September has retained some high daytime temperatures which are set to make it 1.4C or so above Central England Temperature (CET) long term average. New York city also experienced some warm Autumn days with 29C Tmax recently, followed closely by London with near 25C Tmax temps over the last weekend of September. This month has also been exceptionally dry with Reigate recording just 20mm (tbc). The inevitable happens this coming weekend as Autumn arrives, albeit fashionably late. The chart below shows upper air temps dipping as cooler polar air arrives from 5 October (note upper air temps are 1500m, so don’t panic about the scale!).
The charts below also show a defined change for Reigate and the SE over the first weekend in October and into Monday as high pressure and largely rain-free warm settled conditions this week give way to LOW pressure, frontal rain bands and cooler breezy-er conditions delivered by a lively jetstream from the Atlantic. The Icelandic LOW mentioned in previous posts will, at last, nibble through the anticylcone sat over the UK for so long. It may not be quite curtains for HIGH pressure and warmth for the SE yet and some recovery is hinted at later, but it looks like the persistent dry and settled conditions will push off this weekend and be replaced by more mobile Atlantic action.
For Reigate, the change afoot starts gradually, with the odd light shower possible Wednesday, Thursday seeing a rise in pressure again and a continuation of warm and dry calm conditions to end the week. However, a glance to the north west will show an active cold front descending SE during Friday and arriving over SE and Reigate during Saturday morning. It is likely to push through by afternoon and leave brighter fresher conditions through Sunday, which looks not a bad day at all for Reigate, albeit cooler.
The jetstream chart for Thursday and next Monday shows a significant shift south directly over the UK. This is forecast to enhance the trough over Scotland and dig it deeper into the south of the UK during early next week. Breezy conditions are likely into early next week with Monday having possible country-wide rainfall, especially heavy in the south as the warm air lingers CLOSE by to the south and interacts unfavourably with the colder air mass that could produce a lot of rain. The upper air charts below show how close the warm air lingers to the south of the UK.