Convective Outlook
VALID 06:00 UTC Tue 12 Apr 2022 - 05:59 UTC Wed 13 Apr 2022
ISSUED 08:13 UTC Tue 12 Apr 2022
ISSUED BY: Dan
A rather messy forecast evolution is anticipated on Tuesday, as a plume of relatively high (albeit still notably lower than mid-summer standards) Theta-W plume continues to be drawn northwards across central and eastern Britain on the forward side of a weakening longwave trough - becoming especially focussed over eastern England by the afternoon, ahead of the cold front. Pulses of showery rain will accompany this plume, spreading northwards and while a few lightning strikes would be possible almost anywhere, very deep, moist profiles are expected which tends to limit buoyancy and hence lightning risk. Therefore, the best potential for lightning will be largely associated with surface heating and convergence during the afternoon and evening hours.
As such, this outlook as hugely dependent on cloud breaks, insolation and surface heating. A few areas of interest may evolve; ahead of the cold front, if frontal rain can hold sufficiently west across the Midlands there could be some substantial heating across East Anglia/East Midlands with scope for TMax of 19-21C. Should this occur, forecast profiles suggest potential for several hundred J/kg CAPE within an environment with strong deep layer speed shear (40-50kts) - most pronounced during the evening hours as a 500hPa jet streak arrives. Low-level winds may back somewhat as a sea breeze develops (most pronounced in Lincolnshire perhaps). One inhibiting factor is a warm nose at ~800mb, serving as a cap until late afternoon/evening when cooler air aloft arrives. Given the uncertainty over cloud cover, have refrained from issuing any higher threat levels but this area would be one worth monitoring for the potential for organised thunderstorms late in the afternoon and into the evening.
Meanwhile, a rather flabby surface low is likely to be located over the Irish Sea near NW England by the evening, and the light low-level winds and associated convergence/orographic forcing over Wales, W+N Midlands and NW England could result in a few scattered heavy showers or thunderstorms late afternoon and into the evening hours. There will likely be some reasonable speed shear, more especially in the upper portions of the cloud layer which may aid with venting and cell longevity. This again is dependent on sufficient surface heating, and some concern about how deep convection can grow. Any showers/storms could linger well into the evening, especially the N + E Midlands and N England. A low-end SLGT has been issued, but it would not be surprising if very little lightning occurs.
Peninsula convergence and/or orographic forcing may help develop a few heavy showers over Devon/Somerset and the Bristol Channel region during the second half of the afternoon and into the evening hours, as a light onshore flow develops from the Bristol Channel. Finally, across western Ireland beneath the main upper trough axis a few pulse-type showers/weak thunderstorms may form as low-level convergence develops in the rather slack regime.
In all cases, activity will be slow to decay during the second half of the evening (there may even be a transient uptick in some areas for a time).
