Storm Forecast
Valid: Sun 12 Apr 2020 06:00 to Mon 13 Apr 2020 06:00 UTC
Issued: Sat 11 Apr 2020 16:25
Forecaster: TUSCHY

A level 1 was issued across NE-Morocco and NW-Algeria for isolated large hail and severe wind gusts.

SYNOPSIS

A mid-level vortex just west of Ireland at 06Z drops south towards the western Bay of Biscay. This feature is accompanied by a broad LL vortex, which tightens during the day while moving to the S.
An intensifying and progressive upper trough crosses Scandinavia from W to E and causes a sharpening cold front which runs from Scotland to N-Germany to Finland during the night. It continues to surge S/SE until 06Z. Resulting col over N-CNTRL/NE France becomes a focus for enhanced convergence with better BL moisture beneath a steadily lowering/cooling mid-troposphere.
Resistant troughing over Portugal faces a warm subtropical ridge downstream over N-Algeria and wind fields at upper levels strengthen during the day in response to a passing subtropial jet streak and a structuring polar front jet over Spain.

Elswhere, either ridging or an hostile environment preclude DMC activity over many parts of Europe.

DISCUSSION

...N-Morocco and NW-Algeria...

Until noon, some elevated CAPE and prevailing anticyclonic mid-level flow bring scattered showers and thunderstorms with graupel/some hail.
Beyond noon, ingredients improve with N-ward fanning EML plume atop a moist marine layer over N-Morocco/along the coast and assist in elevated CAPE magnitudes of 500-1000 J/kg. Forecast hodographs show impressive loops and lots of streamwise vorticity but most of that remains capped. Effective shear is more in a multicell regime and hail/strong to severe wind gusts are forecast. Of interest is constantly strengthening synoptic-scale forcing and upper divergence, which hint at clustering convection during the evening/night and hence a gradually lowering severe threat. This nocturnal cluster moves offshore while rounding the subtropical ridge. It brings some heavy rain to the level 1 area, too. Lightning areas were expanded accordingly to the E/NE.

...Portugal, France, Belgium and W-Germany ...

Ingredients show very weak shear and variable CAPE over a broad area. Best thermodynamics exists over Portugal and NW-Spain with 400-700 J/kg MLCAPE. Expect scattered to widespread CI with a diurnal maximum during the afternoon. Graupel/isolated hail and gusty winds are forecast. Slow motion may cause local flash flood problems.

S-CNTRL UK is affected by a leisurely northward shifting warm front during the daytime hours in response to falling pressure to the SW before being pushed S/SW during the evening/night as a cold front. Pooling BL moisture beneath cold mid-levels cause weak MLCAPE and SBCAPE in the 300-600 J/kg range. Most thunderstorms bring graupel and a few wind gusts but I don't want to exclude an isolated tornado event with better LLCAPE and augmented streamwise vorticity in the lowest 3 km just along/south of the front. This activity weakens beyond sunset.

Elsewhere expect diurnal driven orografic thunderstorms on an isolated to scattered scale, which slowly drift to the E/NE. Isoalted pre(cold) frontal activity during the night is possible but nothing severe is forecast and this activity remains too spotty for a lightning area/expansion. Excluded aggressive GFS this time due BL moisture problems.