BBC monthly outlook
Summary
Changeable weather with long-range uncertainty
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Saturday 12 September – Sunday 20 September
Briefly warm and dry, but rain likely later.
As a ridge of high pressure begins to build in eastern Europe this weekend, a couple of frontal waves will move across Scotland and Northern Ireland. This will bring some heavy rain for western Scotland along with strong winds at times. Further south though, it will be a mostly fine and dry weekend with some sunny spells and light winds.
By Monday, the high to the east will build enough to shift the winds for the UK to a southerly direction and push fronts off to the north. The result: a warm, dry, and mostly sunny couple of days. The warmth will likely peak on Tuesday in the south-east, with highs reaching into the upper-twenties. Although 30 Celsius is not completely out of the question, a lot of factors would all need to line up to get that warm.
As we start Wednesday, the high pressure to the east will begin to decline and shift away further east into Russia. This will cause us to lose our warm southerly winds and the temperatures will begin to moderate. At the same time, a cut-off low to our south-west will be able to drift slightly closer and will likely bring some rain later in the week to southern and south-western areas. By the following weekend, temperatures will have moderated to near-normal, but perhaps a touch cooler than normal for Scotland with north-east winds developing.
Monday 21 September – Sunday 27 September
Drier for most with rain at times in the south.
As we enter the last full week of September, we should see a large-scale pattern shift in the weather across northern Europe. High pressure is expected to build across the north of the continent, including the UK, which will keep things more settled and drier. However, temperatures will stay near or a bit below normal with an easterly or north-easterly breeze across the North Sea. Expect some cloudier conditions on eastern coasts at times too, especially in Scotland.
While we expect the high pressure to stick around throughout the week, there is a lot of uncertainty in the forecast since the UK is sat right on the edge of the high. Low pressure in France and Spain will occasionally bring some rain into southern areas during the week, but this will likely be more an exception than the rule. However, there is a risk that high pressure will be further east and low pressure could become more dominant.
The strong high is in part due to plenty of warm air from tropical storm Paulette, currently in the tropical Atlantic. This is contributing to the high degree of uncertainty in the forecast, so confidence is low for this week.
Monday 28 September – Sunday 11 October
Changeable weather, but often unsettled.
By the end of September and into October, the forecast becomes trickier. In recent weeks, the computer models have been unusually poor at this range. This is mainly due to the extremely active Atlantic hurricane season this year, as these storms are so large and powerful, they can have downstream effects on the weather pattern over Europe, thousands of miles away!
In general, we expect the high pressure from late September to gradually shift east and allow low pressure to become the more dominant feature into October. However, high pressure will often be lurking nearby in the Atlantic or central Europe, so the wettest and windier weather will tend to be in the northern half of the country. Occasional fronts will still bring some rain to everyone though, but southern areas will likely be a bit drier than normal.
The downside is that all this comes with the rather large caveat that if any tropical cyclones from the Atlantic influence our weather pattern, high pressure becomes more likely. So again, confidence is quite low at the moment, with a chance that a high pressure system could develop to the west or overhead and bring some drier weather. Atlantic hurricane season peaks in September and October, so we will likely continue to see storms well into mid-October.
Further ahead
As we continue to watch Atlantic hurricane activity, next week we will have a closer look into high pressure and drier weather for late September.
https://www.bbc.co.uk/weather/outlook