Issued 2018-07-19 15:21:22
Valid: 20/07/2018 00z - 21/07/2018 00z
A weakening diffuse upper-level trough will edge east across the British Isles, towards the surface an occuluding frontal system will move slowly SEwards across northern and western areas, weakening as it does so, before stalling across northern and western England into Saturday morning. A warm and humid airmass will be in place ahead of this frontal system on Friday across SE England and E Anglia, unstable towards a likely breeze convergence zone - which will support heavy showers/thunderstorms. In addition, a shortwave moving NE over northern France, bringing a potential area of thunderstorms N/NE, may glance Kent during early morning then again afternoon/evening.
*UPDATE 10AM FRIDAY 20TH JULY*
... CENTRAL S + SE ENGLAND, E MIDLANDS, LINCS, WERN E ANGLIA ...
Alot of middle level cloud has been spilling up from France this morning, cloud debris from overnight convection over northern France. This will limit convective potential across the SE of England and much of eastern East Anglia today. However, to west of London, cloud breaks / clearer skies will allow rising temperatures inland, with temps reaching high 20s Celsius combined with breeze convergence inland and associated moisture pooling (Precipitable Water / PWAT on forecast soundings reaching 33mm) forecast to increase CAPE modestly to around 200-500 j/kg during peak heating towards the convergence. Rising temperatures and lift along the convergence may trigger heavy showers and some thunderstorms.
Models converge on convergence zone most likely a zone broadly setting up between Hampshire NE through central S England across E Midlands and Lincs/North Cambs early afternoon before shifting slowly E towards SE England into the evening. Although vertical shear will be very weak, back building from cold pool generation of cells developing along convergence and slow-movement of cells will present the risk of localised flooding issues given hard and dry ground - with rainfall rates in excess of 30mm/per hour locally possible. Hail, perhaps isolated large along with frequent cloud-to-ground lightning may also accompany any storms.
Thunderstorm re-developing over northern France during Friday may again clip Kent in the afternoon and evening, as they will be elevated, locally heavy rainfall leading localised flooding and frequent cloud-to-ground lightning will be the main risk, also small hail and gusty winds may accompany these storms.
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.