Valid: Fri 16 Mar 2018 06:00 to Sat 17 Mar 2018 06:00 UTC
Issued: Thu 15 Mar 2018 23:43
A level 1 is issued for N Syria and N Iraq for large hail and to a lesser degree for severe convective wind gusts and excessive convective precipitation.
Low pressure over NW Russia connects with an extensive, multi-core low pressure system over W, central and S Europe. A blocking anticyclone forms over the Norwegian Sea, and extremely cold air flows westward at its southern flank. A sharpening frontal boundary along 50°N between the Netherlands and the Ukraine separates this Siberian air from mild, moist subtropic air to the south. A shallow frontal wave travels from Austria into the central Ukraine along this boundary.
The main mid-level jet stream is displaced far to the south and runs from N Algeria towards Turkey.
... inland Croatia, S Hungary, N Serbia into W Romania ...
Enhanced vertical wind shear (~15 m/s across the lowest 3 km, ~10 m/s across the lowest 1 km) with mostly unidirectional wind profiles is present in the warm sector of the mentioned frontal wave. A dry intrusion will result in clearing skies as soon as the warm front departs and in some synoptic lift, though it will largely be compensated by beginning cold air advection. Daytime heating should result in 100 or 200 J/kg of CAPE.
A complex interplay of mild and moist inflow from the SE sector, warm and drier downslope winds from the SW sector and cool frontal cross-circulations from the NW sector makes it difficult to anticipate where and when convective initiation will happen. It appears most likely over E Croatia around noon. Showers may organize into multicells or a few (splitting) supercells and will travel eastward. Marginally large hail and/or hail accumulations on the ground are possible. If a storm manages to favourably interact with a convergence zone, a tornado is not ruled out under enhanced low-level shear and low cloud bases.
Limiting factor which precludes a level 1 is the shallow depth of the convection, which may even be too low for lightning activity. Convection will decay around sunset.
... greater SW Europe ...
Mild and moist air beneath rather cool mid-levels allows low-end CAPE under weak vertical wind shear. Scattered, disorganized storms are expected. They will stay mostly non-severe, though small hail and isolated instances of a hail cover on the ground are possible due to the seasonably low freezing level height.
Activity will be mostly daytime-driven over the continent, whereas lift ahead of the main mid-level trough may keep it going into the night at a low intensity level over the W Mediterranean Sea and parts of Italy.
... E Turkey, N Syria, N Iraq ...
Steep lapse rates in response to daytime heating allow a few hundred J/kg of CAPE in the range of a thermal low. Scattered to widespread storms are expected to erupt in the afternoon, supported by synoptic lift ahead of a short-wave trough embedded in the westerly mid-level flow. 0-3 km shear around 15 m/s is sufficient for storm organization. Large hail is the main risk. Localized flash floods or isolated downbursts are not ruled out in addition with mature storms. Activity will gradually decay after sunset.