Storm & Convective Forecast
Issued 2016-11-21 10:34:49
Valid: 21/11/2016 06z to 22/11/2016 06z
CONVECTIVE / STORM FORECAST - TUES 21ST-NOV-2016
A sharpening upper trough digs south across the E Atlantic to the W of UK and Iberia driving a deeply cyclonic S/SWLY flow aloft UK on Monday, while a fairly deep depression slowly arrives from the SW across SW England. A cold and stable NEly flow will cover N Britain to the north of a warm frontal boundary lifting north across England and Wales during the day, bringing persistent rain. To the south of the boundary, S/SWly flow will become increasingly unstable toward southern coastal counties.
…S ENGLAND …
Increasingly colder air arriving aloft from the SW atop of a moist S/SWly flow off the English Channel will create an unstable airmass across southern coastal counties Monday. Lift of this unstable airmass from an approaching upper shortwave trough/vort max (seen as dry slot over Brittany on WV imagery) and cold front approaching from the SW will likely force a line heavy showers and thunderstorms which will move NE from Channel across southern counties, mostly east of Devon and S of M4/London.
30-40 knts of deep layer shear indicated by GFS across S England will be sufficient for organised squall line segments capable of bringing isolated strong wind gusts but of more concern is potential for intense/heavy rainfall bringing localised surface water flooding to already saturated ground following heavy rains over past few days.
Also, a tornado or waterspout can’t be ruled out forming near SE coasts where low-level shear looks enhanced ahead of approaching cold front. Have issued a MARGINAL risk along southern coastal counties to east of Devon for risk of flooding and low risk of tornado and strong convective wind gusts
Otherwise, storms may also be accompanied by hail and gusty winds. Some isolated storms may move inland from south coast too, though unlikely to have any severe weather.
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.