Friday
A shallow area of low pressure will move away to the northeast of Britain. Showery bursts of rain are likely, these could be heavy in places, mainly eastern Scotland and eastern England as far south as about Lincolnshire. A scattering of showers are likely to drift from Wales into the Midlands and Eastern England. Rather a lot of cloud about but sunny spells are expected to develop. Reasonably mild for the south and east with highs of 21 to 24C, cooler north and west at 17 or 18C.

Friday night
Showers largely fading away in the south, to leave a dry albeit rather cloudy night here. Some showery bursts of rain continuing in central and northern Scotland, possibly merging to form more persistent rain through the early hours of Saturday over the far north. Lows of 13 to 16C generally over Scotland and western England and Wales, with a muggier night in the southeast, temperatures of 18 to 20C here.

Saturday
High pressure begins to build from the west over the weekend. Some persistent rain possible over northern Scotland through much of the day. Chance of a local shower elsewhere, but many places will be dry and sunny spells becoming quite widespread through the day too, although some low-lying fog patches may occur. Top temperatures 17 to 22C, warmest in the southeast.
 

Sunday 12/9/21
High pressure establishes across the northern half of Britain resulting in cool start with fog and mist patches in the morning, before a fair day ahead with sunny spells for Scotland and Northern Ireland. Further south looks likely to be cloudier, with some showery rain in places but plenty of dry conditions too. Ireland may see some spells of heavy rain push northwards, but currently confidence in the extent of this is low. Highs 15 to 20C generally but perhaps up to 24C in the southeast of England.