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Storm & Convective Forecast

Issued 2018-05-21 21:54:04
Valid: 21/05/2018 00z - 22/05/2018 00z

THUNDERSTORM OUTLOOK - DAY 1 TUES 21 MAY 2018

Synopsis

Upper level high over Scandinavia will retreat east across western Russia on Tuesday, meanwhile an upper ridge will build NE across the far west of British Isles and NE Atlantic. Weakness in the geopoential height field across S England and subtly lower surface pressure towards near continent combined with surface convergence will support the development of scattered thundery showers on Tuesday across southern Britain.

... S and SE ENGLAND, W MIDLANDS and SE WALES ...

Elevated instability drifting out of Belgium Monday night may support scattered heavy and perhaps thundery showers, with some lightning, pushing over the south N Sea in across parts of SE England and E Anglia early on Tuesday, before activity wanes as by mid-morning.

Then, with heights rising as upper level ridge builds NE across N and W Britain on Tuesday, thunderstorms will be less widespread than Monday with the risk generally confined to S England - where subtly cooler air aloft, higher temperatures and more abundant moisture will produce steeper lapse rates and most unstable airmass. Models indicate modest CAPE values during peak heating in the afternoon of around 400-800 j/kg - which will support isolated thunderstorms which will likely be triggered by surface breeze convergence and some orographic influence. With weak flow aloft - no severe weather is anticipated, any storms will be slow-moving and will drift towards the S or SW, with the main risk being locally high rainfall totals leading to surface water flooding. Some hail is also possible. 

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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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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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