Overnight, a plume of MUCAPE exceeding 1000 J/KG, and potentially reaching 1500-2000 J/KG, is expected to move along the cold front of a low-pressure system passing over southern and central England. This raises the risk of frequent lightning and severe thunderstorms. The cold front, located on the eastern side of the low pressure over western central southern England, will shift northward, spreading across southern England, the Midlands, and East Anglia before moving into the North Sea.
A mesoscale convective system (MCS), or potentially two MCSs with isolated storms in between, could curve northwestward from France and Benelux into southern England, potentially lasting into the Midlands. The complex wind patterns around the low-pressure system could bring further storms to southern England in the morning, moving northeast, with the remains of the MCS heading southwest through the Midlands. Later in the day, additional storms could develop in the southeast (SE) with surface-based CAPE possibly exceeding 1500 J/KG, leading to isolated pulse storms or another MCS depending on deep-layer shear. This could move across the English Channel into SE England, with further surface-based storms possible in the Midlands where cloud breaks occur. The environment is expected to be moist and buoyant, allowing energy to build rapidly once cloud cover dissipates.
There is significant model disagreement over the storm risk tonight. Some models predict an MCS will form over northern France and move toward southern central England, potentially weakening over the Channel, although frequent lightning may still occur if it survives the crossing. The system may transition to an elevated storm as it reaches 800+ J/KG of MUCAPE, although the post-boundary layer is likely to be saturated, which may limit lightning frequency.
Further east, a larger MCS could develop over a pool of 1500+ J/KG of MUCAPE, with lightning potentially semi-frequent due to the high CAPE values, although saturation may limit its intensity. This system may push into Suffolk and move northwest, with isolated elevated storms forming over the SE, where 800+ J/KG of MUCAPE exists but with weaker forcing than in southern central England or the North Sea. These storms also present a risk of organized lightning and could strengthen the MCS as it pushes inland. There is potential for the two MCSs to merge if stronger forcing occurs, with the UKV model being the most likely to predict this scenario, though it has its limitations. The AROME model, meanwhile, suggests that an MCS could form over land in SE England, initially producing strong storms before expanding east and west into a full MCS, potentially impacting London and the Midlands by early morning.
The MCS is expected to move northwest into the Midlands and potentially as far north as Yorkshire. Most models indicate the MCS will not weaken until mid to late morning, with heavy rainfall expected across much of England. Rainfall totals may exceed 50 mm in 24 hours in some areas, with extreme model runs suggesting over 80 mm in isolated locations, raising a significant risk of flooding. Model disagreement remains over where the heaviest rainfall and most frequent lightning will occur, but widespread heavy rain is likely due to large PWAT values and extensive stratiform rain within the MCS. The specific stratiform rain configuration will depend on wind flow patterns within the MCS, which may shift around the low-pressure system. Some models peak rainfall totals in East Anglia and the eastern Midlands, where low-level shear could enhance convective rainfall, even though stratiform rainfall may be slightly weaker. This may also increase lightning activity in these regions, but saturation and available energy will be key factors.
Later in the morning, the MCS could slow down over the western Midlands and near the Welsh border, with a balance between convective and stratiform rain leading to heavy stratiform rainfall. This raises the risk of flash flooding, particularly in the eastern Midlands.
Hail is another risk, particularly in the SE, where storms may develop lower in the atmosphere, with relatively weak low-level shear but abundant MUCAPE extending into higher levels. Low freezing levels advecting from France into the south as the cold front passes increase the chance of low-end severe hail, especially over the SE.
During the afternoon, another round of storms is possible in the SE as a trough pushes up from northern France, bringing 1500+ J/KG of MUCAPE, largely surface-based. Enhanced forcing could lead to deep convection, with cloud tops reaching 20,000-25,000 feet. Despite high saturation levels, a drier zone trailing the trough could reduce saturation issues and support organized lightning and heavy rain. Strong lapse rates could also contribute to small or low-end severe hail. Model differences regarding moisture levels at different heights could affect the lightning risk, with the ECM model showing drying near the boundary layer, while the higher-resolution ICON-D2 model shows more moisture in the mid-levels before drying occurs higher up.
Storms may also develop in the Midlands and Lincolnshire if cloud breaks occur, with SBCAPE values exceeding 1000 J/KG. Drier profiles compared to the previous night may enhance lightning activity, but cloud cover will be a critical factor in determining how much energy can build and how active storms will become.
Thanks for @EagleEyeStorm for providing his excellent, in-depth convective discussion.
You can download the kml file of the convective outlook here:
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