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Storm & Convective Forecast

Issued 2016-08-26 23:02:33
Valid: 27/08/16 06z to 28/08/16 06z

CONVECTIVE/STORM FORECAST - SAT 27TH-AUG-2016

Synopsis

Upper trough to the west of the British Isles will slowly advance eastwards during Saturday. Ahead of the trough, a plume of warm and moist air, characterised by WBPT/Theta-w values reaching 16-18C, advects north in across S England early Saturday. A warm front will move in across S England by 12z Saturday marking the boundary of warm and humid air moving in from the south at the surface.

… E WALES, S ENGLAND, MIDLANDS, E ANGLIA, N ENGLAND …

An elevated mixed layer (EML) characterised by steep mid-level lapse rates will move north in tandem with plume of warm/moist air below. Isentropic lift of the plume ahead of the warm front may create sufficient instability in the mid-levels above a stable cooler and drier boundary layer to support development of scattered heavy showers and perhaps isolated elevated thunderstorms early on in the day across southern counties of England. These early showers and isolated storms tending to drift NE across The Midlands and Eastern parts of England during the day north of the warm front above a stable cool/dry boundary layer. Any elevated storms could produce frequent cloud-to-ground lightning locally and also a risk of flash-flooding. So have issued a marginal risk for severe weather.

Drier and sunnier conditions returning to S/SE England late AM into PM, where surface will likely remain capped, but with strong surface heating of humid airmass, where skies clear, yielding towards 1000 j/kg CAPE. There is a small risk that surface breeze convergence and surface heating will be sufficient to overcome the cap to produce some isolated thunderstorms here later afternoon/early evening. Any sfc-based storm that does develop may  organise, given 30-40 knts of DL shear, to produce large hail (2-3cm), strong winds gusts and torrential rain leading to flash-flooding. Large CAPE modelled, enhanced LL shear near/along warm front and low LCLs look favourable for storm rotation that may lead to a tornado too. Have higlighted a SLIGHT risk for this potential for severe weather near the warm front from central S England NE across S Mids, Nern Home Counties, Lincs and E Anglia. BUT it should be stressed that surface-based storm development is rather uncertain given cap, cloud clearance and forcing (or lack of).

Further N and W, increasing large scale ascent by upper trough approaching from the west, lift from shortwave trough moving in across the SW early evening and increasingly cyclonic and strengthening mid-level flow may trigger some surface-based storms along the edge of unstable plume across E Wales, Midlands and parts of N England during the evening. These storms could produce large hail, gusty winds and a risk of flash-flooding, but given uncertainties over extent, will stick with the MGNL risk for here for now



Issued by:

Issued by the Netweather forecast team whenever there is a risk of storms or severe convective weather, these discussion based and in depth forecasts will highlight the areas at risk and give an in depth description of the risk and the factors surrounding it.

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Key - what do the risk levels mean?

Thunderstorms - Severe thunderstorms unlikely to occur, slight risk of hail, gusting winds and localised flooding.

Slight Risk - A slight risk of severe convective weather exists. Hail >2.0cm diameter, wind gusts exceeding 50mph or 5-10% risk of a tornado within 50 miles of a point, or > 30% risk of localised flooding.

Moderate Risk - A moderate risk of severe convective weather exists. Hail 4-5cm in diameter, or wind gusts of 60-70mph, or a 10-15% chance of a tornado within 50 miles of a point, or >50% risk of localised flooding.

High Risk - A high risk of severe convective weather exists. Rarely, if ever used on the UK. Hail >5cm, or wind gusts in excess of 80mph, or >20% risk of a tornado within 50 miles of a point, or >70% risk of localised flooding.

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