Storm Oscar will develop across the Celtic sea and is likely to move and settle in the southwest of Ireland from Friday night to Saturday. In this period, longer spells of heavy rain are likely across parts of Ireland and southwest England.
An occluded front will move north into parts of southwest England, Wales and Ireland on Saturday, 300-500 J/kg mid-level CAPE might allow some elevated thunderstorms to be embedded in some of the heavy rain, especially across southwest England. However, these thunderstorms are unlikely to survive inland as some showers might weaken due to evaporation.
The latest model runs forecast inland parts of England might exceed 26C, perhaps reaching 28-30C in places. The combination of high moisture and temperature allows high CAPE values, which are most likely to exceed 700 J/kg across central southern England and the Midlands, perhaps reaching 1000 J/kg in places. This will allow thunderstorms to break out across the northeastern edge of the occluded front. High cross-model agreement and considering the MUCAPE and 850mb temperature, two AOIs have been issued even though it is still 3 days away, one across parts of England and Wales, and another across western parts of Ireland. Two border areas of SLIGHT risk have also been issued, covering most of England and parts of Ireland, as there are risks of potent thunderstorms breaking out in multiple places.
Skew-t diagrams suggest a low risk of high precipitation supercell in the slight risks areas, or training effects might occur in places, especially across Wales and West Midlands, hence two areas of severe weather risk have been issued, which mainly focus on the risk of localised flooding and large hail.
Considering the amount of uncertainty of the advance and the angle of the occluded front, an enhanced risk has been held off at the moment due to the lack of confidence, which means there will be a chance of upgrading the warning in later updates.
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