Storm Forecast
Valid: Mon 20 Jun 2016 06:00 to Tue 21 Jun 2016 06:00 UTC
Issued: Mon 20 Jun 2016 07:01
Forecaster: TASZAREK/PUCIK

A level 3 was issued for W Romania, E Hungary, NE Serbia, mainly for large to very large hail, damaging wind gusts, excessive precipitation and tornadoes.

A level 2 was issued for Belarus, E Lithuania, W Ukraine, W Romania, E Hungary and Serbia mainly for large to very large hail, damaging wind gusts, excessive precipitation and tornadoes.

A level 1 was issued for area extending from W Russia up to W Balkan Peninsula mainly for large hail, severe wind gusts and excessive precipitation.

SYNOPSIS

Cut-off over Italy weakens and geopotential heights are constantly rising over most of the S, E and SE Europe. A thermal boundary (a quasi-stationary wavy front) within northwardly moving tropical highly unstable air mass extends from Baltic countries, trough E Poland, E Hungary up to Croatia. A main polar jet stream is located between N Atlantic and Benelux. A sub-tropical jet streak stretches longitudinally from S Italy up to Belarus on the north-eastern flank of the fading Italian trough. In its northern section, a low-level jet develops over Belarus within a passage of the shortwave trough. SW Europe is under influence of a broad ridge while another high is located over W Russia. The main thunderstorm area runs along thermal boundary between tropical and polar air mass. A few more or less pronounced short-waves induce temporarily enhanced synoptic-scale lift. Severe and extremely severe thunderstorms are expected in the corridor from Serbia up to Belarus, where a a highly unstable warm sector is overlapped by a jet streak and thus strong vertical wind shear.

...area extending from W Russia up to W Balkan Peninsula...

Rich boundary layer's moisture on the eastern side of the quasi-stationary frontal boundary combined with ongoing evapotranspiration and regional confluent flow results in mixing ratios up to 13-14 g/kg. Given an overlap with vertical lapse rates exceeding 7 K/km , ML CAPE develops mostly up to 1500-2000 J/kg and locally even > 3000 J/kg. A jet streak covering warm sector on the eastern side of the frontal boundary results in a high DLS up to 25-30 m/s and locally provide a rare and an extreme overlap with high thermodynamic instability (DLS*WMAX values ~ 2000 m2/s2) in the corridor from W Ukraine and Serbia. Any thunderstorm developing in this environment may very quickly become an isolated supercell (SRH values from 250-400 m2/s2) with a potential of producing giant hail, damaging wind gusts and significant tornadoes given increasing LLS up to 10 m/s in the late afternoon hours. In addition, a strong airflow at 700 hPa and resulting 15-20 m/s MLS may potentially organise convection into bowing features where hurricane-force wind gusts are possible. Although there is not a source of significant large-scale lift, a long-lived squall line cannot be ruled out. In this scenario, thunderstorms would produce a widespread damage due to severe wind. In the northern area of lvl 2, although a thermodynamic instability is lower, a similar scenario with an extremely severe supercells is possible. Tornado threat seems to be high in this area due to a more favourable wind profile with extreme SRH values (400-600 m2/s2) and stronger LLS during the day (> 15 m/s). In addition, the area is under influence of the passing shortwave. One or two MCS in the late evening hours are possible. In addition to afromentioned phenomena, a moist vertical profile with PW exceeding 30-35mm may result in a torrential rain produced by supercell, multicell clusters and MCS in the evening hours. A highest threat is confined to a meso-low located over E Poland where a motion of the thunderstorms will be slower.

The main limiting factor for this forecast is whether a CI will take place. Most of the NWP model scenarios agree on the CI due to local convergence zones and orographic lift in the corridor from Serbia to W Ukraine. However, remaining cloud-cover from the nighttime thunderstorms may inhibit diurnal heating and thus do not allow thermals to pass CIN zone. Although we are aware of this risk, a level 3 is issued due to possibility of extremely severe thunderstorms in an extreme overlap of high DLS and very high CAPE. The more uncertain areas with environment favourable for severe storms include E Poland and E Belarus where dew point are lower and NWP models are more conservative in terms of CI over this areas. Therefore, we include these areas in the lvl 2 areas, but they cover 15% probability for lightning.