Verification: Throughout Sunday 12th Of May, CAPE levels rose excessively which led to the huge outbreak of thunderstorms. Over 1500J/KG of CAPE was recorded specifically in parts of Wales, Midlands & SW England which allowed intense cells to develop. Several reports & radar signatures suggested some of these active storms had super-cellular characteristics involved as well as photogenic cloud structures. Quick to mention, Glasgow also saw several rounds of storms which we should've issued an AOI or a Low MDT Risk. Confidence wasn't to high in this case which is why we stuck with SLGHT. Frequent lightning storms affected most parts of the MDT zone issued especially the HIGH Risk. We could of also extended the HIGH further N/NE, but once again confidence wasn't to high as we wasn't particularly sure on how long these storms will last. Gladly the storms finished its track right on the border of the MDT zone. Overall, a very strong forecast by the team which we will definitely be up there on our best verifications list. Rating: 9/10
Discussion: A low-pressure system across the Northern Atlantic will move eastward into Iceland over the weekend. This will introduce southerly winds into France and the British Isles. These southerly winds will converge with the outflow of the high pressure across Scandinavia, producing multiple troughs over the weekend.
One of the disrupting upper trough will move northward into southwest England during early Sunday, potentially bringing some non-convective showery rain locally. MUCAPE of 1500-2000 J/kg will not be utilised during the early morning due to a shallow capping which requires 22-23°C to break, creating a loaded gun scenario.
As temperatures are expected to reach 22-23°C during the late morning/early afternoon, heavy and thundery showers will break out across Devon and Somerset (potentially Cotswold area aswell), moving northward during the first parts of the afternoon, then north-eastward during the later afternoon/early evening. Frequent or very frequent lightning is possible due to the low saturated profile and a sufficient amount of CAPE. One or two supercells could develop in places. Latest models are suggesting cells which is exposed in slightly stronger deep-level shear could develop into a supercell, which potentially create large hail and frequent lightning, this is most likely to develop across North Devon/ NW Wales.
Moderate low-level shear across parts of southwest England and south/mid Wales can enhance updraft rotation, which increases the lifespan and intensity of the thunderstorms. This can also allow temperature and moisture gradients to increase, further boosting updrafts and allowing sustained hail growth. However, slightly weaker deep-level shear may limit the potential for significant hail growth, reducing the size of hailstones. Nonetheless, hail of 1.5-3 cm is possible during the afternoon.
Elsewhere, weak low-level shearing could result in outflow-dominant pulse storms that collapse and create substantial cold pools; daughter cells then form nearby, and this whole process distorts the shape and location of the convergence zone (CZ), which may shift and wriggle north/east at times through the afternoon and evening, dictating where new cells develop. The greatest concern is flash flooding from slow-moving torrential downpours, which could occur in several places; indeed, some areas could locally receive accumulations exceeding 50mm.
These thunderstorms are expected to develop in an area with high PWAT (>30 mm) and topographic lift. This could enhance instability as moist air is forced to rise over mountainous areas in Wales, then cools and condenses, releasing latent heat and further destabilizing the atmosphere. Topographic lift can also create localized areas of enhanced upward motion and convergence, especially on south-facing slopes. These areas could become focal points for thunderstorm initiation and may experience more intense and persistent thunderstorm activity compared to surrounding areas. Coupling with high PWAT, localized flooding is likely. Isolated tornadoes are also possible due to topographically lifted environments. A moderate risk has been issued to highlight the area with the high chance of very frequent lightning due to topographic lift.
During the early evening, thunderstorms might merge into a longer spell of heavy rain when they move into Northwest England, but the remaining CAPE and stronger deep level shear across northern England should be able to support embedded thunderstorms within the heavy rain as the profile is not too saturated. Frequent lightning is possible locally, especially across parts of northwestern England.
Isolated thunderstorms could also develop across parts of Scotland, Ireland, and Northern Ireland, bringing localised flooding in places. Some frequent lightning is possible across west of Dublin but the likelihood is too low to issue an moderate. Wrote By: @Handry_Outlook
Technical Discussion: Two shortwave troughs, one over Ireland and providing energy to Wales and eventually NW England should force widespread clustering of thunderstorms with frequent lightning and 1-1.5 inch hail potential. A small capping is in place meaning that for the first part of the afternoon, until it reaches 23+ degrees and vigorous convection can take place with enough forcing being built up under capping for clustering of storms likely in some form.
