Warm start to week before wetter and windier weather arrives

A brief spell of mild, settled weather will greet much of the UK at the start of this week, but conditions are set to turn markedly more unsettled as a strengthening jet stream drags a succession of Atlantic fronts across the country. It’s a pattern that forecasters say carries a distinctly autumnal feel — though the bigger question is whether a September heatwave could still be waiting in the wings.

Warm start, but don’t get too comfortable

Temperatures across southern and central England are expected to reach 22°C to 24°C on Monday and into Tuesday, with lighter winds and some decent spells of sunshine. Scotland and Northern Ireland will be cooler, sitting closer to 15°C to 17°C, but still dry for most. Yet this relatively pleasant picture won’t last long. A deepening area of low pressure tracking in from the Atlantic is set to clip northern and western parts of the UK by Wednesday, dragging rain and gusty winds inland.

Jet stream shift drives the change

The deterioration is being driven by a significant shift in the jet stream, which is forecast to buckle southward and intensify through the middle of the week. That change pulls storm systems off the Atlantic onto a more direct track towards the British Isles. Wind gusts of 45 to 55 mph are possible across exposed western coasts and hills by Thursday, with rainfall totals of 20 to 40mm likely over parts of Wales and northwest Scotland in just 24 hours. For some upland areas, that figure could be higher still.

It’s the kind of setup that signals summer is well and truly loosening its grip.

What forecasters are saying

“What we’re seeing is a classic late-summer transition where the jet stream reasserts itself after a quieter, more blocked pattern,” said a Met Office spokesperson. “The signal for unsettled conditions through the second half of the week is quite strong, and people in western areas in particular should be prepared for some disruptive weather.” The spokesperson added that flood-prone areas should keep a close eye on local warnings as the week progresses.

Could a September heatwave still happen?

So is that it for any warmth this month? Not necessarily. Long-range models have been hinting at a potential ridge of high pressure building back across the UK somewhere between September 18th and 24th. And while confidence at that range is low — forecasters stress it’s far from guaranteed — there’s enough consistency in the data to suggest above-average temperatures could return for a time. Some models are even flirting with the idea of 28°C or higher in parts of the south, which would qualify as a genuine late-season heatwave.

But September has a habit of teasing. Plenty of autumns have dangled the prospect of a warm finale before delivering nothing more than a grey anticlimax. The wetter, windier spell arriving midweek is confirmed. The heatwave? Watch this space.