On Sunday, an upper ridge will stretch from the North Sea and down across western Europe, while a cut-off upper low loiters over the Atlantic to the west of Biscay. Between these two features, a relatively high Theta-E airmass will cover much of the British Isles providing the potential for another day of deep convection and thunderstorms. The exact forecast evolution is very uncertain, with a considerable amount of inter and intra-model variability. As such, broad themes will be discussed but unlike Saturday when there was a clear foci for thunderstorms this will be more challenging for Sunday.

At the start of the forecast period, showery outbreaks of rain and embedded elevated convection may be ongoing across parts of Scotland (western areas especially), and perhaps also across parts of the English Channel / southern England, both associated with areas of PVA embedded within the rather weak south/southeasterly flow aloft. In these areas rain should gradually dissipate and/or clear through the morning, and while cloud cover across the British Isles is uncertain a degree of surface heating is anticipated given largely medium/high level cloud present (away from Ireland, western Scotland and SW Britain where fairly extensive low cloud/coastal fog is possible). The presence of a stout EML over central/eastern Britain will require near-surface temperatures to exceed 27-28C to eradicate surface-based CIN, while farther west towards Wales/West Country less capping is likely with lower initiation temperatures (23-24C, for example). The main caveat here is cloud cover and whether surface temperatures can reach/exceed these values, whereas in eastern Britain greater insolation should allow near-surface temperatures to reach/exceed required values. However, even if SBCIN can be eroded in eastern Britain, the strengthening upper high in the vicinity and associated very dry, subsided mid-level profile will probably work against any deep convection. Towards the Midlands, Wales, SW England, forecast profiles are moister with less evidence of subsidence and therefore more conducive to deep convection. Given the rather slack flow, convective initiation will probably rely heavily on both surface boundaries/upslope flow and any subtle features aloft - and this is where large discrepancies are evident in various NWP guidance. 

A small surface low over SE England on Sunday morning is expected to migrate/expand slowly northwestwards into the Midlands, and this will influence the low-level wind flow while also maximising low-level convergence in its vicinity. Therefore the exact shape/track of this low will be instrumental to areas most likely to see shower/thunderstorm activity. In a broad sense, scattered heavy showers and thunderstorms may develop over portions of Wales (aided by upslope flow/sea breeze) and across the Midlands/Cen S England during the afternoon hours, persisting into the evening. Modified forecast soundings yield MLCAPE of 1,000-1,500 J/kg CAPE, locally higher, with cloud tops potentially to 40,000ft. However, given weak flow aloft (<10kts at 500hPa) effective shear will be rather weak, leading to rather pulsey/outflow-dominant slow-moving storms with cold pools aiding the development of daughter cells nearby. Nonetheless, the magnitude of CAPE will be sufficient for some fairly active thunderstorms, capable of producing marginally-severe hail in the strongest cells. Of greater concern, however, is the potential for surface water flooding, especially across the Midlands, Wales and West Country/Cen S England where PWAT of 30-40mm, moister profiles (less evaporation) and a tendency for storms to become prolonged/morph into clusters. The well-mixed boundary layer will also be supportive of wind gusts 40-60mph locally near the most intense cells. A SVR has been introduced primarily for the risk of surface water flooding, especially in urban areas, and to a lesser extent for isolated large hail. This whole scenario is also complicated by potential for PVA to arrive from France, partly enhanced by previous day convection, which could result in a much larger area of heavy convective rainfall across parts of S/SW England and south Wales - this aspect is uncertain.

Elsewhere, scattered heavy showers and thunderstorms are likely to develop over hills in northern England and portions of Scotland on Sunday afternoon and evening, drifting slowly to the north or northeast. Some of these storms could be fairly active lightning-wise, with scope for some marginally-severe hail over northern England and northern Scotland in particular. In Ireland/Northern Ireland, despite increasing instability in response to surface heating, a warm nose in the 600-700hPa layer and rather extensive low cloud (at least initially) may inhibit much in the way of deep convection, hence have refrained from including any higher threat levels at this stage. Depending on how things evolve on Sunday afternoon/evening, some clusters of showers/thunderstorms over Wales/NW England may migrate northwestwards across the Irish Sea on Sunday night towards the Isle of Man/SW Scotland/east coast of Ireland. In addition, there have been hints of mid-level showers drifting into SE England (and perhaps East Anglia) on Sunday night within the residual instability plume - although whether much lightning occurs with this activity is uncertain. Finally, there is scope for an uptick in showers/thunderstorms over eastern Scotland during the early hours of Monday, although limited cross-model agreement casts a lot of uncertainty over this potential.