Storm Forecast
Valid: Sun 08 Aug 2021 06:00 to Mon 09 Aug 2021 06:00 UTC
Issued: Sat 07 Aug 2021 21:00
Forecaster: TUSCHY

A level 2 was issued across the S/SE Black Sea mainly for excessive rain and to a lesser extent for large hail.

A level 2 was issued across parts of Austria into extreme N/NE Italy mainly for large hail and excessive rain.

Both level 2 areas are surrounded by level 1 areas with similar hazards but with lower probabilities. In addition a few severe wind gusts are possible over Austria.

A level 1 was issued over parts of the Baltic States mainly for hail, severe wind gusts and an isolated tornado risk.

SYNOPSIS

The static pattern continues over Europe. A broad cyclonic circulation remains centered over Scotland with numerous waves circling its S/E fringes. A robust low amplitude wave crosses Benelux NE while weakening.
A low latitude trough well down into the subtropics is anchored just W of Portugal/Morocco. A downstream subtropical anticyclone over Tunisia/Libya intensifies during this forecast and acts as a blocking feature for any approaching upper trough from the W. This anticyclone serves as focus for another building heat wave over the CNTRL Mediterranean with H5 geopotential heights exceeding 3 sigma during the end of the forecast.
Another upper trough is forecast over far SE Europe with no real net motion forecast until 06Z.

Numerous surface fronts will serve as foci for enhanced CI with an occluding cold front over E Poland into the Baltic States stretching all the way down to the Alps. Another quasi-stationary front is analyzed over far W Russia into the Ukraine.


DISCUSSION

... Georgia to N/NE Turkey, Armenia into N-Azerbaijan ...

The elongated and meridionally aligned trough axis sits over the E Black Sea to the Sea of Azov with only weak mid-level waves crossing its E fringe during this forecast. The most pronounced QG forcing is forecast over Turkey with the slowly eastward crawling upper trough axis.

The BL air mass features rich moisture content with LL mixing ratios in excess of 14-16 g/kg and surface dewpoints in the upper tens to lower twenties (e.g. 20-24 C in the Kolkheti Plain). Lapse rates aloft are not impressive but a bit enhanced between the Caucasus Mountains and Lesser Caucasus with prevailing southerly flow, which pushes MLCAPE in the 1-2.5 kJ/kg range when crossing this enhanced BL moisture field - CAPE field is maximized over the Kolkheti Plain.
DLS will be in the sub-10 m/s range, so pulsating convection will be the main issue, probably growing upscale into one or two slow moving clusters. During CI, intense updrafts probably contain large hail but with time, excessive rain becomes the main hazard.

Another focus for excessive rain arises next to upper trough axis, which breaks apart into numerous pieces (over N-Turkey). Any of these pieces can enhance LL onshore flow with a very moist and unstable airmass interacting with the Pontic Mountains. That's why GFS/IFS show variable QPF maxima in excess of 100 l/qm/24h, dependant on where any of those circulations evolves.

A broad level 2 for excessive rain and to a lesser extent for large hail was issued to cover this risk. A few significant flash flood events are possible, especially along the Pontic Mountains due to the complex orography.

... N Italy into Austria ...

Once again, a passing wave interacts with a moist/unstable air mass and creates a favorable environment for organized convection.
Models keep CI mainly along the orography next to the passing wave with strongest forcing. Although CI is less likely further S over N Italy, not much capping and some QG forcing also affects this area during peak heating. One caveat will be the intrusion of drier (downslope) air from the W.
Kinematics remain excellent with DLS in the 25-30 m/s range and 0-3 km shear approaching 20 m/s in a few places (especially within the level 2 area). Strong veering with height and increasing strength of the background flow create elongated and curved hodographs.

CI is forecast beyond noon with a mix of multicells/supercells. One concern is the intense 0-3 km moist 20 m/s inflow into these cells, which supports temporal back-building and excessive rain. Otherwise, numerous rotating updrafts contain large hail and severe wind gusts. Main limiting factor for a more robust outbreak will be the southward sagging cold front with a cooler/more stable postfrontal air mass - convection could turn more elevated while racing to the NE. The hail risk however could extend all the way into W Slovakia.

An nocturnal cluster with elevated convection will continue E/NE and will cross Slovakia into S Poland. Heavy rain and isolated hail will be the main issue. Dependant on the magnitude of MUCAPE, a spotty large hail event can't be ruled out (especially if IFS verifies). This risk however seems too low in the poor man's ensemble to upgrade for now. Moisture advection into S Poland ahead of this wave has to be monitored closely as not much BL modification is needed for a more pronounced severe wind gust/hail threat into S Poland.

... Baltic States ...

A progressive wave affects the Baltic States during daytime and is accompanied by a weak surface depression. Strong forcing compensates meager CAPE and a forced line of convection should cross the Baltic States from S to N with the most vigorous convection over Latvia into Estonia. Strong to severe wind gusts and hail will be the main hazard. In addition an isolated tornado event can't be ruled out with strong LL speed/directional shear ahead of the LL depression.

... Denmark into S Sweden ...

An active thunderstorm day with scattered pulsating convection is forecast. Heavy rain will be the main issue. Towards S-Sweden, MLCAPE in excess of 1000 J/kg may also support a few hail reports. Currently the anticipated risk is too low for a level area.