The forecast evolution for Monday daytime is very uncertain and rather complex, and hence carries low confidence. However, confidence is much higher by Monday night, especially compared with the convective potential on Sunday for example. 

On Monday morning, a few elevated thunderstorms may be ongoing across parts of the Irish Sea and northern England, along the rear side of the frontal rain. This will continue to lift northwards into southern and eastern Scotland and out across the North Sea as the day progresses.

In its wake, a second shortwave impulse will arrive by late morning or the afternoon, and so a second round of thunderstorms is possible, particularly over northern England - but possibly some areas farther south too, depending on the timing/phasing of this shortwave. At a glance, forecast profiles look generally capped to surface-based convection, and so it is quite likely much of this activity will be elevated. That said, isolated surface-based convection cannot be ruled out, especially where elevated convection can root within the boundary layer, or orographic lift / upslope flow plays a role in thunderstorm development (such as the Pennines).

The environment is likely to be well-sheared, with both reasonable speed and directional shear present, enabling cell organisation. Cloud clover is likely to be a major issue, but if sufficient surface heating can occur, some fairly substantial CAPE may develop. As such, hail up to 2cm in diameter and flash flooding may occur. This potential may be somewhat contaminated by hints of some frontal (dynamic) precipitation running up from the south at times too, along the frontal boundary.

Scattered thunderstorms are likely to develop over parts of Northern Ireland and the Republic of Ireland on Monday afternoon / early evening. Cloud may also be an issue here, so storms will tend to develop where the best surface heating occurs combined with areas of low-level convergence (such as sea breeze). With closer proximity to the upper low, the environment will be less-sheared and so "pulse type" convection is most favoured - but with weak steering winds, slow storm motion could result in prolonged heavy downpours leading to some localised flooding. Hail up to 1.5cm in diameter may also accompany the most intense cells.

Periods of frontal rain will affect central, southern and eastern parts of Scotland for much of this forecast period, reinforced by embedded convective elements for North Sea coastal counties in particular - especially later in the afternoon and into the evening. Some embedded lightning may occur locally, with the main threat being from localised flooding.

During Monday daytime a heat low will form over Spain and western France, with low-level winds on the northern flank of the low becoming more backed with time. As a result, advection of a very warm, moist airmass will occur towards the Bay of Biscay which will serve to increase the horizontal thermal gradient in the area.

By late afternoon / early evening, a shortwave will approach from the southwest, forcing a wave to develop along the frontal boundary and increasing the potential for dynamic rainfall. On the eastern flank of the frontal wave, isentropic upglide will result in scattered elevated thunderstorms developing over NW France during the evening hours. Strong deep directional and speed shear will allow thunderstorms to become organised, with unimpeded warm, moist inflow from easterly low-level winds. Upscale growth into an MCS (mesoscale convective system) is possible as the thunderstorm complex drifts NNE / NE across the English Channel towards CS / SE England, then later past London and into East Anglia. Hail, gusty winds, very frequent lightning and localised surface water flooding are all hazards associated with this complex.

The exact track of these thunderstorms is a little uncertain, and an adjustment of ± 40 miles may be required to the MDT area. However, it must be noted that most lightning activity will tend to be focussed on the eastern flank of the precipitation envelope, with a period of more generic frontal rain likely on the western flank as the wave runs north along the frontal zone.