Extended Forecast
Valid: Tue 30 Apr 2019 06:00 to Wed 01 May 2019 06:00 UTC
Issued: Sun 28 Apr 2019 19:27
Forecaster: TUSCHY

A level 1 was issued across NE-Bulgaria, E-Romania and S-Moldova mainly for isolated large hail.

A level 1 was issued for parts of the S-Ukraine mainly for an isolated tornado and large hail threat.

SYNOPSIS

An upper trough rotates from the CNTRL-Adriatic Sea to the NE and approaches Romania during the late afternoon/evening hours from the SW. With another trailing impulse SW of that vortex, a positive tilted upper trough configuration exists over the CNTRL Mediterranean. Downstream SW-erly flow affects most of SE Europe. A LL vortex evolves over Romania during the day and lifts NE during the night while intensifying gradually (on a modest rate).
Ridging over W Europe and the ongoing advection of a cold/dry air mass over N/NE Europe preclude thunderstorms over most parts of the mentioned regions.

The evolving Romanian depression forces a developing cold front to move east towards and over the W-Black Sea with a structuring warm front over the S-Ukraine. Elsewhere synoptic-scale fronts don't impact today's thunderstorm outlook.

DISCUSSION

... Bulgaria, Romania, Moldova and the S-Ukraine ...

An evolving warm sector over E-Romania, NE-Bulgaria, Moldova into far S-Ukraine features improving BL moisture with weak onshore flow towards the E-Transylvanian Alps and the E-Carpathian Mountains (probably with thinning BL moisture depth further inland and hence prone to diurnal mixing). Despite rather weak mid-level lapse rates, most models show widespread 400-800 J/kg MLCAPE.

Forecast hodographs reveal veer-back-veer (VBV) structures over NE Bulgaria/S-Romania, turning more unidirectional further north. More pronounced curvature is expected in the vicinity of the warm front over the S-Ukraine.

These ingredients favor a few rotating storms along the warm front during the afternoon hours with a low-end tornado and hail risk. Despite weak LL flow, deviating storms may ingest enough SRH for transient LL mesocyclones in a low LCL environment.
Further south, VBV profiles add uncertainties to how much effective helicity and shear will be available and current thinking is that only a low-end risk of splitting supercells exists. Organized multicells with isolated large hail and locally severe winds gusts are forecast.
E-Romania into S-Moldova however could see a few splitting supercells with the mentioned unidirecional flow and shear pattern but this activity could become more elevated next to the coastal areas. We expanded a hail-level 1 into those areas.

During the night, clustering thunderstorms turn elevated with a subsiding severe risk.

... SE-Spain and NE-Algeria ...

The passing upper trough sparks isolated to scattered thunderstorms along the mountains of the highlighted areas. CAPE/shear overlap may provide temporal storm organization with an isolated large hail threat, but the overall risk won't support a SWODY-2 upgrade right now.