Storm Forecast
Valid: Sat 01 Jul 2017 06:00 to Sun 02 Jul 2017 06:00 UTC
Issued: Sat 01 Jul 2017 07:28
Forecaster: TASZAREK

A level 2 was issued for CNTRL Ukraine and parts of Russia mainly for large to very large hail, severe wind gusts, excessive precipitation, and in lesser extent for the possibility of significant tornado event.

A level 1 was issued for a part of SE Europe mainly for large hail, severe wind gusts and in lesser extent for excessive precipitation.

A level 1 was issued for Alpine area mainly for marginally large hail and in lesser extent for excessive precipitation.

SYNOPSIS

Jet stream stretches from N Africa through Italy, Balkan Peninsula, Ukraine and SW Russia. A weakening and eastwardly moving long-wave at 500 hPa covers SW and CNTRL Europe. Thunderstorms with a limited potential of producing severe weather are expected on the eastern flank of this system in the belt from W Mediterranean up to Poland. Storms with a more pronounced severe weather potential are expected to fire-up over CNTRL Ukraine along the sharp thermal boundary with a moderate instability and jet streak above.

DISCUSSION

...Ukraine, SW Russia, N Balkans...

Elevated mixed layer with steep lapse rates (> 7 K/km) from N Africa overspreads SE Europe. A sharp horizontal thermal boundary extends from Serbia up to SW Russia. Warm section is heavily capped and only within strong diurnal heating, moisture pooling and large-scale lift is capable of producing a thunderstorm. Such ingredients are available over CNTRL Ukraine where due to strong evapotranspiration and moisture convergence, BL mixing ratio jumps up to 11-12 g/kg providing 1500-2000 J/kg CAPE. Lower moisture (9-10 g/kg) is available on the southern edge of the convergence zone over CNTRL Romania and S Serbia, but is still sufficient to provide 600-800 J/kg ML CAPE. This whole boundary is covered with a strong airflow in both low and mid levels. This results in a quite impressive shear values exceeding 30 m/s in 0-6km layer and 15 m/s in 0-3km layer. Low-level jet with 10-15 m/s 0-1km shear values is also observed over CNTRL Ukraine. Given such kinematic environment and instability over Ukraine, any developing cell will have a possibility to evolve into supercell capable of producing large to very large hail (very steep lapse rates good shear), severe wind (low-level jet good storm organization) and torrential rain (> 40mm PW). If discreet supercells will evolve (that given strong CIN is likely), kinematic environment with S/SE LL inflow will be supportive for tornadoes. LLS exceeding 12.5 m/s along with strong veering wind profile (400-600 m2/s2 0-3km SRH) and decent instability may even result in a significant tornado event. However, there are 2 big limiting factors for this scenario. First is relatively high cloud-base that may inhibit tornadogenesis. Second, the area of favourable overlap of ingredients is very shallow and it is unknown if any cells will move over this zone, but if they do, a possibility for a strong tornado should be involved, especially in the evening hours when cloud bases will drop. Some of the NWP scenarios indicate a line of a discreet supercells clustering in the late afternoon hours into large MCS. In such system a potential for severe and damaging wind gusts along with excessive precipitation will be enhanced, especially given strong airflow at 700 hPa and Serly winds ahead that may provide a good organization of the system. In the current scenario isolated supercells are expected to develop over CNTRL Ukraine in the afternoon hours (S part of lvl. 2), and moving NEwardly should cluster into MCS and enter Russia in the evening (N part of lvl. 2).

This forecast has however a few key limiting factors that my result in the lack of CI and thus any storm activity:
- QG-lift is not very strong, passing shortwave trough is very shallow and PVA is moving mainly parallel to the boundary, mostly on its cool side.
- CIN is very strong and given weak QG-lift storms may not overcome cap
- NWP models overestimate LL moisture that decreases instability and lower probability for SB CI

Convective initiation is also very uncertain over Serbia and Romania given weak CAPE and even weaker large-scale lift. Some NWP model scenario indicate however initiation along the convergence zone in the late afternoon and nighttime hours. Although instability will be weak and storms will be most likely elevated, an enhanced 30 m/s DLS may support few severe wind or large hail incidents. Level 1 over W Ukraine, parts of Poland, Slovakia, Hungary and NW Romania was issued mainly for storms that will evolve on the cool side of the boundary and within the support of strong shear (but weak instability) may have a potential to produce few severe wind and large hail events.

...Alpine area...

Although this area is located in the cool sector of the trough and expected instability with BL moisture is rather low (ML CAPE 400-600 J/kg), enhanced lapse rates and 15 m/s DLS may result in a few marginal large hail events (especially over areas with higher elevation). Mesoscale NWP models are quite positive with a widespread CI over this area and thus lvl. 1 is issued. Cells should remain mostly isolated and cluster locally into multicells. Given their weak motion a local excessive rain cannot be ruled out as well. However, the general severe weather potential for this area seems to be low.