VALID 06:00 UTC Sun 16 Jun 2019 - 05:59 UTC Mon 17 Jun 2019
ISSUED 23:37 UTC Sat 15 Jun 2019
ISSUED BY: Dan
An increasingly elongated upper low will reside to the west of Ireland, creating strong southwest flow aloft over the British Isles. Cool air aloft combined with diurnal heating of a reasonably moist low-level airmass (dewpoints 10-13C) will yield 300-700 J/kg CAPE. Scattered showers and a few thunderstorms are expected to develop by midday and through the afternoon / early evening, particularly across N / NE Scotland, the Midlands into northern / eastern England, and across the Republic of Ireland and into Northern Ireland.
... SCOTLAND ...
For Scotland, shear will tend to weaken by the time the majority of deep convection occurs, and so a more "pulse type" mode is expected - though any showers/storms that can develop early, primarily over Highland, may benefit from reasonable shear to become somewhat-organised. Nonetheless, given cold air aloft and the depth of convection likely to be deepest here than elsewhere in the British Isles, some small hail will be possible from the most intense cells, and perhaps a couple of funnel clouds where showers/storms develop over zones of low-level wind convergence which will likely develop during the afternoon hours around the Moray Firth coastline. Slower storm motion here may result in some localised surface water issues.
... ENGLAND ...
Elsewhere, a strongly-sheared environment is expected allowing updrafts to become longer-lived and cells somewhat organised as they track generally to the northeast. A couple rounds of showers/thunderstorms will be possible across England, associated with a pre-frontal trough early afternoon, and then perhaps the approaching warm front late afternoon / early evening. The most intense cells, including the potential for a supercell, may produce hail close to 2cm in diameter. Low-level winds will be a little more backed as storms approach the east coast, and so here would be the best potential for an isolated tornado.
... REPUBLIC OF IRELAND / NORTHERN IRELAND ...
Perhaps the best overlap of favourable conditions for organised thunderstorms (in the British Isles) will exist over Ireland, although the forecast evolution here is less-clear due to a potentially messy precipitation mode as a new Atlantic frontal system approaches. Assuming sufficient cloud breaks and surface heating can occur, then scattered showers and a few thunderstorms may affect parts of N / NW Republic of Ireland initially - early afternoon - spreading northeastwards across Northern Ireland with time, with perhaps a second round developing near Limerick and environs later in the afternoon / early evening as the left exit of the approaching mid-level jet streak overspreads the area - these then migrating northeast across the northern half of Leinster during the evening hours.
A couple of supercells will be possible, with the potential for hail up to 2cm in diameter, and perhaps a tornado where low-level winds become locally backed, enhancing the low-level shear and SREH. Due to limited confidence in forecast evolution, have refrained from issuing a SVR for now.