Convective Outlook
VALID 06:00 UTC Tue 20 Jun 2023 - 05:59 UTC Wed 21 Jun 2023
ISSUED 06:32 UTC Tue 20 Jun 2023
ISSUED BY: Dan
On Tuesday, an area of heavy rain, driven by a pronounced shortwave/dry intrusion engaging with the western flank of a high Theta-W plume (>16C at 850hPa) being drawn northwards from France, will continue to track northwards across England and Wales. This will contain a messy mixture of stratiform and convective rainfall, the latter most likely on the eastern flank with relatively unimpeded inflow of unstable air. As such, there will be an ongoing risk of very heavy rain and/or lightning, especially on the eastern edge of the main area of rainfall. A few elevated showers/storms may also developing ahead of the large mass of rain over NE England and SE Scotland. As the rain clears northwards, a slack surface low will slowly drift northeastwards across central or eastern England, while increasing cloud breaks should lead to better surface heating. Given the lingering moist plume across eastern England, there is scope for substantial CAPE to build here (potentially 1,500 J/kg MLCAPE) if near-surface temperatures can get close to the mid-20s Celsius. This, coupled with strong SSW flow aloft on the eastern flank of an upper trough near western Ireland, will create an environment supportive of organised (and potentially severe) thunderstorms. Surface winds will be backed on the north and northeast flank of the surface low (perhaps moreso in Lincolnshire/East Yorkshire), yielding 20-30kts effective shear and giving the potential for rotating updrafts/supercells.
The thermodynamic profile is supportive of large hail, 2-3cm in diameter and potentially larger should a well-organised thunderstorm develop, while substantial low-level shear - especially to the northeast of and close to the surface low centre/effective triple point - coupled with low cloud bases poses the threat of an isolated tornado. Given storms will tend to mature as they drift northeastwards, this places Lincolnshire, Cambridgeshire and Norfolk as most at risk for severe convective weather (perhaps also East Yorks). A broad SVR was introduced for the threat of large hail, and to a lesser extent an isolated tornado. It should be stressed this is a conditional risk depending on the degree of surface heating and shape/location of the surface low, which will be heavily influenced by the track and extent of cloud/rain in the morning. It is certainly possible subdued temperatures could result in little in the way of deep convection over the region. One option is the tail end of the earlier elevated convection could, with time, become rooted to the boundary layer (if sufficient destabilisation occurs to sustain it). In addition, the window of opportunity may be relatively narrow (restricted to the afternoon hours) as the most favourable environment will be migrating increasingly offshore with time. Latest data this morning would suggest the risk of severe thunderstorms in eastern England is less likely, and perhaps largely confined to Lincs/E Yorks if anything were to develop - however, have retained the larger SVR area to highlight the main area of potential.
Elsewhere, scattered showers and thunderstorms will develop in response to diurnal heating, lightning most widespread across Ireland and W/NW Scotland where some marginally-severe hail may be possible (higher CAPE in Ireland, stronger shear in NW Scotland), but a few thunderstorms will also be possible in parts of Wales, SW England, Cumbria and S/SW Scotland, especially later in the day. While most activity will tend to slowly fade through the evening hours, there may be a relative uptick in activity into the early part of the night from E Wales/NW England/West Country into the Midlands, aided by a shortwave/cold pool aloft migrating northeastwards across the area.
