Summer ripped apart by jetstream this weekend!

September 4, 2013 — Leave a comment

This weekend a big LOOP in the jetstream is set to rip up the summer heat still lingering over the southern part of the UK. In a dramatic weather battle in the upper atmosphere above our heads the southerly blowing limb of the jetstream will bring POLAR air to west of the UK to fight it out with the continental tropical heat lingering over the south and east. Heavy rain and strong winds will be the result of this battle but the exact location of the heaviest downpours is tricky to be precise about. Broadly speaking, the heaviest rain will start in the South of the UK on Friday and move North, rotating over to the NW and falling heaviest over the northern hills as the LOW pressure drifts NW during Saturday and Sunday. The South could have comparatively drier days after any heavy rain on Friday, but stay tuned for details on that.  Friday rainfall for Reigate could exceed 10mm starting in the afternoon – so possibly very heavy rain for a few hours Friday pm, with lighter rain through Saturday and possibly none at all on Sunday as the LOW moves away to the NW.

Very warm air drifting up from the continent over England by a northward blowing jetstream will be forced to rise over the markedly colder polar air invading from the NW through Friday and Saturday. The difference between these air masses is very marked: the warm 28ºC surface air currently over Reigate equates to -12ºC at 5000m while the invading cold Polar air mass is -28ºC at 5000m, which will push down surface temperatures on Friday to a Tmax of only 16ºC! It is this contrast between the air masses which is a hallmark of autumnal weather and the key to creating lots of rain: polar air meets tropical air, forcing it skyward, forming rain with gusty winds spiraling round LOW pressure: typical autumnal scenario (except this weekend’s scenario is actually quite unusual: called a trough disruption with the surface low drifting off in an unusual direction: from SE to NW, unlike usual LOWS which track west to east across the UK along a zonal jetstream).  Wind speeds for Reigate this weekend could gust at 25mph at times on Saturday. No thunder is now forecast this weekend as the polar air is relatively stable, being on a return leg to the Poles.  

Thereafter, as you can see from the ensemble below, next week looks to remain cool and unsettled but with some improvement in the south possible later in the week as pressure could rise by next weekend, but no return to hot conditions is expected.

ensemble sept 4

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