Convective Outlook Issued: Enhanced risk 01/05/2025 06:00 - 02/05/2025 05:59

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.

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