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Convective Outlook - UK/Ireland - Tues July 20th

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Author Topic: Convective Outlook - UK/Ireland - Tues July 20th  (Read 77 times)
martinastro
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« on: July 20, 2010, 12:58:20 am »



Valid: 20/07/2010 06:00 - 21/07/2010 06:00
Headline: ...THERE IS A RISK OF THUNDERSTORMS FORECAST...

Synopsis

Broad upper trough to the west of the UK and associated SW'erly jet will slowly edge east across the UK over the next 24-36 hours. At the surface, a waving cold front will lie across SW England, The Midlands and NE England by midday and will be slow moving through the afternoon and evening before eventually moving east to clear E England during Wednesday morning.

SW ENGLAND, E WALES, W MIDLANDS, NE ENGLAND ...

Impulses moving in fairly strong SSW'erly flow aloft ahead of approaching sharpening upper trough moving SE will develop waves along slow-moving cold front. There remains some uncertainties from this evening's 12z numerical model output and now the 18z GFS with regards to potential instability likely to develop just ahead of cold front, GFS has been increasingly reluctant with each run to develop instability. However, presence of warm/hot and moist airmass to SE of front and modelled wind convergence just ahead of front across SW England, the Midlands up to NE England indicates potential for forced ascent of humid airmass that will steepen lapse rates allowing isolated thunderstorms to develop across these areas in the afternoon and evening. Any storms that develop will benefit from 30-40 kts of deep layer shear and may organise into multicell clusters/line segments, possibly evolving into a MCS, capable of strong wind gusts and hail. Some modest storm relative helicity (SREH) may develop from The Midlands up to Yorkshire, as surface winds back ahead of front and along with LCL heights falling below 1000m and the increasing wind convergence in this area, a tornado cannot be ruled-out. However, given lack of instability modelled by GFS and lack of any dry mid-level air modelled to arrive in what looks like rather saturated profiles, severe threat looks too uncertain to issue a SLIGHT risk at this time.

IRELAND and SCOTLAND ...

Post frontal rPm airmass here with increasingly cold air moving in aloft from the W will generate steepening lapse rates with surface heating ... so scattered diurnally driven heavy showers and thunderstorms are likely to develop through the afternoon/early evening. 30-40 knots of deep layer shear across Scotland in particular will help organise storms into multicells with risk of hail and gusty winds otherwise no severe weather is anticipated.

... E ENGLAND ..

Again ... model uncertainty this evening seems to point to reluctance to develop any convection/t-storms across this area overnight into Wednesday morning as the cold front moves through. However, given some lift along/ahead of front of humid Tm airmass and nocturnal destabilisation of this airmass through differential thermal advection, there will be potential for isolated, mainly elevated thunderstorms in the overnight period and first half of Wednesday morning, though no severe threat is anticipated with any storms...credit: Nick Finn, Netweather.tv

« Last Edit: July 20, 2010, 01:00:11 am by martinastro » Report Spam   Logged



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