Convective Outlook
VALID 06:00 UTC Sun 03 May 2020 - 05:59 UTC Mon 04 May 2020
ISSUED 19:34 UTC Sat 02 May 2020
ISSUED BY: Dan
A rather complex pattern will evolve for Sunday, as an upper ridge over the British Isles flattens and westerly flow strengthens significantly in the mid/upper levels, this in part due to the tightening gradient as an upper trough swings southeastwards across Scotland. At the surface, a weak frontal boundary will linger close to southwest England, introducing more cloud into southern Britain with hill and coastal fog and some light rain and drizzle. The exact behaviour of this front is a little uncertain, perhaps unusually so given the short lead time now, with the GFS (and WRF derivatives) rather bullish in pushing this zone of thicker cloud and light rain across more of southern Britain than most other NWP guidance.
Either way, scattered showers are likely over Scotland throughout the morning, with others developing elsewhere across northern England, and perhaps parts of N Wales / Midlands / E Anglia, primarily aided by diurnal heating and low-level convergence zones given the slack flow. For the most part, forecast profiles suggest a warm nose will exist ~550mb, which will serve to restrict the depth of convection, otherwise the environment is well-sheared above the cloud-bearing layer. With a shallow surface low developing by the afternoon, low-level winds will become increasingly backed, blowing from the ESE and veering to the SW at 850mb and W at 600mb. This turning with height, plus increased vorticity due to surface convergence, could aid in the development of a few funnel clouds or perhaps the odd weak tornado. Weak instability (100-200 J/kg CAPE) and restricted cloud depth suggests any lightning will be very isolated and the risk overall is considered very low (10%).
During the evening and overnight hours, the approaching upper trough over Scotland and associated increasing baroclinicity may result in convection amalgamating into an area of showery rain over parts of northern England and the east Midlands, perhaps also into East Anglia.
