A tongue of warm, moist (high Theta-W) air will be drawn northwards across southern then central Britain on the northern/eastern flank of a surface low generally located over the Celtic Sea, and this will provide the main focus for any thunderstorm development. Overnight soundings suggest rather moist profiles with skinny CAPE over SW England, and this casts some uncertainty over how much lightning will occur. Realistically, isolated elevated thunderstorms could develop anywhere along the high Theta-W tongue, from SW Eng - Channel Islands at 06z, south Wales - Dorset at 12z, Irish Sea - north Wales - Kent at 18z, and Cumbria - Norfolk at 00z. Model guidance often struggles in these situations as subtle changes aloft relative to model expectations can make a big difference as to whether deep convection is able to develop or not. A SLGT has been introduced where a blend of guidance suggests a slightly higher probability of thunderstorm activity, but confidence is low.

The extent of medium-level cloud cover will also affect the degree of surface heating that can occur; given a deeply-mixed boundary layer, forecast profiles suggest surface temperatures exceeding 25C will be required to develop surface-based convection. However, by late afternoon an increasingly pronounced convergence zone is expected to develop from north Wales through the Midlands towards London and north Kent (broadly M40 corridor), lasting into the evening hours. Moisture pooling along this boundary, with dewpoints of up to 17C, may just be sufficient for isolated, deep convection to occur later in the day. While speed shear will be fairly weak, substantial backing of the low-level winds near this boundary may provide sufficient rotation to a rapidly ascending updraft to allow some organisation for a time. Even if surface-based deep convection is unable to initiate, there will still be an ongoing risk of isolated elevated activity near this boundary in either case.

As nocturnal cooling of the boundary layer commences, any surface-based convection will become increasingly elevated through the evening hours - but there could be an uptick in coverage for a time elsewhere across the Midlands and perhaps northern/eastern England later. Also monitoring developments over France in the afternoon/evening as surface-based thunderstorms may attempt to drift across the English Channel towards SE England - however, a combination of both relatively cold SSTs and changing steering flow during the evening hours may ultimately mean lightning activity remains over France.