Convective Outlook
VALID 06:00 UTC Sat 09 May 2020 - 05:59 UTC Sun 10 May 2020
ISSUED 06:53 UTC Sat 09 May 2020
ISSUED BY: Dan
Upper ridge continues to sit over the British Isles on Saturday, although being gradually eroded by both a sharpening upper trough approaching from the Norwegian Sea and by a cut-off upper low diving into Iberia from the Atlantic. The net result is a gradual falling of heights aloft through the day. A rather complex upper flow is also present on Saturday, with a southeasterly at 700mb potentially advecting some weak elevated instability / convection across the Channel Islands first thing on Saturday morning, this 700mb flow then veers round to the southwest upon reaching the Bristol Channel / M4 corridor - all the while the flow at 500mb is largely uniformly southwesterly.
... MAINLAND BRITISH ISLES DAYTIME / EVENING ...
Once again, diurnal heating will yield 300-700 J/kg CAPE by the afternoon and evening hours across parts of England, Wales and Ireland, potentially higher over S / SW Ireland. Given the upper ridge in place and the lack of any pronounced shortwaves, as with previous days convection initiation and sustainment will be heavily reliant on a combination of low-level wind convergence and orographic forcing - and hence any isolated showers that do develop are most likely near high ground.
Lightning activity will be somewhat dependent on the depth of convection - and while slight cooling may have occurred at the 500mb level early in the day, it seems likely that this layer may warm slightly once again during Saturday afternoon, acting as a warm nose and restricting the tops of shower clouds to below this level. Nonetheless, forecast profiles look slightly more encouraging for some deeper convection on Saturday than compared with the past two days, more especially across S / SW Ireland where mid-level lapse rates will be steeper. As a result, some sporadic lightning is possible here - and a low-end SLGT has been issued - with a few isolated lightning strikes possible elsewhere (particularly Wales / W Midlands). Showers will gradually fade from mid-evening onwards as diurnal heating subsides. Some hail will be possible from the strongest cells, more especially in Ireland.
... CHANNEL REGION SATURDAY NIGHT ...
Depending on the exact shape of the upper pattern, the mid-level steering flow could allow some elevated convection to drift northwards from France across the Channel Islands and general English Channel region during Saturday night. However, it is not clear how much (if any) lightning activity will be present with such showery outbreaks of rain, given relatively weak instability and existing uncertainty surrounding how far north this may spread in either case.
... NORTHERN ISLES SATURDAY NIGHT ...
Behind a cold front, an incursion of Arctic air will spread rapidly southwards atop relatively warm SSTs. This will generate a 100-200 J/kg CAPE, and numerous wintry showers are then likely to affect the area, particularly during the early hours of Sunday. However, the depth of convection will be fairly shallow, and this suggests that any lightning will likely be isolated at best.