The main shortwave trough over Wales helps force an ejection of moisture converging towards Wales as the afternoon heat builds. Whereas cooler air is trapped above the capped layer and as the surface heat builds the lapse-rates increase significantly leading to trapped surface buoyancy increasing. The mixed-layer buoyancy also rapidly increases with the mid-upper levels with rapid decreasing Geopotenttial height at the 500+ hPa layers especially. So once the capping is broken then convection is likely to be very vigorous and quickly form.
A convergence zone, perhaps two, along the trough wherever it forms is likely. Initiation should take place over northern Wales and a second band in southern Wales, both moving NE, potentially forcing up into one line.
The moisture convergence with winds flowing up the Irish sea and also from the SE arrive in the afternoon in NW Wales. This is where models are potentially converging on bringing some of the strongest storms to with especially strong hail potential, perhaps severe level. Flooding possible given the incredible potential for moisture combining here along with orographical forcing.
There's slightly weaker moisture advection into southern Wales, with less coming in off the Irish Sea. But much the same risks as this moves NE are likely. It's less likely than the NE Wales risk though and if it gets pulled in too early, it might not go on to hit the Midlands at all. Hence theres still only a moderate risk here.
Saturation profiling looks to be where most of the moisture is trapped in the surface layer with the surface wind flow and increasing dewpoints with evaporated moisture trapped at the surface layer and upper-level moisture divergence happening clearing out the moisture before the storms. This is where the capping is helpful and that increases the lightning potential due to less saturation for the profile. This despite cooling upper-levels, the divergence is slightly stronger than the cooling.
Scattered showed a and storms are possible to the north and south of the main clustering (s) where forcing may be weaker but the energy is still there, especially in areas around the south Midlands below the southern MCS, where it depends on the forcing strength whether they become part of a cluster or not.
There is the potential for formation into a fully fledged MCS, but the relatively weak deep-layer shear may mean that a visible band of storms doesn't form and instead areas of storms close together and merging appears more likely. This merging may induce shifts in momentum where merging cells or nudging cells may interact with a stronger cell, more likely or not if a Supercell could form though as I'm not sure how much budgets affect non-Supercellular storms compared to the merging cells more known momentum inducing interactions, and lead to severe hail and a weak tornado is possible. Along with temporarily increasing lightning potential. But with the lightning looking potentially very frequent, that may not make that much of a difference anyway.
These storms should move NE into the late evening and could last into the night in the NE of England. Along with a few afternoon storms clustering up similarly in Scotland potentially as well. But they should be slowly weakening from the late evening onwards as the surface energy potential decreases and environment the storms are pushing into becomes more stabilised and also the severe potential appears to weaken andor fully go out by the late evening.
Cells trailing behind the MCS are possible after it's left areas especially on eastern extent as the lapse-rates are allowed to re-strengthen as surface heating is still occurring. Moisture also feeds those trailing cells from the far SW where it combines with lapse-rates in Wales ish area. Rising moisture feeding into the surface heating and colder air feeding lower and lower in the atmosphere but surface heating weaker. Stronger low-level and deep-layer shearing but weaker energy and forcing.
Supercell potential would be better if initiation if the MCS is slightly earlier and allowed for better heating, instead it remains about even with the MCS's with the southern part having a Supercellular risk given the rising warm air into the southern portion and open air below allowing a few momentum induced to southern portions of the system with cells combining into the system. Potentially the best hail chance along this portion of the system. Lift with rising motion is increased and energy lifting with that rising motion. Therefore updraft speed is increased and with cells pushing into the southern part of the likely MCS, either one, the increased updraft width will also help hail production and if it weren't for the mess with not knowing where it will be, it would likely have high risk lightning frequency, very frequent lightning possible and more severe risks associated with the southern part of either of these two or combined systems.
In eastern Ireland, there's a shortwave trough that converges moisture there on the far western ecrwnt of the Theta-E ejection where WAA should allow for a few storms to form. Weak clustering and fairly frequent lightning are possible but not currently enough support for specific bands of storms for an area of interest to be added.
Small hail and heavy rainfall are possible here, but nothing appearing significant. It's essentially a weaker version of the UK risk with less severe potential and similar energy with weaker forcing. Unlikely to be upgraded to a moderate as this looks to be the last update, unless there's a substantial model output upgrade for that area.
In the SE of England, there's an area of interest with a sliding lobe of forcing allowing showers from France to push along the forcing band into the far SE. However, given that they're likely to be in the evening and surface based, it's unlikely that any lightning activity will sustain across the channel and instead it's something to keep an eye out on for sporadic lightning and small hail.