Storm Forecast
Valid: Mon 18 Jul 2016 06:00 to Tue 19 Jul 2016 06:00 UTC
Issued: Mon 18 Jul 2016 03:20
Forecaster: VAN DER VELDE

Levels 2 and 3 were issued for N Ukraine, E Belarus and W Russia mainly for severe convective wind gusts and large hail, and to a lesser extent tornadoes.

A level 1 was issued for E Belarus mainly for excessive convective rain chances.

A level 1 was issued for NE France, S Germany and Slovenia/Austria border mainly for slight chances of large hail.

SYNOPSIS

A mid-level ridge / anticyclone over France is flanked by a broad upper trough from Scandinavia to the Black Sea. Warm, humid air converges into an almost stationary frontal zone especially between central Ukraine and the Belarus/Russia border with MLCAPE values of 1500-2500 J/kg.

DISCUSSION

...Ukraine, E. Belarus, W Russia...

At 00Z, surface mixing ratios of 13-15 g/kg (Td 17-20°C) are observed over N Ukraine, SE Belarus and adjacent W Russia. The 00Z Gomel sounding indicates very steep lapse rates from low levels to 550 hPa and 1000 J/kg CAPE, which will increase during the day with diurnal heating. GFS MLCAPE using a 1000 m deep layer exceeds 2000 J/kg with ICAPE over 4 MJ/m² thanks to the deep moist boundary layer. The LCL is at 2000 m over Russia decreased to below 1000 m over Belarus southward into Ukraine in the cold air advection region. Storms on the western boundary will be slow-moving storms with chances of excessive convective precipitation and flash floods. Storms in the warm air advection regime marked by the level 2 and 3 area will develop in about 15 m/s 0-6 km shear vector magnitude and over 250 m²/s² of 0-3 km storm-relative helicity, which will turn a number of storms into supercells which can produce very large hail and severe wind gusts, and clustering of storms will generate squall lines with significant severe wind gust potential. The environment contains Delta-Theta-E values of 20-30 Kelvin thanks to the warm and humid low levels (high theta-e) but cold and dry mid levels (low theta-e). The evaporational cooling of large precipitation masses can accelerate downdrafts and cause microburst wind phenomena.
Low level shear is adequate to moderate (7-15 m/s 0-1 km shear) and will favor bow echo formation. It also allows for local tornadogenesis. Weak capping over a deep layer for much of the day suggests early storm development. This may diminish available CAPE, but the strong moist/warm air advection from the southeast may compensate sufficiently.

...S Germany, NE France, Austria-Slovenia border...

With roughly 10 m/s deep layer shear and 500-1000 J/kg MLCAPE (250 J/kg Slovenia), some large hail may be produced by some storms. The lift conditions for initiation are very limited by the high pressure nearby, so will likely depend on orographic enhancement.

...C and W France...

Dry air with steep lapse rates from the south meet with moist air in this area, where 1000 J/kg MLCAPE can result. There is also more than 10 m/s DLS. However, storm initiation will be very difficult by the lack of low level lift close to the N France anticyclonic center, with divergent winds, and mid level subsidence just east of the ridge axis.