Extended Forecast
Valid: Thu 16 May 2019 06:00 to Fri 17 May 2019 06:00 UTC
Issued: Tue 14 May 2019 20:53
Forecaster: TUSCHY

A level 2 was issued across parts of the N-Ukraine and far S Belarus mainly for large to very large hail, severe wind gusts, tornadoes and excessive rain.

A level 1 surrounds the level 2 for similar hazards with lower probabilities.

A level 1 was issued across S-Romania, Bulgaria into S Greece mainly for large hail, severe wind gusts and an isolated tornado.

A level 1 was issued across N-Turkey mainly for large to isolated very large hail.

A level 1 was issued across Sicily, S Italy and surroundings mainly for severe wind gusts, isolated large hail and a tornado risk.

SYNOPSIS

An extensive blocking high pressure area covers N-Europe. Further south over CNTRL and S-Europe, a broad cyclonic gyre with numerous embedded smaller-scale vortices results in unsettled conditions with regionally augmented thunderstorm probabilities. Downstream of this vortex, WAA affects most of SE Europe. Ingredients for organized convection improve in this air mass.
A wavy front from Belarus to the E and another one from the W Ukraine to W-Turkey frame that plume of unstable air and serve as foci for organized thunderstorm probabilities.

DISCUSSION

...Ukraine to S-Belarus ...

Convection debris from the previous night should exit the area to the north with anticipated diabatic heating until noon/early afternoon. Persistent confluent low-tropospheric flow and pooling moisture along the synoptic-scale front result in improving BL moisture and ML mixing ratios in excess of 10 g/kg. This moisture content offsets meager mid-level lapse rates to some degree and global models agree in 500-1000 J/kg MLCAPE during the afternoon/evening. Forecast soundings reveal reasonable fat CAPE profiles in the hail growth layer and 35-40 kn DLS for large hail and this risk is attenuated by the forecast curved hodographs, which indicate a supercell risk. A very large hail threat (in excess of 5 cm), although not zero, may be softened by moderate CAPE values and borderline DLS for significant hail (diminishing to the CNTRL-Ukraine). Despite the hail threat, severe wind gusts and a tornado threat accompany mature supercell.
During the evening onwards, a noticeable increase of the 1-2 km AGL flow is forecast, which causes enlarged LL hodographs. A resulting excessive rain risk due to temporal back-building but also due to upscale growing convection into a large northward moving thunderstorm cluster exists. Despite issuing a SWODY-2, we upgraded parts of this area to a level 2 because of a concentrated severe risk but also an isolated extremely severe risk (hail and tornado).
The thunderstorm cluster shifts into Belarus during the night with an ongoing heavy to isolated excessive rainfall threat. Bot level areas may see modifications if a SWODY 1 will be issued.

... E-Romania, E-/CNTRL-Bulgaria to N-Greece ...

These regions reside along a quasi-sationary frontal boundary within a broad WCB regime. A moist air mass is present but weak mid-level lapse rates keep final MLCAPE magnitude rather benign over Romania into NE Bulgaria, before improving a bit towards CNTRL Bulgaria and NE Greece (with 500-1000 J/kg MCLAPE).
Romania may suffer from lingering convective debris from the previous night and hence air mass recovery remains a bit questionable that far out. An upgrade may be needed if earlier recovery is more likely in further model data.
To the south, 15-20 m/s unidirectional shear indicates a risk of organized multicells and splitting supercells. Large hail and severe wind gusts will be the main hazard, although an isolated tornado risk with deviating thunderstorms can't be ruled out - the latter risk is enhanced over CNTRL/S Bulgaria with a slightly better backed low-tropospheric flow regime. This activity weakens beyond sunset.

... N-Turkey ...

Deepening LL moisture beneath mid-level lapse rates in excess of 8K/km cause MLCAPE in the 1000-2000 J/kg range. Synoptic-scale forcing may be a bit weak, but passing short-waves and the orography should support at least isolated to scattered afternoon/evening storms. Although effective shear remains modest with elevated convection, degree of CAPE and 10-15 m/s shear should support large to isolated very large hail with mature updrafts. We kept this event in a level 1 for now due to uncertain nature of CI and weak/modest shear, but the hail risk may result in a subsequent upgrade later-on, in case of a SWODY-1 issuance.

... Sicily, S-Italy and surroundings ...

A progressive and well structured upper trough crosses this area from NW to SE/E with lots of synoptic-scale forcing and a rapid mid-level cool-down. A developing cluster is forecast between S-Sicily and Malta, which moves rapidly E/NE during the forecast. 15-20 m/s 0-3 km shear indicates a severe wind gust risk with that MCS and a broad level 1 was issued. Isolated large hail will be also possible.
The rest of Sicily and S-Italy is placed beneath cold mid-levels and in a weak geopotential height gradient regime, so slow moving convection with hail and heavy rain will be the main hazard. With augmented 0-2 km CAPE and weak background shear/enhanced background vorticity, numerous waterspouts are possible in this region. Hence a broad level 1 was issued for both events.

... N-Spain ...

Another passing trough induces active evening/overnight convection, but weak CAPE/shear should preclude organized DMC activity. Marginal hail and gusty winds will be the main hazard.