The band of rain now over Northern Ireland, Argyll and the Clyde will push east through this evening. Rain is close to certain around the Clyde and the Central Belt, and it should reach the Southern Uplands, the Borders and Northumberland by late evening; Cumbria and the Solway follow after midnight, likely rather than certain. Shetland stays wet until late as well. Wales, the Midlands and the south-west may catch a shower, and East Anglia and the south-east stay dry. The night will be mild, two to three degrees above normal, with around 15C on the Irish Sea coasts.
One system governs the whole week. A deep Atlantic low stalls out south-west of Iceland and stays there, leaving a long fetch of Atlantic air across the country and a front lying west to east across northern Britain. Waves of rain run north-east along that front, and Wednesday's is the wet one. It will sit over the north from the small hours until mid-afternoon, longer than yesterday's forecast had it, which cleared most of it before lunchtime. The heaviest few hours run from before dawn to late morning over Cumbria, Lancashire and the Yorkshire Dales, then over Northumberland, Durham and Tyneside before clearing into the North Sea. Rain will be widespread and persistent from Northern Ireland right across northern England and Wales, and it reaches into the Fens. It eases towards the south-east from there, with about a 70% risk of rain in Norfolk and a 40% risk around London. Expect 20 to 25mm in the wettest spots between the Lancashire coast and the Dales.
The north Wales hills take just as much. A second core of rain builds over Snowdonia through the morning, and the Ffestiniog hills should end the day as wet as anywhere in Lancashire, while the south Wales coast sees far less. This is a band rather than rain wrung out of the high ground, so which side of it you are on matters more than height: Settle will do no better than Morecambe.
The wind that day runs two ways at once. Pressure sags over northern England, though not into a low: this is the slackest gradient of the week. The flow comes from the east over Scotland and Northern Ireland and from the south-west over Wales and southern England. It will be light in most places, the exception being Anglesey and the north Wales coast at 35 to 40mph, wet and blustery together. Newcastle will manage only 16C under the rain, against 22C in Herefordshire and around Exeter.
Thursday should turn warmer and much quieter, with the country well out on the Atlantic low's southern flank in a freshening south-westerly. Cloud is likely to be extensive rather than broken, and where it thins over the west Midlands and the Severn valley, figures should get to around 24C. Showers reach further south than the warmth suggests. Scotland, Northern Ireland, north-west England and Wales all carry a 60 to 80% chance of catching one. The risk thins going south-east through Yorkshire, the Midlands and the south-west, and East Anglia and the south-east are drier still. Thursday night should barely cool at all in the south-west, with Truro near 18C, very mild for early September.
Friday's wind will be a national one rather than a northern one. By then the low has wedged itself into the gap between Iceland and the Faroes, and the squeeze across the country that afternoon is the tightest of the week. Gusts of 35 to 45mph will reach well inland, in the Midlands and the Home Counties as much as over the Pennines and southern Scotland. That is down on yesterday's forecast, which had 45 to 50mph over the northern hills. A cold front runs across northern England around dawn and clears the east coast by early afternoon, so the rain will be brief. Ahead of it, Suffolk, Essex and the London area could reach 24 to 25C around midday, with the afternoon five degrees or so down on that.
Saturday should turn quieter again, with pressure rising from the south-west and the wind easing to 10 to 15mph across England and Wales. How fast it happens is the one thing unsettled: the American GFS builds the ridge harder and sooner than ECMWF, with the figures here following the Met Office model in between. Brighter skies may come with it, best over eastern England and eastern Scotland. Showers could hang on over the north-west Highlands and the Northern Isles, and a few may reach Wales, the south-west and the Home Counties, at a 60% chance of one at best. Inland spots in the Pennines and central Scotland should get down to about 6C before dawn.
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