Verification An MCS (Mesoscale Convective System) tracks NW from France towards Portsmouth to Brighton which brought prolific lightning & intense rainfall! As this also approached the coast, further strong active storms developed across SE England & Slightly inland across CS England which also brought prolific lightning. We forecasted this very well as it was an unusual setup, so we are very happy with our verification. Definitely one of our top forecasts! Overall Rating: 9/10
Discussion:
During Wednesday morning/afternoon, Cape will build in Ireland to around 200-500J/KG which will once again spark some intense & heavy showers with the risk of isolated lightning strikes. One or two thunderstorms may develop across Ireland, however confident on this remains relatively low.
Our attention then shifts towards Wednesday night/Thursday early hours a Theta E plume will advect into CS/SE England which will rapidly destabilise allowing active & intense thunderstorms to develop across France which will continue to advect north. Further rapid thunderstorm development can be expected across N France & English channel becoming elevated in nature.
CAPE looks to be around 800-2000J/KG allowing these storms to give frequent & prolific lightning across the English channel. These storms will run in quite a strong deep layer sheared environment allowing these storms to become quite long living & organised.
These elevated intense storms will then track into CS/SE England bringing prolific lightning, hail & flooding. As these main storms approach, their is the possibility of explosive developments of thunderstorms ahead of this. These storms will also give prolific lightning as well as all the other severe risks including hail & flooding. These storms could also develop further inland, infront of the main storms which is why the MDT Risk has now been extended further north & east.
During the early hours of Thursday morning (0400 - 0700), most thunderstorms would have died down in activity turning into thundery rain. However due to the high CAPE levels present, further storms may initiate throughout the morning. A separate outlook will be issued tonight or tomorrow morning for Thursdays afternoon risks!
However, their is still a huge amount of uncertainty on where exactly these storms will hit therefore we have not issued a HIGH warning.
An active & severe night ahead!
Technical Discussion: An upgliding theta-E ejection on the east side and parallel to a northwestward moving warm front should allow for an MCS to approach the south coast overnight with Supercellular activity possible mainly with feeding cells on the eastern side of the storm. Frequent lightning, 40,000 feet tall storms, some severe hail and some flash flooding are all possible. Linear development should take place after initial surface development over France evolves from surface to elevated during the evening. With dry warm air, or on slightly less capped models a warm nose, intruding at the 925hPa level, a surface inversion layer forms and without surface heating as night falls, convection is very likely to transition to elevated well past the boundary layer where significant ascent begins to be forced with the development of stronger lapse-rates and a potential vorticity lobe stretches out to help with development into a full on linear MCS. As it hits the south coast, the far eastern extent of the MCS may develop more significant potential with slightly lower LCL’s and energy pushing in more from the southeast into the Kent and Sussex coast generally on most but not all models. Some models have the eastern extent a lot further west between Brighton and the Isle of Wight, but those models are beginning to become outliers compared to most. This is where 3000+ J/KG of MUCAPE is possible for the development of these cells and the eastern extent is more likely to see frequent lightning given more depth of MUCAPE and more MUCAPE generally. The movement from northern France is northwest towards the central south to be hit first by the first part of the MCS but that appears to split or attempt to detach from what becomes the main MCS and weaken over the channel as it gets embedded into the warm front but may remain as thundery rain. That is not the main risk and occurs late in the evening. It is also not guaranteed to become much and may just be a feature of the warm front itself. Whereas the overnight risk comes next with MCS development further SSE, orientated less parallel and more head on towards the south coast moving northwestwards from northeastern France andor Belgium towards the central southern and southeastern coasts. Then it should move NW and further inland towards southern parts of the Midlands. With very lightning favourable saturation and significant MUCAPE, likely 1500-2000+ J/KG and cloud heights possibly up to 12km as it appears likely to hit central southern and southeastern coasts. This is where most risk comes from. The AROME and UKV are very strong on this occurring with pre-MCS cells forced with the change in wind direction forcing wind convergence in a line between Belgium and the SE on a few models and if this does occur, then very strong energy is available for pre-MCS cells to form and frequent lightning is very much a possibility along with severe hail especially as it eventually combines with the MCS. The most unstable buoyant layer has lifted indexes of around –9, which is extremely rare for UK risks and show the vigorousness of convection, hence the only reason why this isn’t a high risk is because of the worry over where the MCS hits. It would be in the SE but the Swiss models refuse to back down and they have been some of the best models, instead they focus on the central south with some convection extending east past Kent once it gets towards London way. A large inflow layer with significant energy and large buoyancy means that despite strong low-level shearing, hail is possible between 1 and 1.5 inches. This especially evident where cells feed momentum where they combine or if a Supercell does form. Rainfall totals could also be fairly upwards of 30mm in places and a lot of that falling in a short amount of time with a lot of moisture available. Therefore there exists risk for flash flooding. Overnight, initiation may occur north of the MCS as a forcing band develops perhaps due to gravity waves as the very tall potential of these storms gives that risk. Similarly, storms could trail behind to the south and east if the MCS is particularly strong enough. Some of which may last well into the morning because of the significant energy available. Eventually it should weaken wherever it is around 4am to 6am into thundery rain. This is where the potential weakens and any trailing cells are the main risk but only really on the UKV. There is still a lot of difference between the models but frequent lightning they almost all agree on. Along with some severe risk and generally quite significant storms. Along with the upscaling into an MCS at least of sorts on pretty much all of the models. Wrote By Our Team Member: @/EagleEyeStorm