Convective Outlook UPDATED: 01/05/2025 06:00 - 02/05/2025 05:59

UPDATE 1: A moderate risk and a high risk have been issued for parts of Wales and the Midlands for Thursday as severe thunderstorms with supercellular characteristics are expected to develop once the convective cap is eroded. Large hail up to 4 cm is possible, and is particularly likely, especially within the high-risk zone. Latest data are suggesting a strong confidence that thunderstorms will initiate across the HIGH risk zone due to orographic lift, as well as across Bristol/Chepstow due to localised sea convergence, similar to the initiation of thunderstorms across Cardiff/ Newport on 12/05/2024. Although there is still some uncertainty with the widespreadness of thunderstorms tomorrow, a moderate risk has been issued to cover areas most likely to be prone to severe thunderstorms, mainly for potential supercell(s) which are capable of producing large hail. Lightning frequency might fall below the high-risk threshold, but a high risk has still been issued due to the high likelihood of severe thunderstorms.

As thunderstorms become elevated during late afternoon/early evening and drift eastward, strong gusts of up to 55 mph (just reaching the SVR threshold) could be embedded within these storms. A further update is possible tomorrow morning to adjust the moderate/high risk zone if necessary.

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A dry surface ridge and a pronounced inversion will force elevated thunderstorms to develop on Thursday afternoon. A warm theta-e plume with a saturated layer near 850 hPa creates an environment favorable for clusters of thunderstorms. Steep lapse rates and moderate shear will likely make these storms frequent, active, and long-lasting.

As high pressure dominating the UK weakens and a surface low in the Bay of Biscay advances northward, a warm theta-e plume will move northward into southwest England along a warm front on Thursday morning. The dry ridge may initially limit convection in southern areas.

As a cold front over Scotland sinks south and collides with the warm theta-e plume, isolated thunderstorms may develop across the Scottish Borders during the early afternoon.

In southern England, a strong dry ridge pushes the boundary to mid-levels, initially capping convection. The cap is expected to erode around 14-16 Z as temperatures reach 25°C in parts of Devon and Somerset, with sea breeze convergence along the Bristol Channel triggering thunderstorms in south Wales and the southern West Midlands.

A similar scenario is expected in Ireland, where orographic lift will initiate thunderstorms between 13-15 UTC. MUCAPE of 1,000-1,500 J/kg across England supports robust storms, while 500-700 J/kg in Ireland, combined with orographic lift, may produce moderate thunderstorms. Moderate directional shear will support organized, tall single-cell or small cluster storms once the cap is broken.

As thunderstorms initiate, their outflow boundaries will enhance low-level shear, potentially allowing additional storms to develop, possibly reaching as far east as Cambridge by late afternoon. This eastward expansion depends on sufficient frontal lift and model consensus. Precipitable water (PWAT) of 30-40 mm suggests heavy rainfall is likely, especially in slow-moving storms. Steep lapse rates will support hail up to 1.5 cm in the Midlands and southwest England, with slightly larger hail possible in the east Midlands where steeper laspe rate and shear may enhance storm intensity.

Thunderstorms will remain active until they merge into a line of heavy rain during the early evening, likely driven by outflow pooling or the advancing cold front.

This convective outlook is likely to be updated on Wednesday night, with a high likelihood of upgrading and expanding the enhanced risk area.

You can download the kml file of the convective outlook here:

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