Mesoscale Discussion
Valid: Sat 21 Jun 2025 15:00 to Sat 21 Jun 2025 18:00 UTC
Issued: Sat 21 Jun 2025 14:29
Forecaster: TUSCHY

A Mesoscale Discussion (MD) was issued for the highlighted area to update an increasing severe risk during the evening into the overnight hours (beyond the MD's validity). The MD area equals a level 2 upgrade.

Latest WV data reveals a gradually N-ward departing broad/weak lead impulse, which caused scattered elevated convection over most of UK during the past hours. Clearing behind that impulse spreads NE into W UK and should assist for some diabatic heating in the following hours. A pronounced WV drying at roughly 45N 14W indicates the southern tip of the main upper trough axis, which is about to lift NNE towards the MD area. A meridionally aligned and structuring/wavy cold front is analyzed just W of the MD area, which awaits the impulse from the SW before starting a motion to the E thereafter.

Latest surface data is more or less in line with local model data and rain-cooled airmass from elevated convection in the MD area should see some recovery with 2m temperatures in the mid twenties and dewpoints in the upper tens to (spotty) lower twenties. Constant deepening of the prefrontal moisture over Wales into W-CNTRL UK is forecast next to improved advection due to a weak warm jet (with latest lidar revealing roughly a 10kt increase from the S in the lowest 1.5-2.5 km AGL). Despite some displacement of best BL moisture with an eastward advecting EML, mentioned setup still supports MUCAPE in the 1000-1500 J/kg range (modest entrainment could lower those peaks a bit). 12Z Camborne/Aberdaron feature a warm 2 km layer AGL, but not much CIN remains if modified with expected 2m T/Td values during the evening hours. SE UK experiences BL mixing with suppressed dewpoints, but some recovery is forecast when going into the evening.

Shear improves along the frontal zone, with LIDAR Crug-Y-Gorllwyn and Predannack showing 50kt at 3 to 5 km AGL. Hence NWP forecast shear of 20 (+) m/s in the 0-3 and 0-6 km layers seems reasonable. Latest 12Z hodographs offer elongated signatures in the lowest 4 to 5 km AGL with straight profiles. However, 0-3 km SRH increases with mentioned uptick of low/mid-level winds in ongoing WAA regime and forecast hodographs show rather impressive 0-2 km SR inflow with lots of streamwise vorticity.

Expect CI during the evening hours (model spread of 16Z to 18Z) in the W/SW parts of the MD area, where a mixture of CI due to the approaching IPV maximum and along sea breeze fronts/the synoptic-scale cold front within the WAA regime occurs. Not much cap modification is expected with only marginal cooling from the W, so diabatic heating during the next few hours dictates the risk of how widespread surface based CI will be. This risk however seems possible within the MD area.

Initiating and rapidly organizing cells (into multicells and a few supercells) pose a (large) hail threat with isolated hail diamater in the 3 to locally 5 cm range not ruled out. Any near surface based cell would also pose a strong to severe gust and tornado risk. Maturing mid-level mesos might overcome lingering weak CIN for effective near surface based inflow. LCLs remain in the 600-800 m AGL in the western part of the MD with more than 1 km AGL to the E/S. Magnitude of CAPE/shear space also supports a strong and long-tracked tornado event, which includes towns like Manchester, Liverpool, Leeds and further N to Sunderland.
Betimes, increasing CIN and upscale growth into clusters (also supported by splitting cells) induce a more dominant heavy rain threat with some hail and gusts before the activity shifts out to the sea.

Latest nowcast trends including more aggitated Cu development over SW UK and consistent NWP signals for CI in an environment with EHI values in excess of 3 all increased confidence for a level 2 upgrade (which equals the MD area).