MON 29 JUL 2013: A MODERATE chance of thunderstorms https://fbcdn-sphotos-c-a.akamaihd.net/hphotos-ak-frc1/1000897_484694824947644_1105276872_n.png Synopsis: Sharp upper longwave trough continues to pivot and migrate northeastwards across the British Isles on Monday, cooling the mid and upper levels. A series of shortwave troughs embedded in the flow will also move around the base of the upper trough, while a strengthening jet moves across southern Britain during the second half of the forecast period, ahead of a new Atlantic frontal wave. Discussion: Showers will be ongoing through the early hours of Monday as a couple of shortwave troughs pivot northeastwards across the British Isles. Through Monday daytime, diurnal heating accompanied by the progress of the sharp upper trough and cooling aloft will generate 500-800 J/kg CAPE widely, and locally in excesss of 1000 J/kg. Consequently, numerous showers and a few thunderstorms are expected to develop, and with converging winds inland (such as SW peninsula through south Midlands towards The Wash/northern East Anglia) will allowing showers/thunderstorms to organise into distinct lines/bands at times, with shower training over similar areas likely. The most active thunderstorms are likely to be mid afternoon onwards through the evening, particularly across eastern England where the best parcel depths are expected (hence the MDT here), although here most thunderstorms will tend to decay by 18z-20z, although continuing to remain active across the North Sea overnight. Several models simulate a few convergence zones inland across the Scottish Highlands in particular, so this area and environs likely to be aided by orographic forcing have been including in a MDT for a heightened chance of lightning activity. The approach of the jet and exiting of the upper trough is likely to lead to reduced lightning chances across W/SW Ireland through the afternoon, and SW England by late afternoon/early evening - hence it is questionable whether to include these areas in a SLGT given the reduced instability during peak period for daytime heating. However, have kept the SLGT here for the possibility of showers during the morning and early afternoon turning locally thundery. Fewer/weaker showers are expected across SE England due to the short land track. Given dry and cool air aloft, and sufficient instability, hail between 1.5-2.0cm in diameter is possible in any stronger cores, particularly across eastern areas. PWAT in mid 20s mm and shower training may result in some locally large rainfall totals. Link ............... ukasf.co.uk/storm-forecasts/263
Posted on: Sun, 28 Jul 2013 20:19:48 +0000
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