Storm Forecast
Valid: Fri 24 Jul 2020 06:00 to Sat 25 Jul 2020 06:00 UTC
Issued: Thu 23 Jul 2020 00:30
Forecaster: TUSCHY

A level 2 was issued across parts of Italy, Slovenia and Croatia mainly for large to isolated very large hail, severe wind gusts and a tornado threat. Heavy rain is possible as well.

A level 1 with similar hazards but lower probabilities encompasses the level 2.

SYNOPSIS

A mobile upper-level trough over SE-France shifts east towards N-CNTRL Italy until the evening hours and towards the Adriatic Sea thereafter. Modest mass response in the lower/mid-troposphere occurs over the N-Adriatic Sea with falling heights/surface pressure and mixed signals in a potential closed circulation over the northern part of the Adriatic. No clear signal of any frontal passage exists so accompanying mid-level cold pool overspreads a seasonable warm and moist air mass south of the Alps, resulting in widespread destabilization.
A deep/extensive upper-level cyclonic vortex sits over NE Europe and pushes an extensive cold front east towards W-Russia with rather cold mid-levels aloft resulting in widespread weak to modest instability build-up.
Another sharp trough enters the scene from the west during the second part of the night and attendant synoptic fronts affect Ireland, Scotland and UK until 06Z.

DISCUSSION

... Parts of Italy all the way to W-Romania ...

The overall picture favors several rounds of strong to severe convection over many parts of the level areas. As the upper trough draws near during the start of the forecast period, models show either rapidly developing or already ongoing severe convection over N-Italy with 500-1000 J/kg MLCAPE and 10-20 m/s DLS, increasing from E to W. Would not be surprised to see already long-lived convection with large hail and severe wind gusts marching either more east towards the N-Adriatic Sea or further SE towards the CNTRL Adriatic Sea along a CAPE/thetae gradient.
Thereafter abundant forcing and diurnal heating/orography cause rapid thunderstorm development further east from Slovenia to W-Hungary and N-Montenegro. DLS around 15 m/s and 1000 to locally up to 2000 J/kg MLCAPE will be adequate for organized multicells and isolated supercells with large hail, severe wind gusts and heavy rain.
Forecast hodographs ahead of the trough feature rather straight signatures with some better curvature in the lowest 3 km AGL over Slovenia and Croatia, favoring rotating updrafts with all kind of severe including a tornado or two with sufficient LL moisture/shear overlap. Mesoscale boundaries or ageostrophic response to any developing LL vortex over the N-Adriatic Sea certainly dictate the tornado threat.

Betimes, convection grows upscale into numerous eastward pushing MCSes, which affect the CNTRL and eastern parts of the level areas. Large hail, severe wind gusts and heavy rain remain the main hazard. Dependant on the strength of the low/mid-level vortex over the N-Adriatic Sea, an excessive rainfall risk could arise over parts of Slovenia and Croatia during the overnight hours. Could become a late start for severe convection over NE Bosnia and Herzegovina into N-Serbia but organized overnight MCS event is possible.

Towards the S - CNTRL Italy - CIN remains a bit stronger, but given upper-level support and increasing BL moisture, CI should occur as well. Even higher CAPE and DLS around 20 m/s indicate a good chance for supercells with isolated very large hail, severe wind gusts and excessive rain. In addition a tornado event is also possible given amount of LL shear and moisture. Hence the level 2 was expanded south.

A few strong to isolated severe storms are also possible north of the level 1 area (including SE-Germany and the Czech Republic) but could see some issues with a thickening cirrus canopy from the activity to the south and a resultant mixed mode of partially elevated/surface based activity. Isolated large hail should be the main hazard mainly driven by local MUCAPE in excess of 1kJ/kg despite weak tropospheric shear.

... Other lightning areas ...

Scattered to widespread thunderstorms occur which should stay sub-severe (excluding an isolated event). The lightning areas are a compromise of current models with modifications possible in the future.