Convective/Thunderstorm Discussion - Spring 2023
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Convection with thunderstorms possible producing heavy rain and hail possible moving W to E tomorrow and coming up against a good convergence zone in the NE, E and SE. Temperatures not as high maybe as today with lower Dp's, Lapse rates decent with some LLS and DLS available especially along the coasts on the Eastern side of the country. Upper cold pool colder than today so wondering if that will offset the fact that temps might be degree or two lower in general ? Would expect it to kick off in the afternoon and could peak in the afternoon or late afternoon along NE, E and SE coastal areas. The N /NE might see the most storm activity.
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From Convective Weather
Day 2 Convective Outlook
VALID 06:00 UTC Sun 30 Apr 2023 - 05:59 UTC Mon 01 May 2023
ISSUED 20:14 UTC Sat 29 Apr 2023
ISSUED BY: Dan
A pronounced upper trough will crest the western Europe upper ridge as it slides eastwards across the British Isles on Sunday. A moist low-level airmass will persist across most areas, with the exception of central and northern Scotland, with dewpoints typically 10-12C. Ireland/Northern Ireland will have the benefit of early occlusion debris clearing eastwards leading to better insolation and surface heating, coupled with being under the upper trough (and associated cold pool) during peak heating hours. As such, an unstable environment is anticipated here, yielding generally 500-1,000 J/kg MLCAPE and resulting in numerous heavy showers and thunderstorms developing through the day, especially in Ulster and Leinster aided in places by orographic forcing and some areas of low-level wind convergence as a surface trough develops through the afternoon. Modified forecast soundings reveal fairly unidirectional winds with height leading to rather weak cloud layer shear (<20kts), but potential for cloud tops to reach around 30,000ft (ELTs around -50C). As such, some fairly active thunderstorms will be possible, perhaps more especially in the north and east of Northern Ireland. Cells initially will probably be steered towards the east or northeast on the forward side of the upper trough, but as the trough departs across Britain during the evening the steering flow may take cells more to the southeast later in the day and through the evening. While shear may not be particularly strong, some fairly robust updrafts in the most organised cells could produce hail up to 1.5cm in diameter (sub-severe). Thunderstorms may spread into or develop over portions of southern Scotland later in the afternoon and into the evening hours.
Over England and Wales, rather extensive cloud (and outbreaks of rain) associated with occlusion debris may limit surface heating and therefore casts a lot of uncertainty over whether deep convection can develop. Ahead of this, some modest surface heating is possible in eastern England for a time, and may just be sufficient to develop a few heavy showers/weak thunderstorms in conjunction with breeze convergence in the area. The risk area will shift southwards from Northumberland towards Lincolnshire through the afternoon, and is still a possibility until around mid-evening for Lincolnshire/The Wash, but confidence on much lightning activity occurring is rather low. Elsewhere, some modest buildup of CAPE is possible across northern England, Wales and SW England in the vicinity of the occlusion in response to diurnal heating if sufficient cloud thinning/breaking occurs as the leading edge of the upper trough/cold pool arrives from the west, and may provide the focus for additional heavy showers/weak thunderstorms - but again, low confidence. In addition, during the evening and night activity over eastern Ireland will eventually migrate across the Irish Sea into Wales/NW England albeit probably with a weakening trend in lightning activity.
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