Slight risk of Severe Thunderstorms

There is a Slight risk of Severe Thunderstorms.


Issued 2015-07-03 09:47:04
Valid: 03/07/2015 0600z to 04/07/2015 0600z


Slow-moving upper trough over the mid-N Atlantic to the west of the British Isles will become increasingly negatively-tilted over the next 12-24 hrs. Nose of 100knt jet at 300mb rounding the base of the trough will push NE across the western UK this evening with corresponding increase in SWly flow generally aloft. A sharpening shortwave trough will eject NE towards the UK from the base of the upper trough. At the surface, hot and moist airmass characterised by high theta-w values (16-18C) currently over France will advect back NW across the UK this evening and overnight to the south of northward moving warm front. Surface cold front moves in from the SW tonight, which combined with shortwave trough moving in from the SW and general cooling aloft, will create large scale ascent and destabilisation of the of high theta-w plume.


A shortwave trough is very evident on water vapour imagery this morning between the Azores and NW Iberia around 15W, this will continue NE today and will later engage high theta-w plume spread NW from France across southern/central Britain and Ireland this evening and overnight. Over-running of the shortwave and associated cooling aloft atop of warm moist plume will create steep lapse rates and strong instability in the warm sector between warm and cold fronts moving N tonight. GFS indicates up to 800-1200 j/kg MLCAPE towards Sern and Ern England in the warm sector spreading N across England and Wales , somewhat less further west towards Ireland. Large scale ascent created by approaching shortwave, cold front and falling heights will support storm development initially over SW approaches, The English western English Channel/N France before storms develop further east then spreading in a broad line N and NE across S England, Wales, Ireland, Midlands and eventually N England and N Ireland later in the evening and overnight.

Storms will likely organise quickly, given 40-50 knts of 0-6km deep layer shear indicated with winds veering and strengthening sharply with height. This will support bowing line segments or even a few supercells, capable of producing large hail given fairly high CAPE values and also a damaging wind threat – so have issued a general SLIGHT risk across central, S and E England and Wales – where CAPE is highest. Winds backed easterly to the SWly flow aloft will also generate large Storm-Relative Helicity (SREH) values across S England later this evening, which suggests a tornado or two is not out the question if mesocyclones form – particularly before midnight when there maybe still sufficient diurnal warmth. Have included a hatched area rather than a higher risk here for the tornado threat, for now, as well as the risk of large hail and wind damage … given uncertainties over exactly where and when storms will form before moving NE across England and Wales later in the evening and overnight – the greatest uncertainty lies with eastward extent of storms given less forcing for ascent and more capping of the atmosphere towards SE England, despite more favourable severe parameters.

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