The upper high over the North Sea and western Europe will begin to expand across Scotland on Monday, while a cut-off upper low continues to linger over the Atlantic west of Biscay. Once again a relatively high Theta-E airmass will cover the British Isles within a fairly weak southeasterly flow aloft, providing another day with potential for showers and thunderstorms. One concern is the quality of airmass over central England given substantial overturning in the wake of Sunday afternoon/evening thunderstorm activity, although continued advection from France/the Low Countries over the following 12-18 hours should be sufficient in yielding an unstable environment once again. Otherwise, a very similar setup is anticipated to that on Sunday, with strong surface heating lifting near-surface temperatures to the high 20s/low 30s Celsius and dewpoints of 17-19C yielding MLCAPE in the 1,500-2,000 J/kg range, with SBCAPE values likely to exceed 3,000 J/kg in places, potentially yielding the most volatile environment of the year so far. In the absence of any substantial upper-level forcing, convective initiation will likely be reliant on upslope flow/sea breeze and surface boundaries to create forced ascent and generate deep convection. Forecast soundings suggest the EML of recent days will be somewhat shallower, with slightly lower convective cloud bases and therefore requiring slightly lower near-surface temperatures for initiation. That said, there are hints of a warm nose above the mixed layer at ~850hPa that may act as somewhat of a cap, but convective cloud could potentially reach 40,000ft/-60C. 

Current thinking is an area of showery rain over Wales/Irish Sea, courtesy of an area of PVA, will continue to drift westwards across Ireland. In its wake, the first showers/thunderstorms could develop as early as 11-12z, tied to orographic forcing in northern Scotland and Wales, and perhaps in the vicinity of low-level convergence in eastern England. A slack surface low will cover much of central/southern Britain, deepening gradually through the day and resulting in increased low-level convergence towards its elongated centre with time. Numerous showers and thunderstorms are expected to develop through the afternoon and evening hours, across a swathe of Wales, the Midlands and down towards the London area, while also extending into parts of NW England. Additional thunderstorms are likely in northern and western Scotland. In all areas, PWAT in the mid-30s mm, slow storm movement and the potential for multiple thunderstorms to train over similar areas will bring the risk of surface water flooding, especially in urban areas. This is the greatest hazard, but the magnitude of instability (despite limited shear) may also allow isolated hail 2-3cm in diameter to occur in the strongest cells, and a couple of SVRs have been included to highlight both of these risks. Once again, some strong downburst winds may be possible locally, with scope for 40-60mph gusts. Thunderstorms will grow in size and coverage with northwestward extent through the Midlands, Wales and NW England, perhaps accelerating forwards as cold pools develop, hence the inclusion of a broad MDT and elongated HIGH risk area.

Eastern coastal counties of England will be subject to greater boundary layer mixing as a strengthening easterly breeze develops, coupled with a more substantial EML aloft, therefore resulting in greater capping. In SW England, rather dry mid-levels will probably limit potential for deep convection. The evolution in Ireland is less-clear, given potential for cloud and showery rain to drift westwards through the day. It is possible this may contain some embedded elevated convection, while the best potential for surface-based convection probably exists over western Ulster into eastern Connacht, but very much dependent on the degree of surface heating. There may continue to be showers/thunderstorms that develop well into the evening in parts of Ireland. In addition, thunderstorms over Wales will probably attempt to drift across the Irish Sea towards eastern Ireland, although probably with a weakening trend.