Storm & Convective Forecast - Issued 12/04/2012 10:30
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Valid: 12/04/2012 10:30 - 13/04/2012 06:00
Headline: ... THERE IS A RISK OF THUNDERSTORMS FORECAST ...
Upper trough over the UK and surface low across southern UK will move slowly east during Friday, with an unstable airmass in place across southern UK.... S ENGLAND, SE WALES, MIDLANDS and E ANGLIA ...
An increasingly moist airmass, with dew points reaching 8C, will advect north across Sern UK beneath cold mid-levels of the upper trough. This will create steep lapse rates across this region, with 200-300 J/KG CAPE forecast to build as surface heating occurs in the strong April sunshine, which will drive heavy showers and, increasingly, scattered thunderstorms by the afternoon - as we approach max heating. Deep-layer shear will be rather weak, given winds are light and blowing in a similar direction at all levels, so no organised severe weather is anticipated. Any storms maybe accompanied by hail and slow-moving torrential downpours that may lead to localised flooding.
Increasing 500mb temps from the SW later today will mean risk of storms will quickly subside by early evening as lapse rates weaken.
Nick F - Forecaster for Netweather
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.
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.