Storm Forecast
Valid: Sat 02 May 2020 06:00 to Sun 03 May 2020 06:00 UTC
Issued: Fri 01 May 2020 22:53
Forecaster: PISTOTNIK

A level 1 is issued for parts of Belarus, the Ukraine and Moldova mainly for excessive convective precipitation.

A level 1 is issued for parts of Italy, Albania, Greece and S Turkey mainly for large hail.

A level 1 is issued for parts of SE Turkey, Iraq, Iran and Armenia mainly for large hail and severe convective wind gusts.

SYNOPSIS

A broad mid-level trough stretches from Iceland to central Europe and gradually extends SE-ward. Its rear side is flanked by a strong jet stream from France across the central and E Mediterranean region to the Near East. In the range of the main trough, cool maritime air from the NW is advected across the British Isles, central Europe, and increasingly into E and SE Europe as well.
Iberia and European Russia are under the influence of mid-level ridges and surface anticyclones with fairly warm air.

DISCUSSION

... central, E and SE Europe, including most of Turkey ...

Diurnal heating creates a few hundred J/kg CAPE and allows scattered, mostly daytime-driven thunderstorms across wide areas. Weak vertical wind shear will generally limit the severe weather risk.
Towards the east, the leading cold front across Belarus and the central Ukraine is the main source of initiation. Strong convergence and front-parallel storm motion from south to north may promote a few excessive rain events despite rather modest CAPE (on the order of 500 J/kg) and low-level moisture (expected 2m dewpoints between 10 and 12C). Otherwise, synoptic lift is largely absent, and thunderstorms will mostly be tied to orographic features or to the a sea breeze front in Moldova, E Romania and E Bulgaria.
In the cooler maritime air further west in central Europe, CAPE will be confined to 100 or 200 J/kg. Showers and low-topped thunderstorms will mostly be connected to travelling vorticity maxima. The most pronounced short-wave trough will cross Germany and surrounding areas in the convection-relevant time frame.

... Italy to Albania, Greece, far-south Turkey, Near East ...

The southern fringes of the CAPE reservoir overlap with the mid-level jet, which enhances vertical wind shear across deep layers (0-6 km: 15-20 m/s, in Italy even up to 25 m/s). A vorticity maximum crosses the W Balkans and Greece in the afternoon to evening. Another one - formerly affecting Germany - will enter N Italy after 18 UTC. These features will likely support scattered storms. Otherwise, the lack of synoptic lift will leave it to smaller-scale features like upslope flow convergence, drylines on the downwind side of mountain chains, and sea breeze fronts to achieve convective initiation.
Some storms will likely be multicellular and pose a main risk of large hail. Their coverage and their ability to detach from the mountains are difficult to assess, as the lowland moisture may often be of limited depth and therefore prone to premature mixing. If onshore/upslope flows can resist the downward mixing of dry mid-level air, pockets of bottled-up moisture could support CAPE up to 1000 J/kg. In that case, a few storms propagating off the mountains into the favorable lowland environments could organize into supercells, and one or two cases of very large hail are not ruled out.
In the Near East, very deep and dry boundary layers increasingly shift the main risk from large hail to evaporation-driven downbursts.