Negatively-tilted upper trough extends SE from southern Greenland area across N Atlantic into western Europe with an upper level high over NE Europe. A warm and humid SE flow across the UK ahead of upper trough to the SW and W, becoming increasingly unstable as upper/surface low pressure and associated cold front moves in from the SW.
… ENGLAND, WALES, S SCOTLAND and N IRELAND …
A conveyor of warm and moist air spreading NW ahead of cold front moving up from the south this afternoon will become increasingly unstable with surface heating in sunny spells and contribute to a corridor of 300-700 j/kg CAPE by mid-afternoon which will move north across Wales, central and N England ahead of north moving cold front. Combination of upper trough moving NE and surface cold front moving N will create sufficiently strong lift to support lines of heavy showers and thunderstorms to develop and drift N – mostly from the M4/London northwards through the afternoon and evening – reaching far N England/S Scotland after dark. 30-40 knots of deep layer shear, increasing S/SEly mid-level jet and locally strong convective updrafts developing could combine to allow a few organised thunderstorms to develop capable of producing hail up to 1-2cm, frequent lightning, strong wind gusts and intense rainfall leading to localised flooding. The greatest risk of this occurring across N Wales, Midlands, Lincs and northern England. This activity could reach S/SW Scotland and N Ireland this evening/tonight for a time before weakening and becoming confined to off the west coast of Scotland.
Further south, drier and brighter weather this afternoon/evening will follow a few morning/early afternoon heavy showers tracking north, with isolated lightning possible, as cold front moves away north. Then overnight further heavy showers and a few isolated thunderstorms look to spread in across southern counties by the end of the night.
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.
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.