UPDATE 15:08 UTC No changes to overall expectations, have extended the LOW across remainder of SW Britain for potential elevated instability this evening/tonight

General evolution sees a large upper low becoming anchored to the southwest of Britain, with 2 slow-moving frontal zones being the main focus for some convection - one across northern Scotland, and another over southern Ireland/southwest Britain. 

At the beginning of the forecast period, a triple-point frontal system will run NW-wards over S/SW Ireland during Sunday morning. Backed surface flow coupled with low LCLs, and strong forcing and DLS, suggests a risk of a tornado, especially if any rigorous convection can occur close to or to the rear of the triple point in an environment with notably steep lapse rates. There would only be a short window of potential on Sunday morning, before the main cold front begins to bend back farther to the SW. This is also dependent on the exact position and timing of these frontal features, some models keeping the main area of interest offshore to the SW, and as such confidence is too low to issue a SVR threat area for now.

Shortwave trough rotating around main parent upper low will engage with occlusion across northern Scotland, perhaps allowing some convective elements to become embedded within. However, of more interest will be the post-frontal environment, with slack surface flow/convergence and some element of orographic forcing serving to develop a few scattered showers with a generally low risk (5-10%) of lightning.

Backing flow ahead of frontal zone will advect higher WBPT airmass from western France, with an increasing signal for elements of elevated convection to develop within the frontal zone on Sunday evening/night. As is often the case with such events, there remains some uncertainty as to the true likelihood of lightning activity and for now we remain with just a LOW threshold. In either case, such elevated convection that may develop over the English Channel and advect towards IoW/Hampshire/Dorset will probably weaken as it heads NW-wards towards SE Wales.