How ex-hurricanes can often shape our weather in September, even if they don’t come near


After the 10th, rain fronts rolled in off the Atlantic while the remnants of Hurricane Danielle made an impact in the southwest near Iberia. Initially, this pumped some hot air through Spain and western France, resulting in September’s high temperature record being broken in France, with 40.7°C recorded in south-west France. With very warm air also reaching the south for some time, culminating in London on the 12th with 27ºC. But this was followed by rain on the 13th and 14th in the south as the Atlantic frontal system merged with heat and moisture pumped north by ex-Danielle.

From the 15th, however, pressures began to build from the west as an anticyclone was established over the mid-North Atlantic. This strengthening anticyclone likely helped Hurricane Earl increase upstream flow downstream across the mid-North Atlantic.

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High pressure near the west in the second half of last week brought drier and calmer conditions for many. High pressure then moved east across NW Europe and has dominated UK weather for the weekend. However, the high has also pumped cool arctic air south into much of mainland Europe, resulting in below-average temperatures in late September, particularly at night. During frost in rural areas.

The UK has also seen some cool days and cool nights since late last week. But the anticyclone that was concentrated over south-west Britain earlier in the week has drifted south-east towards Europe. This allowed warmer tropical sea air to flood Britain yesterday and today before a cold front moved south-east, bringing rain showers in the north before reaching the south tonight, with rain lingering in the extreme south-east for a period on Friday before breaking out cleared up.

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Yesterday I talked about how Hurricane Fiona, steaming north toward Atlantic Canada over the weekend, will once again increase high pressure north downstream over the North Atlantic. The strengthening will be stronger than Hurricane Earl in early September, so the high-pressure ridge over the North Atlantic will be stronger, but the downstream trough to Europe will also be deeper and stronger.

Hurricane Fiona, heading into the Canadian Atlantic, will help strengthen the North Atlantic High and Europe Low downstream and bring cool polar air to Europe

This will mean a sharp dip in polar air over the UK, Ireland and northern Europe early next week after a cold front pushed south on Sunday evening and Monday morning, bringing a band of gusty rain with a cool and rainy polar current from the north or north-west moving during for much of the week thanks to deep upper trough and low pressure near the east and high pressure in the west.

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The cold front is pushing south on Monday

Cold polars will tumble southeast into Europe through next week, bringing a chilly end to September

A cool, restless and autumnal end to the month is in sight. Temperatures are currently generally between 1 and 2°C above the 1991-2020 average thanks to a warm first half through September, but the upcoming cooling in the last 5 to 6 days of September may reduce the above-average temperature anomaly, perhaps nearing the end on the average of the month.



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