Issued 2017-12-28 23:12:49
Valid: Friday 29th Dec 6am to Saturday 30th Dec 6am
Broad long-wave trough of cold polar air extends from the arctic across the N Atlantic and digs into central/southern Europe, a strong zonal jet stream across the Altantic into western Europe before diving SE into the central Med. A potent 200mph jet streak arrives across S Ireland and SW UK on Friday morning on the rear-side of an upper shortwave moving east across the UK. At the surface, an occluded cold front will surge east across England and Wales in the morning ahead of a low moving E across Scotland.
... S IRELAND, S WALES, SW & S ENGLAND ...
WV imagery Thursday evening shows a marked dry-air intrusion surging east to the W of Ireland, this band of dry ar descending from the tropopause on the poleward flank of a 200mph jet streak is characterised by strong positive vorticity advection and cyclonic shear. This combined by lift of warm moist SWly flow conveyor at the surface by cold front surging east will allow some strong convective bands to develop, given rather steep lapse rates as cold air overruns the frontal boundary aloft. Some thunderstorms are likely to be embedded within convective squall lines or segments along the front and also scattered thunderstorms are possible over sea/near coasts in the post frontal polar maritime airmass as cold air moves over relative warmer seas.
.... MARGINAL RISK of severe convective weather for S England ....
.... SLIGHT RISK of severe convective weather for S Ireland, S Wales, SW England and Channel Islands ....
1. Line of convection, perhaps a squall line with thunder, brief intense rain and some hail, looks to mark the occluding cold front pushing east during the morning ... strong winds aloft could be brought to the surface as strong convective gusts reaching 50-60mph. Also strong low-level helicity ahead of the front as winds veer, could allow a brief tornado to develop. A MARGINAL risk covers much of Sern England for this low risk of severe weather along front and also convection following.
2. Post frontal Polar maritime airmass will be more unstable and bring more significant risk of severe weather given steep lapse rates and stronger deep layer and low-level shear as dry air intrusion/jet streak surges aloft over backed moist surface winds. Primary threat will be damaging convective gusts with stronger cells, perhaps isolated 60-70mph gusts across S Ireland, S Wales, SW England and Channel Islands, as strong winds descend in convective downdrafts of any cells building into dry air aloft. Also, strong low-level helicity will favour rotating updrafts with stronger cells, so a tornado or two can't be ruled out. A SLIGHT risk has been issued for S Ireland and SW UK.
Issued by: Nick Finnis
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.