Valid: 31/07 0600Z - 01/08 0600Z
Headline: ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FORECAST...
Synopsis
Upper low to the west of Ireland with increasingly negatively tilted upper trough moves in towards SW and W UK during Thursday, engaging a slow moving front over western UK and causing waves to run north along the frontal boundary which will lie Cornwall-Anglesey-Belfast at 12z Thurs. Further east, increasingly moist and unstable southerly flow advects across much of the England, with an upper trough moving NNE across southern and eastern England during the morning to lie southern Scotland to E Anglia by 12z. Cold front will lie Liverpool to Southampton by 00z Friday.
SLGT RISK OF SEVERE TSTMS FOR SW ENGLAND, WALES ...
Slow moving waving cold front lying down the western side of the UK will bring outbreaks of locally heavy rain NNE during the day across the area, arrival of upper trough with increasing deep layer shear as jet streak rounds the base of upper trough will enhance forced ascent and bring the risk of isolated t-storms along the frontal boundary - most likely places to see a t-storm being SW England and Wales where lapse rates will become sufficiently steep and PVA increases on arrival of left exit of jet streak. Up to 24 m/s of deep layer shear and up to 15 m/s low-level shear is indicated across SW England and Wales by the afternoon, frontal cloud band will likely limit insolation and therefore possible severe threat, but reluctantly give a slight risk of severe weather, however, given there's a small risk of rotating strong updrafts (with environment of moderate to strong wind shear) that form in any convection to produce an isolated tornado. Also, dry air intrusion spreading across SW UK with arrival of upper trough will combine with strengthening deep layer shear to increase threat of hail and marginaly severe gusty winds in any storms.
...N. IRELAND, NW ENGLAND...
Same frontal boundary bringing outbreaks of heavy rain may include isolated embedded t-storms, though overall risk is somewhat lower than further SW - given weaker lapse rates forecast here.
...CENTRAL SERN/SE ENGLAND, E MIDLANDS, ERN ENGLAND, NERN ENGLAND...
Elevated plume of warm moist air with theta-w values of up to 16C is indicated to advect north across the eastern side of England during the morning along with rising sfc dew points - trough moving north destabilising the plume. Some thundery showers breaking out from Ac cas across central-S and SE England at first - with risk of an isolated t-storm, then late morning and through afternoon increasing threat of heavy showers and thunderstorms developing north of London across eastern and northern England. Although it is unclear whether convection will remain elevated or root into the boundary layer -vertical shear looks undirectional, though some speed shear along with convergence will perhaps organise storms to bring threat of gusty winds down to the surface - severe potential looks intially too low - but increasing shear in the evening may bring a marginal severe threat if storms can root into boundary layer - this potential will be monitored. Any storms that develop, particularly over the Pennines may bring high rainfall totals in a short space of time leading to localised flooding
Issued by: Nick F - Senior Forecaster for Netweather
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.
