Thunderstorms expected over Midlands northwards across northern England and southern Scotland
VALID: 06:00 UTC Thu 25 Jul 2019 - 05:59 UTC Fri 26 Jul 2019
ISSUED: 21:12 UTC Wed 24 Jul 2019
ISSUED BY: Dan (Twitter)
A rather rare setup is expected to develop across the British Isles on Thursday. A negatively-tilted upper trough will be located to the west over the Atlantic, placing the British Isles on its forward side with a shortwave rounding its base and lifting northwards during Thursday evening.
SE ENGLAND THURSDAY AFTERNOON
Prior to this, poleward advection of a very warm airmass will occur characterised by a deep elevated mixed layer (EML), creating some remarkably steep mid-level lapse rates and acting as a cap to any surface-based convection. However, surface temperatures of 36-37C would be sufficient to erode this low-level cap and while an isolated surface-based thunderstorm cannot be ruled out over SE England during the second half of the afternoon, it is considered a rather low risk given a very dry profile and large dewpoint depression (cloud bases would likely be near 8,000 ft!). A slight warm nose at 800mb may also serve to inhibit deep convection.
CS + SW ENGLAND / WALES / MIDLANDS / NW ENGLAND THURSDAY DAYTIME
Elsewhere, gradual moistening at the top of the EML (in the 600-700mb region) may lead to a few scattered elevated showers or thunderstorms developing over CS / SW England around noon, these then drifting northwards across Wales / Midlands / Irish Sea / NW England through the afternoon. The depth of convection will likely be quite shallow, so lightning may be somewhat sporadic and very hit-and-miss. Given the very deep hot, dry airmass beneath the cloud base (also approx 8,000 ft, but note this is from elevated convection rather than the surface-based convection mentioned earlier) any rain would likely evaporate before reaching the ground.
NW MIDLANDS / N + NW ENGLAND / S SCOTLAND THURSDAY EVENING
Elevated showers / thunderstorms will arrive in the north Midlands and N / NW England mid-late afternoon. There is scope for these to become rooted within the boundary layer and / or other surface-based thunderstorms to develop more widely through the evening hours, leading to a rather complex and messy convective evolution with the potential for a mixture of both elevated and surface-based thunderstorms. Either way, thunderstorms are expected to increase in coverage as they continue to drift northwards, with the best multi-model agreement by the time they reach southern Scotland - hence the inclusion of a HIGH risk area.
Strong deep layer shear, with some backing at the low-levels, suggests a couple of supercells will be possible, capable of producing very frequent lightning, large hail 3-4cm in diameter and, if storms can remain surface-based, the risk of a tornado or two. A SVR has been introduced to cover the threat of large hail, flash flooding and perhaps a tornado.
E / SE ENGLAND THURSDAY EVENING / NIGHT
As the upper trough continues to approach from the southwest, continued cooling aloft and gradual moistening of the vertical profile, overspreading the poleward advection of the EML below, will result increasing instability by the evening hours with MLCAPE 1,500 - 2,000 J/kg expected. This combined with increased forcing as the shortwave approaches has the potential to develop scattered elevated thunderstorms during the evening and night hours, particularly from the 700-750mb layer (bases around 7,000 ft). Initial focus would be SE England, with thunderstorms also developing over France and drifting NNW-wards towards these areas. Additional elevated thunderstorms may also develop farther north across eastern England and into eastern Scotland as the night progresses.
However, it is noted that model guidance is less clear-cut than with the thunderstorms on Tuesday night, particularly as profiles may remain quite dry and forcing fairly subtle. As such, there is some uncertainty over just how many thunderstorms may develop on Thursday evening / night, and exactly where and at what time. Winds will likely be fairly unidirectional with height, but there may be some workable shear to help cell organisation. The magnitude of CAPE/shear and fast vertical motions brings the threat of some large hail, although the size that reaches the ground will be subject to partial melting through the hot, dry layer beneath the cloud base. Lightning could be very frequent in the most intense storms, with the risk of some very strong gusts of wind.