Slight risk of Thunderstorms

Netweather.TV has issued a slight risk for severe Thunderstorms.

Issued 2015-09-15 22:22:58
Valid: 16/09/2015 0000z to 17/09/2015 0000z

Longwave upper trough overspreads NW Europe, with a number of shortwave troughs rotating around it within a belt of very strong upper winds, containing jet streaks of 180mph+. One shortwave trough and left exit of a jet streak combine to deepen a low over Biscay Tuesday/early Weds which contains the tropical airmass remnants of ex-TD Henri. This fairly deep low for early Sept tracks NE across SE England/E Anglia Weds afternoon, the unstable warm sector on east side of the low crossing N France and clipping SE England and E Anglia Weds PM.


Unstable warm sector over France, characterised by theta-w values of 16C+, spreads across far SE UK Wednesday afternoon, with GFS indicating 400-800 j/kg CAPE available. Left exit region of strong jet stream will overspread this warm sector over SE UK, producing strong divergence aloft and thus convergence at the surface to fill the void. This strong forcing for ascent of the warm moist airmass combined with some 40-50knts of 0-6km deep layer shear will support well-organised thunderstorms, perhaps even one or two supercells, capable of producing severe convective weather.

Main severe risk appears to be excessive rainfall (15-25mm in a short space of time) from thunderstorm downpours given high moisture content of atmosphere in warm sector. Also there could be isolated incidences of large hail (1-2cm in diameter) from stronger storms. Low-level shear is likely to be fairly strong for a time as winds will be backed ahead of the low moving NE in relation to SWly flow aloft … so combined with moist surface airmass and likely low LCLs (cloud base), this will create conditions ideal for supercells with mesocyclones/rotating updrafts which maybe capable of producing a tornado. There is also a risk of isolated strong convective gusts (40-50mph). Given these severe risks, but uncertainty over how far NW unstable warm sector will advance over SE UK and thus how widespread storms will develop, have issued a SLIGHT risk for now.