase with sunshine and showers, the showers heavy and thundery in many places. However, we may well see frontal rain straddling northern England and southern Scotland, which when accompanied by east winds will make it decidedly cloudy with cool days and warm nights near the east coast. Northern Scotland should be dry and sunny in the west but cloudy in the east. Towards midmonth as the low moves away eastwards I expect a colder, mostly dry bright regime establish from the north.

The second half of the month is uncertain but I think high pressure will always be close by. In the third week, it is likely to be situated primarily to the east and south of the country, a solution with fairly strong support from the model ensembles and teleconnections, and this means predominantly warm dry sunny weather for most of England and Wales but cloud and rain for northern and western Scotland and possibly north-west England and Northern Ireland also. In the last week, we are likely to see more of a colder northerly type with high pressure often centred to the west of the British Isles. I predict a September CET of 15.3C, it will be a "quietly" warm month with consistent rather than outstanding warmth, only partly offset by a couple of northerly outbreaks. Temperature anomalies will be largest in central and south-eastern England, with northern and eastern Scotland and north-east England probably nearer average.

Most parts are expected to have a dry sunny month. Rainfall and sunshine may be near normal in south-western Britain and Northern Ireland, and some east-coast districts in SE Scotland and NE England may fail to report sunshine excesses. But for most other parts we're looking at sunshine excesses of 10 to 30% and rainfall deficits of around 40%.

Forecast update issued by Ian Simpson (aka Thundery wintry showers) on 2nd September 2010.