Valid: Thu 10 Aug 2017 13:00 to Thu 10 Aug 2017 17:00 UTC
Issued: Thu 10 Aug 2017 13:24
Recapitulation of last night:
A remarkable outbreak of extremely severe storms occurred in the warm air advection regime over central Europe. Numerous supercells with very large hail formed over N Italy and Austria in the evening, complemented by weaker but still organized storms over SE Germany (Bavaria) and the W Czech Republic (Bohemia), where cooler air prevented surface-based activity. After midnight the storms merged into a large bow echo which caused widespread wind damages from the E Czech Republic into central Poland before it decayed in the morning.
Current situation and probable development of the next few hours:
The storm system left a long outflow boundary roughly along a line Udine (IT) - Graz - Vienna (AT) - Olomouc (CZ) - Warsaw (PL), which separates cooler air and mostly W-erly winds to the west from hot air and mostly SE-erly winds to the east. This boundary has been further sharpened by differential heating (clouds and showers to the west versus sunshine to the east) in N Italy and Austria, while plenty of sunshine and a return of warm air advection with backing winds are constantly softening it over the Czech Republic and Poland.
The pronounced temperature and moisture gradients are also reflected by the CAPE values from the 12z soundings: 0 J/kg in Germany, around 800 J/kg in Prague, Vienna and Milano, around 1500 J/Kg in Prostejov and Budapest, and up to 4000 (!) J/Kg in Udine. Station data suggest that CAPE well in excess of 2000 J/kg is probably present in NE Italy, parts of Slovenia, Croatia, W Hungary and extreme east Austria (increasingly capped towards the SE). Compared to the model forecasts, the hot air with abundant CAPE seems to be shifted about 200 km to the east.
Two foci of convective activity emerge for the next few hours:
(1) Storms originating over the Italian Alps are just crossing central Austria. They will likely stay elevated as long as they are in Austria (2m temperatures of 22-24°C in their path), but in case they survive, they would have increasing chances to become surface-based in the quickly warming conditions over the Czech Republic. This scenario is what most of the NWP models would see.
(2) The second focus for initiation is the stationary outflow boundary with its extreme temperature gradient (>10K/100km) further southeast. First storms are forming at this boundary over E Austria and should intensify as they can follow this boundary.
In general, the overlap of robust CAPE and very strong vertical wind shear (including directional shear) suggests that any storm that becomes surface-based poses a risk of severe to extreme weather events - mainly hail in case of discrete storms, mainly wind in case a convective line forms!