Thunderstorm Watch - 2024

Looks like I'm just on the edge of it then for Thursday, was originally warning for Wednesday here, then Thursday but now not forecast here.
 
A convective outlook has been issued

Valid: TMR 06:00 - FRI 05:59

An outbreak of severe thunderstorms is becoming increasingly likely across southern and eastern England on Thursday afternoon, potentially as severe as the thunderstorms that occurred across Wales and Northwest England on May 12, 2024. This event is expected to affect densely populated areas, making it a significant and impactful occurrence.



During early Thursday morning, isolated showers will be triggered by a convergence zone in Northern England. These showers will intensify as a moisture-rich plume of warm theta-E air mass already covers most of England. This will create some CAPE, allowing convection to begin in the early morning. Scattered thunderstorms could be embedded within the showers, particularly from Manchester eastward to York. However, this depends on the level of saturation in the atmosphere. If the showers remain discrete, they could produce frequent lightning locally, but this scenario is less likely. A broad slight risk has been issued for parts of Northern England and the Midlands to cover this potential.



It's important to note that destabilizing plume events are inherently uncertain due to their complexity, leading to an element of unpredictability. There is a risk that one or two (super)cells could develop across or just offshore Kent early on Thursday as warm air interacts with a shortwave. The location of these cells depends on the position of the convergence zone. Any thunderstorms that develop could contain frequent lightning and some hail, given the high CAPE (around 1000 J/kg). High PWAT (40 mm) and slow-moving cells may cause localized flash flooding. Due to the uncertainty, a slight risk has only been issued for Kent to highlight the morning threat.



The main event, which could cause significant disruption, is expected in the afternoon. The remnants of the morning showers across Northern England and the West Midlands could redevelop into thunderstorms in the early afternoon due to diurnal heating. With 1500-2000 J/kg of MUCAPE, there will be sufficient energy for strong thunderstorms to develop. Large hail, potentially reaching 1.5-2.5 cm in diameter, will be the primary hazard in the early afternoon across Gloucestershire, Swindon, and Oxfordshire due to high MUCAPE and strong updrafts. However, weak deep-level shears will limit the lifespan of these cells.



As the showers and thunderstorms move eastward into eastern England in the late afternoon, 35-40 knots of low-level and deep-layer shear combined with 1700-2000 J/kg of MUCAPE will enable severe thunderstorms to develop. This will allow strong updrafts, potentially producing large hail up to 4 cm in diameter. According to the latest CAM models, there is strong agreement that a band of discrete cells will develop along a convergence zone extending from London, Surrey, Sussex, Hertfordshire, and Bedfordshire, potentially reaching as far north as Cambridgeshire. The lack of significant forcing will cause the thunderstorms to move slowly. Very high PWAT (40-45 mm) means these storms will contain a lot of moisture, raising the risk of some areas seeing more than 40 mm of rain within 3 hours, and potentially 70-100 mm locally within 6 hours due to training effects. This could lead to localized severe flooding, similar to the Brettenham storm on July 25, 2021, where 150-200 mm of rain fell in less than 6 hours. High-resolution models suggest the possibility of a supercell developing across Surrey and Sussex in the late afternoon, increasing the potential for large hail, flooding, and perhaps a tornado. Therefore, a moderate risk has been issued, which may be expanded or upgraded in the next update. The sea convergence zone is expected to weaken by early night, causing lightning activity and storm cells to diminish.
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Storms and Convective discussion - July 2024

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