A heat dome is returning across Europe as September draws to a close, setting up an unseasonably warm spell that forecasters expect to stretch into early October.
A powerful upper-level ridge building north from the Mediterranean and North Africa is driving a warm plume northwards, with temperature anomalies at the 850 hPa level locally climbing 8 to 12C above long-term averages across large parts of western, central and northern Europe.
The engine behind it is a classic atmospheric dipole. A deep, high-amplitude upper-level trough has established itself over the North Atlantic and is forcing an intense southerly jet stream plume, which carries warm, tropical air deep into the continent.
Beneath the core of the blocking ridge, strong atmospheric subsidence compresses and warms the lower atmosphere and suppresses cloud cover, driving afternoon surface temperatures well into the upper 20s and peaking into the low 30s in places. Two-metre temperatures are running around 10 to 14C above the long-term figures for late September.
Iberia takes the worst of it. Peak daytime highs are forecast above 35C across central and southern Spain and Portugal, in an extremely dry air mass, with temperatures possibly reaching around 40C from Thursday through Saturday.
In France, the hottest days are across the south and centre, with the interior south and south-west pushing towards 36C, central France into the mid-30s, and northern France and the Benelux in the low 30s over the weekend.
Further north and east, Germany is forecast to reach 27 to 29C, nearing 30C in places, warmest from Sunday into the middle of next week. Northern Italy may touch 30C and challenge late-September record highs, with the most anomalous air over the Po Valley, Tuscany, the north-central Balkans and Greece.
Britain and Ireland sit on the other side of the boundary. South-central England will see the mid-20s from Thursday to Saturday, which is high for the end of September, while Ireland and Scotland stay around the mid-teens at most.
The same pattern that delivers the heat delivers the rain. The strong south-westerly jet running between the deep Atlantic low and the continental ridge is steering an atmospheric river of high moisture into the western British Isles, and orographic rainfall totals could exceed 150mm from western Ireland to Scotland and south-western Norway through early October.
The blocking ridge restricts Atlantic weather systems to the far western edge of the continent. GFS and ECMWF ensemble trends keep the pattern in place into October, with the most significant thermal anomaly shifting north to Sweden and Finland.
The word dome has been doing quiet work throughout. Forecasters use it because the ridge behaves like a lid on a pot, trapping warm air at every level and sinking it towards the ground. Read as a description of a building, it holds up unusually well.
The structure has a footprint: Iberia, France, Germany, northern Italy, extending to Scandinavia by October. It maintains an interior climate 10 to 14 degrees above the outdoor figure. It suppresses its own cloud cover, which is broadly what a roof is for. It is expanding, and the ensembles agree that it intends to keep expanding.
It also has an outside. Ireland and Scotland are not in it. They are standing against the west wall in the mid-teens, taking more than 150 millimetres of the weather the structure has displaced, which is what happens to anything left beside a large roof.
The Department of Unnecessary Research notes that the premises are due to extend into Sweden and Finland next week, and that no application has been submitted.