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Storm & Convective Forecast

Issued 2018-09-21 07:06:59
Valid: 21/09/2018 06z to 22/09/2018 06z

THUNDERSTORM OUTLOOK - FRIDAY 21 SEPT 2018

Synopsis

Broad upper troughing overspreads higher latitudes of northern Europe and far N Atlantic with upper high over SW Europe - with a strong westerly jet stream flowing across the N Atlantic and UK, a deep surface depression (Storm Bronagh) continues to move away NE toward Norway. A polar westerly flow covers the UK, which will be unstable to SSTs and diurnal surface heating, supporting heavy showers and a few thunderstorms.

... S SCOTLAND, S IRELAND, N ENGLAND, WALES, MIDLANDS, LINCS ...

A pool of unseasonably cold air aloft will shift east across the UK this morning on the back of shortwave in strong upper westerlies / jet moving out over the N Sea - creating steep low level lapse rates. This cold air atop seasonally warm SSTs will generate instability that will support scattered showers, some heavy with hail and isolated lightning strikes, spreading off coastal areas and well inland this morning across the above areas. As diurnal heating warms the ground, surface heating inland may allow strong enough convection to support a greater/more widespread chance of thunderstorms albeit isolated, particularly into the afternoon. These storms accompanied by hail, gusty winds and perhaps localised flooding exacerbated by recent heavy rain. Any storms should fade after dark.

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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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