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Storm/Convective Technical Analysis Discussion 17/7/2014 onwards

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Storm & Convective Forecast


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Issued 2014-07-20 11:57:00

Valid: 20/07/2014 1200z to 21/07/2014 1200z

Upper trough that has sat to the west of the UK and France over recent days has edged east with trough axis now lying N to S across the UK and France. Slack area of surface low pressure lies across eastern areas and into the Low Countries/Germany, with a waving front just off east coast of England/Scotland this afternoon - heavy showers/t-storms are likely to develop in more unstable airmass here, while surface ridge of high pressure extends north across western British Isles on Sunday, bringing more stable conditions here.




Warm and moist air (dew points of 17-21C) left over from recent Spanish Plume still hangs on across eastern England, despite winds turning west/northwesterly across all areas, bringing in less humidity now further west. GFS builds 600-1000 j/kg CAPE this afternoon across the above areas, in reponse to surface heating of the humid airmass and cooling of upper levels. Surface convergence between W/NW flow and sea breezes from the North Sea, as temperatures rise inland and pressure falls, will aid in the development of heavy showers and thunderstorms from Kent/Sussex north across central and eastern England and into SE Scotland. Already storms are firing across NE England.


Despite weak vertical shear today, storms will persist near convergence zones ... precipitable water (PWAT) values remain high in the east and as cells will be slow-moving, there is a risk of large rainfall totals in a short space of time from any storms, bringing risk of local flooding and surface water on roads. Storms may also produce hail, frequent cloud-to-ground lightning and gusty winds, storms may continue across the E/NE regions overnight too, before fading Monday morning.


Issued by: Nick Finnis

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Midday Herstmonceux (East Sussex) radiosonde ascent sampled a rather unstable atmosphere early afternoon, 700 j/kg CAPE and pretty much little in the way of capping - cloud tops capable of rising to 10,500m or 34,000ft looking at skew-t. Wind barbs on the right of the skew-t suggest SE'erly steering wind at 500mb, though not particularly strong, winds variable and light though towards the surface. PWAT (precipitable water) values of 31mm are rather high too, with alot of moisture loading of the atmosphere:




Strong cells have developed since early afternoon across Kent, E Sussex, Essex and Suffolk coastal areas, Norfolk and further inland Cambs, along sea breeze fronts which separates the synoptic NW'erly flow and onshore E or SE sea breezes developing in response to inland surface heating lowering surface pressure. This sea breeze fronts seems to be shifting further inland, albeit v. slowly, towards the W and NW now.




Watch out for funnel clouds with any storms along these convergence/sea breeze fronts and also flash flooding!

Edited by Nick F

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