Storm & Convective Forecast
Issued 2016-09-25 08:38:33
Valid: 25/09/16 0600 - 26/09/16 0600
CONVECTIVE / STORM FORECAST - SUN 25TH-SEPT-2016
Longwave upper trough moves in from the west across the British Isles. At the surface, N-S aligned frontal system bringing overnight rain clears east coast first thing ahead of low pressure system to NW of Scotland. An increasingly unstable Polar maritime (Pm) flow spreads in from the west through Sunday, with blustery showers accompanied by a risk of hail and thunder affecting the W and NW.
... EIRE, N. IRELAND, N WALES, N ENGLAND, SCOTLAND ...
Upper trough moving in from the west will increase lift of moist maritime flow and introduce increasingly cold air in the mid-levels which will steepen lapse rates. As a result, moderate instability will develop across the above areas, aided by modest surface heating, with 300-600 j/kg CAPE forecast sufficient for hail and thunder to accompany the heavier showers expected to spread in/ develop from the west today. Deep layer shear will be fairly weak, so showers /storms are unlikely to produce organised severe weather, though there is a risk of minor localised flooding, hail and gusty winds from heavier cells.
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.