Storm Forecast
Valid: Mon 18 Apr 2016 06:00 to Tue 19 Apr 2016 06:00 UTC
Issued: Mon 18 Apr 2016 06:16
Forecaster: TASZAREK

A level 1 was issued for SE Poland, E Slovakia, NE Hungary, N Romania, N Moldovia, and SW Ukraine mainly for large hail, severe wind gusts and in lesser extent for tornadoes.

A level 1 was issued for NE Italy, SW Austria and W Slovenia mainly for the large hail and severe wind gusts.

SYNOPSIS

A jet streak bordering cold polar air mass on N and NW Europe with warm and moist air from Mediterranean, SE and E Europe stretches from Morocco trough Italy, Romania up to Ukraine. Within this border, a highest chances for thunderstorm fall on Slovakia, Hungary, N Romania and W Ukraine where an overlap of boundary layer's moisture and vertical temperature lapse rates, along with an available large scale lift will be most favorable. Other places where DMC is possible concern N Italy, where an eastern flank of the weakening mid-level trough will provide a signal for CI in a high shear low CAPE environment. Thunderstorms are also possible over the W Iberian Peninsula where convection may take place in a warm sector of the eastwardly moving trough. Small chances for thunderstorms concern also Scandinavia where a trough shifting from the W Norwegian coast, thanks to an advection of very steep lapse rates and QG-lift, provides conditions for the low-topped convection in a marginal CAPE environment.

DISCUSSION

...SE Poland, E Slovakia, NE Hungary, N Romania, N Moldovia, and SW Ukraine...

High shear / low CAPE setup occurs on this area. Weakening cap in the environment of ~ 10 g/kg mixing ratios along with > 7 K/km lapse rates results in a CAPE up to 800-1000 J/kg, overlapped on its entire area by a strong core of the jet streak with DLS up to 35 m/s and strongly veering vertical wind profile (3km SRH ~ 200-300 m2/s2, curved hodographs). Although a CI may be an issue, a strong westerly signal of PVA should support DMC in the afternoon hours on the Polish/Slovakian/Ukrainian/Hungarian/Romanian border and transform cells quickly into mesocyclones capable of producing large hail up to 3-4cm and severe wind gusts. Due to a persistent large-scale lift and available instability, it cannot be ruled out that cells will start to merge and form a MCS in the late afternoon hours that will propagate to CNTRL Ukraine and N Moldovia. The limiting factor may be not very high CAPE (600-800 J/kg) that usually needs to be better to support the persistence of MCS. Due to an enhanced LLS (~ 8-10 m/s) and low-level helicity (up to 100-150 m2/s2) a tornado event within isolated supercells over Ukraine and N Romania between 16-19 UTC cannot be ruled out. Threat for severe weather phenomena should drop after 18-20 UTC when the convection will transform from the surface-based into elevated. However, lightning activity may stay until nighttime hours over CNTRL Ukraine thanks to the possible MCS.

...N Italy, SW Austria, W Slovenia...

Mixing ratios of 8-9 g/kg overlap with > 7 K/km lapse rates and strong DLS of 40 m/s. Although the CAPE seems to be not impressive (~ 300-500 J/kg) and the hodographs over Italy seems to be not favorably curved for mesocyclones, still some cells thanks to good tilting of the updraft may transform into mesocyclones and produce large hail up to 3-4cm and some local severe wind gusts. Vertical wind profile seems to be more favorable for supercells over SW Austria and W Slovenia where a 3km SRH up to 250-300 m2/s2 will provide a higher chances for a rotation in developing updrafts. However, a limiting factor over these areas may be a CAPE, that according to NWP model predictions will be rather marginal. All level 1 threat area thanks to the approaching mid-level trough receives a strong QG-lift in the afternoon hours, therefore CI is likely, but the question if severe thunderstorms will occur concerns mainly how much thermodynamic instability will be available to sustain the development of updrafts in a such strong wind shear environment without being dispelled.