Convective Outlook
VALID 06:00 UTC Tue 13 Jun 2023 - 05:59 UTC Wed 14 Jun 2023
ISSUED 06:20 UTC Tue 13 Jun 2023
ISSUED BY: Dan
An upper high will be centred over NW Scotland on Tuesday, with heights slowly rising through the day. Around its southern flank, southeasterly flow aloft will advect the relatively high Theta-E airmass that has resided over Britain for the past few days westwards into Ireland. A subtle shortwave/area of PVA, partly enhanced by convection over England and Wales on Monday afternoon/evening, will slowly meander across the Irish Sea and Ireland on Tuesday morning, although largely drifting offshore to the Atlantic by early afternoon. This may provide the focus for showery outbreaks of rain over parts of the Irish Sea during the morning hours, this gradually decaying by late morning. Diurnal heating over Ireland is expected to yield an increasingly unstable environment, with near-surface temperatures of around 20C in southern Ireland to 22-23C closer to Ulster required to surpass low-level capping and generate deep convection. Given predicted TMax in the mid 20s Celsius and dewpoints around 17-19C, 1,400 - 1,700 J/kg MLCAPE should be available fairly widely (with SBCAPE potentially up to 3,000 J/kg) in an environment with weak effective shear (~10kts).
A shallow surface low is expected to develop through the day across southern and central Ireland, with a strengthening east/southeasterly flow on its northern flank over Ulster and northern Leinster creating a NW-SE zone of enhanced low-level convergence from Co Leitrim/Sligo to Co Westmeath. In addition, sea breeze convergence will develop along the length of the west coast, and all of these low-level boundaries (in tandem with the subtle shortwave close to the NW coast) will provide the focus for forced ascent and a scattering of thunderstorms to develop. Thunderstorms should grow in size and intensity as they drift northwestwards, especially across Connacht and western Ulster where they may form into clusters. Given the magnitude of instability, the strongest cells may be capable of producing isolated 2-3cm diameter hail, wind gusts of 40-50mph and, given PWAT in the mid 30s mm and potential for multiple storms to train over similar areas, the risk of local surface water flooding. A SVR was introduced for the risk of flooding and isolated large hail. Showers/storms will ease during the mid-late evening hours, although a few elevated showers may drift into the east coast of Ulster/Leinster overnight.
Elsewhere, a few isolated to scattered thunderstorms will be possible in parts of northern and western Scotland, despite upper ridging in place, aided by sea breeze convergence and orographic uplift. Here there may also be a risk of some local surface water flooding. Depending on the trajectory, the dregs of some earlier storm activity over Ireland may eventually drift towards the Outer Hebrides on Tuesday night. Finally, a few isolated showers may develop in parts of mid/south Wales, S/SW England and the West Midlands during Tuesday afternoon/early evening, but given very dry air through most of the troposphere, and a deep, well-mixed boundary layer, it is likely these will probably struggle to gain sufficient height to produce lightning.
