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Thunderstorm Outlook - Tues Sept 30th

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Author Topic: Thunderstorm Outlook - Tues Sept 30th  (Read 129 times)
Martin Mc Kenna
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Maghera, N. Ireland

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« on: September 30, 2008, 01:12:17 am »

From Netweather.

Valid: 30/09 0600Z - 01/10 0600Z

Very strong upper zonal flow affects NW Europe during Tuesday and into early Wednesday, with a strong jet streak up to 160 mph moving across the UK. At the surface is cyclonic westerly flow bringing moist maritime airmass across the UK, with cloud and rain for England and Wales to the south of frontal boundary roughly between Londonderry and Newcastle at 12z, and to the north in the polar maritime air, clearer and showery conditions across Scotland. Cold front clears east across western areas by 00z introducing clearer showery weather.

Presence of low heights towards northern Scotland with cold dry air intrusion overspreading aloft on northern flank of strong jet streak to the south sets up steep lapse rates across this area, with moist polar martime airmass unstable given general forced ascent associated with strong vorticity advection on the flank of strong jet streak with minor shortwaves in the strong upper flow. Isolated t-storms can be expected amongst general heavy showers developing towards windward coasts, accompanied by hail and gusty winds. Decreasing deep layer and low-level shear towards Nern Scotland tends to rule out a severe risk however.

Polar maritime airmass moves from west behind cold front in the late evening, with steepening lapse rates spreading in from the NW allowing an increase in convective activity in the overnight period across the areas above - with isolated t-storms possible towards western coasts. Instability likely to be realised remains uncertain - with GFS downplaying CAPE with at best low values modelled around western coastal areas. However any convection over Ireland, Wales and NW/Wern England will benefit from strong deep layer shear of up to 40 m/s by 06z Weds and low-level shear of up to 15 m/s, so any strong updrafts maybe capable of rotating into a short-lived weak tornado or more probably funnels ... though risk is deemed too low for a slight risk of severe weather given lack of instability modelled.

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