An area of developing low pressure will rapidly deepen and deliver heavy downpours to the west and north on Sunday. Along the leading edge of the cold front, there will be a risk of a strong thunderstorm outbreak as the moisture from the low pressure will aid with surface heating. Strong low/mid-level shear with relatively high CAPE (700-1200 J/kg) allows severe thunderstorms to develop, few might also carry supercell characteristics.
Models are struggling to forecast the speed and the timing of the low pressure as well as the cold front, which will cause the area of thunderstorms to change. Even though, considering the severity of the thunderstorms on Sunday, two enhanced risk has been issued, one across eastern parts of Ireland and Northern Ireland, and one across most of northern England, parts of midlands and parts of Scotland.
Ireland/ Northern Ireland:
In the morning, heavy showers, perhaps with isolated elevated thunderstorms, will clear through most parts of the region, enabling a clearer sky for scattered showers to develop in the afternoon. MUCAPE of 500 J/kg supports some convection, perhaps turning more organised in the mid-afternoon, especially across eastern parts of Ireland and Northern Ireland. A low-end enhanced risk has been issued for some of these areas of risk of widely scattered thunderstorms. Strong shearing with an ideal lapse rate also supports a risk of marginally severe hail (2-2.5 cm) and a risk of a tornado or two, hence a severe weather risk has been issued for these areas as well.
Wales/ Midlands/ Northern England/ Scotland:
A cold front is expected to move from the west to the east, bringing some rain, potentially thundery for most. At first, the cold front might carry some elevated thunderstorms, affecting western parts of England and Wales, these thunderstorms might form a couple of bow echos, bringing a risk of gusty wind in the heaviest showers (50-55mph).
As the cold front weakens as its moves inland, some severe surface-based thunderstorms are likely to break out from Birmingham, tracking northeastward into most of northeast England. MUCAPE of 700-1200 J/kg allows some severe outbreaks of thunderstorms, this is aided by strong shearing as well as warm surface temperature (>26c). There are also some signals of these surface-based thunderstorms will contain supercellular characteristics, which if this occurs, large hail (2-4cm), a few brief tornados, and perhaps some gusty winds as well. Training effect might also bring the risk of flooding in places, as 30-50mm of rainfall is possible within 2-3 hours. A large enhanced risk has been issued to cover the elevated thunderstorms as well as the surface-based thunderstorms, with an AOI covering the area which is most likely to see severe surface-based thunderstorms (and risk of supercells).
This convective outlook will be reviewed on Saturday night (24/06/2023), perhaps upgrading the AOI area into a high risk.
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