Storm Forecast
Valid: Mon 03 Jul 2017 06:00 to Tue 04 Jul 2017 06:00 UTC
Issued: Mon 03 Jul 2017 03:54
Forecaster: VAN DER VELDE

A level 3 was issued for eastern Bulgaria mainly for very large hail and severe convective wind gusts.

A level 2 was issued for Bulgaria and southern Romania mainly for excessive convective rainfall, and for large hail and severe wind gusts in Bulgaria and NE Greece.


SYNOPSIS

The southwestern half of Europe is under the influence of a stable high pressure system, while southern Scandinavia, and the eastern Balkan / Black Sea region are affected by very vigorous troughs that will activate deep convection.
The mid level trough coming from the northern Atlantic Ocean intensifies as it approaches Denmark by 18Z, then ploughs into the southern Baltic Sea region where on Tuesday 06Z it should start to move into Lithuania. CAPE is weak (50-200 J/kg)
A larger trough is moving very slowly across the Balkan, where Bulgaria and southern Romania will be under more than 24 hours of intense dynamic lifting of warm air, with MUCAPE between 1000-2000 J/kg until it is removed around 18Z. The subtropical jet stream blows from SW-NE over this zone

DISCUSSION

..eastern Balkan and Black Sea region...

A strong easterly flow over the Black Sea advects a moisture-rich air mass into Bulgaria and southern Romania (11-15 g/kg mixed over 0-1 km, GFS model), with currently Td of mostly 18-20 C, even 22 degree dewpoints in central Bulgaria. The flow should replenish the supply consumed by nocturnal thunderstorms, until it is pushed eastward by the cold front, which will come with intense thunderstorm activity. Because of opposing low and mid level flow, deep layer shear (0-6 km) will be over 30 m/s and 0-3 km storm-relative helicity ranges between 250 and 400 m2/s2 in the unstable airmass. Therefore, conditions look very favorable for a high concentration of severe storms, with supercells and squall lines bringing very large hail, severe wind gusts (Cordfidi>25 m/s, Delta Theta-e>20K S Bulgaria) and perhaps a tornado (0-1 km shear generally remains below 10 m/s, except along the BG-RO border). The combination of high PW, high column relative humidity, strong convergence and slow storm motion makes severe convective precipitation intensity and duration likely across northern Bulgaria and southern Romania with extensive flash flooding possible. As the system moves into the Black Sea during the evening, a strong cap prevents eastward propagation of the storms, but they likely will remain active along the Romanian and Moldovan coast, fed by strong low level moisture transport vectors.


...southern Scandinavia, southern Baltic Sea, northern Germany and northen Poland...

Low CAPE but strong dynamics and moderate shear in low and mid levels will be present at 18-21Z when the trough enters Denmark, southern Sweden and N Germany. A well-defined surface convergence line seems to be missing according to GFS output while the 3 km theta-e fields show an intensifying mid-level cold front. Shear is mostly friction-induced and reaches over 12 m/s 0-1 km over parts of Denmark, N Germany and N Poland, which could allow an isolated tornado, while gusts seem to remain under 20 m/s. A level 1 does not appear to be necessary.