A complex frontal system associated with a large Atlantic low-pressure system will bring bands of heavy showers across the British Isles from Wednesday night into early Thursday. An occluded front will transport a plume of milder southwesterly winds into many parts of the British Isles on Thursday, followed by modest CAPE values of 400-700 J/kg across the west.
Showers lingering behind the occluded front will likely turn thundery due to 200 J/kg of 3CAPE combined with moderate shear. The atmospheric profile behind the occluded front does not appear saturated, making it easier for thunderstorms to develop. Low-level shear of 35-45 knots and deep-layer shear of 40-60 knots could help sustain updrafts. If these thundery showers organize sufficiently, there is a chance that a bow echo could develop across central parts of ROI and Northern Ireland, potentially bringing locally damaging gusts, hail, and even one or two tornadoes along its 'tail'.
Due to strong cross-model agreement on the likelihood of a bow echo, the first enhanced risk of the year has been issued, mainly highlighting the risk of severe thunderstorms between 07Z and 11Z. Due to strong shear, thunderstorms developing across Northern Ireland and ROI will continue moving northeastward into parts of Scotland during the late morning, maintaining a threat of large hail and tornadoes.
Another lower-confidence risk exists for a second round of thunderstorms across northern England. This will strongly depend on the available moisture remaining in the atmosphere after instability shifts into North Wales and northern England. Given forecasted PWAT values of 15-20 mm, one or two showers could develop and potentially intensify into thunderstorms. Shear will remain strong across the region, meaning there is still a continued tornado threat. If moisture levels are sufficient, one or more squall lines could develop, significantly increasing the chance of tornadoes between broken segments of showers. However, due to uncertainty, only a slight risk has been issued for this scenario.
You can download the kml file of the convective outlook here:
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