Valid: Sat 16 Jun 2018 06:00 to Sun 17 Jun 2018 06:00 UTC
Issued: Fri 15 Jun 2018 22:05
A level 1 was issued for parts of SE Europe mainly for excessive precipitation large hail and severe wind gusts.
Long-wave with a jet stream passes through British Isles and Scandinavia. Shallow convective activity is possible in the warm sector of the wave. SW Europe is under influence of Azores ridge and rather dry air mass, which inhibits convective activity. A broad and shallow high with rich boundary layer's moisture and enhanced lapse rates cover SE Europe. Within the support of strong diurnal heating and weakly sheared environment, slowly moving isolated and pulse thunderstorms should develop in the afternoon hours in the local convergence zones. Locally, storms may be capable of producing large hail up to 2-4 cm, downburst-type severe wind gusts and excessive precipitation leading to local flash floods.
...Balkan Peninsula, Ukraine, Turkey and Caucasus...
This area is dominated by rich boundary layer's moisture (MIXR 12-14 g/kg) which originated over this area due to strong diurnal heating and resulting evapotranspiration, local convergence zones, moisture pooling beneath capping inversions and advection from the Black Sea and Mediterranean from previous days. All these processes consistently increase moisture in the boundary layer and thus thermodynamic instability. ML CAPE by most of the global NWP models is predicted to range from 1000 to 2000 J/kg with local peaks of 2500 J/kg. Very weak airflow and thus vertical wind shear will support weakly organized isolated pulse storms which may locally cluster into multicells. Main threat within these storms will be associated with heavy rain and local flash flooding, especially in the areas with complex orography. Such scenario is supported by high PW (> 40mm) and weak storm motion. Thanks to moderate instability large hail is possible, but mostly up to 2-4 cm as a consequence of weak wind shear. Severe wind gusts of downburst-type are likely in many places, but their range of impact will be highly local. A level 1 is issued primarily for enhanced thermodynamic instability and high PW. Supercells are unlikely in this environments, but local wind field interactions with mountain ranges cannot exclude few such storms (especially near Caucasus). Majority of thunderstorm activity should develop within intensifying diurnal heating in the afternoon hours and fade in the late evening. Some of the NWP models indicate nighttime elevated convection activity over Ukraine and Adriatic Sea.
Shallow convective activity is likely within the passage of the long-wave trough, especially in its warm sector where boundary layer's mixing ratios will increase to 7-8 g/kg. Dynamic steepening of lapse rates will provide jump in ML CAPE to 400-600 J/kg. Along with QG-lift, convective initiation should take place over N part of British Isles around afternoon hours and move to North Sea and S Scandinavia in the evening and nighttime. Although enhanced airflow and wind shear may result in a local severe wind gust event, overall threat is too low to issue level 1 for this area.