Ahead of this, an isolated heavy shower / thunderstorm is possible during Sunday afternoon / early evening in a zone from Cen S England / S + W Midlands / Wales. However, despite profiles exhibiting reasonable speed and directional shear to aid organisation, one or more warm noses may significantly restrict cloud depth and so the risk of lightning is treated with quite a low probability for now. It is more likely that outbreaks of largely dynamic rain will be falling from medium-level cloud over Wales / Midlands, associated with the warm front slowly lifting northward. A few showers are also likely in N and W Scotland, but limited in depth and so the lightning risk is considered quite low.

Main thunderstorm interest is during Sunday night. By Sunday evening, frontal rain will likely already be affecting parts of SW England, Wales, W Midlands and Ireland. A lead shortwave impulse in the strong southerly flow aloft will aid ascent, coupled with isentropic upglide. CAPE values of 500-1,000 J/kg are expected, this instability being released through the course of the night. 

In a semi-random fashion, scattered elevated thunderstorms will develop, although there is considerable uncertainty over exactly where and when, hence a very broad SLGT area. We suspect much of the thunderstorm activity will be primarily focussed along two main Theta-E ridges. This would suggest one round over the East Midlands / Lincs / Yorks, and into NE England later in the night - this may also extend into parts of East Anglia for a time, depending on how quickly the plume destabilises (the earlier, the more widespread the risk is before moving offshore).

A second area of interest would then be Wales, perhaps the W Midlands, moving NW-wards across the Irish Sea towards eastern Ireland - perhaps expanding across to NW England later in the night. Given the magnitude of CAPE/shear, lightning will tend to be very frequent with any thunderstorms that do develop, accompanied by the threat of hail, gusty winds and locally very heavy rain.

Given the large uncertainty about storm placement at present, it is difficult to highlight specific areas for a MDT. We have issued one where there is better multi-model consensus, but it is acknowledged that scattered thunderstorms will develop elsewhere as well. It may well be that two main clusters evolve within the MDT, with a relative minima lightning activity in the middle. We will continue to monitor trends throughout the day, and may issue further updates if necessary.