Extended Forecast
Valid: Sun 16 May 2021 06:00 to Mon 17 May 2021 06:00 UTC
Issued: Fri 14 May 2021 22:54
Forecaster: TUSCHY

A level 1 was issued for large hail and severe wind gusts for the Crimean Peninsula and S-Ukraine.

A level 1 was issued for heavy rain for Slovenia and parts of NE Italy.

A level 1 was issued for isolated hail/strong gusts and a low-end tornado risk for E-CNTRL France to S Germany and parts of Hungary.

SYNOPSIS

Unusually far south displaced frontal zone becomes established between low geopotential heights over the NE Atlantic/ NW/CNTRL Euope and a robust subtropical ridge over Morocco/Algeria. 250 hPa v winds run in the 2-4 sigma space (deviation from the model climate) and reflect the impressive nature of this jet for this time of year (with similar values down to the 700 hPa level). Attached to the low heights is a slow moving negatively tilted trough which affects Greece during the forecast. Downstream, strong ridging covers far E Europe/Russia with ongoing warm/hot conditions.

At the surface numerous frontal boundaries pass W/CNTRL Europe with the one over Poland curving back to N Spain in form of a wavy boundary embedded in the prevailing westerly flow regime. Beside those synoptic-scale fronts, numerous convergence zones should move east. South of this boundary subtropical moisture pushes E and affects the W Mediterranean while a marine air mass with less moisture content emerges from the NE Atlantic and advects E towards CNTRL Europe. With the latter one pushing beneath cold mid-levels, we expect a broad area with low to moderate instability build-up over W/CNTRL and E-Europe.

A continental air mass (additionally dried mid-levels with backward trajectories crossing the Russian anticyclone) mixes somewhat with offshore (more humid) flow from the Caspian Sea, but moisture content remains rather low N of Georgia. This NW-ward pushing air mass creates a belt of enhanced BL moisture along its leading edge with also serves as focus for maximized CAPE build-up during the forecast. This belt runs from the Black Sea to Volga/the Ural mountains.

DISCUSSION

... Crimean Peninsula to the S-Ukraine ...

In response to a developing LL depression over the far E Black Sea a strongly confluent flow regime sets up over the area of interest with low tropospheric dewpoints in the solid mid/upper tens. Deeply mixed air from the greater Caucasus Mountains moves atop the moist air mass and results in elevated 0.5 - 1.5 kJ/kg CAPE within 15 m/s DLS. CI is forecast next to the Crimean Peninsula (off/onshore) right where LL convergence seems to be maximized. Forecast hodographs show impressive veering in this ongoing WAA regime with pure streamwise vorticity for any deviating thunderstorm (to the right). Large hail and isolated severe wind gusts will be the main hazard with these potentially impressive (visually) structured supercells.
Although near BL remains stable stratified, SRH1 in excess of 300 m^2/s^2 and lowering LCLs along the coast of the Black Sea could support an isolated tornado event.

Further east, elevated CAPE exceeds 3000 J/kg but should remain capped during the forecast period.

... W-/CNTRL- to E Europe ...

We put the focus on the overlap of the anomalous wind field beneath the upper/mid-level jet with slim/modest SBCAPE which is true for parts of France, S Germany to Hungary and Slovenia.

The air mass in this belt is weakly capped/well mixed with rather low LCLs and moist (vertical) profiles. Especially over CNTRL France to S Germany a signal for better mid-level dryness exists with respect to enhanced confidence in gust potential. Scattered to widespread CI is forecast within an environment of roughly 15 m/s 0-3 km shear (more or less doubling in the 0-6 km shear layer). Hodographs reveal a rather straight nature so isolated hail/strong to severe wind gusts will be the main hazard.
Nevertheless orography can induce enough (regionally) augmented LL (directional) shear to increase SRH in the range where a tornado event can't be excluded anymore (with LCLs in the 600-900 m range). This can be the case along convectively enhanced line segments or more discrete cells (e.g. Hungary). During the late afternoon, SRH in general increases over France into S-Germany, affecting still ongoing convection (with LLCAPE in the 100-150 J/kg range). Again an isolated tornado event is also possible with this activity (E-CNTRL France to S Germany). CAPE decreases during the night with slim MUCAPE values expected all night long to assist in ongoing/isolated/non-severe thunderstorm potential over France/W-/S-Germany.

Slovenia could see flash flood issues with nighttime convection which seems to train along the frontal boundary/orography. Perc 99 (COSMO-EPS) shows 70 to 80 l/qm which is in line with ICON/IFS and hence a small level 1 was added. Don't want to exclude and isolated tail-end tornado event between Trieste/Pula, where LL directional shear is maximized.

... Bulgaria to Poland/Lithuania ...

A convergence zone adds enough LL lift for widespread CI during the day. DLS in the 10 m/s range should assist in clustering multicells with locally heavy rain and isolated hail/gusty winds.
A bit more interesting is the potential line-up of NE-ward marching convection over far NE Poland into Lithuania and W-Belarus where somewhat better curvature of the hodographs exists which could add to an isolated large hail threat in this area.

... Finland ...

Slow moving convection may pose isolated heavy rainfall chances but overall risk seems too marginal for a level area.

... S-UK ...

Slow moving storms may bring locally heavy rain next to an isolated short-lived tornado event due to better LLCAPE (e.g. S-UK with a zonally orientated convergence zone).