A negatively-tilted upper trough will extend east across the UK from the Atlantic, an intense depression, named Storm Brian by Met Eireann, will track east across Ireland and northern Britain during Saturday, filling as it does so. An unstable Polar maritime airmass will follow cold front clearing eastern England early in the day, with heavy showers and thunderstorms moving in from western coasts and grazing southern coasts.
… IRELAND, WERN BRITAIN, CHANNEL ISLANDS, SERN COASTAL COUNTIES …
Increasingly cold air advection aloft as upper trough spreads east will create steep lapse rates and induce instability over seasonally warm SSTs … supporting heavy showers and some thunderstorms to form and move in across windward coastal areas initially and perhaps inland across western areas and southern coastal counties.
The centre of Storm Brian will lag well behind the occluding cold front in the polar maritime airmass and will weaken as it crosses northern England Saturday PM. However, downward momentum of convective downdrafts in heavy showers or t-storms may enhance wind gusts around coastal areas, (50-60mph / isol up to 70mph). Also, lobes of positive vorticity spiralling around the low as it moves east combined with locally backing winds ahead of the low or troughs in the flow will create strong low-level shear which may allow stronger convective updrafts to rotate to form funnel clouds or even one or two waterspouts or tornadoes near coastal areas – where buoyancy will be enhanced by SSTs. So have issued a MARGINAL risk for windward western/southern coasts of Ireland and windward southern and western coasts of England and Wales.
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.