Valid: Fri 11 Aug 2017 16:00 to Fri 11 Aug 2017 22:00 UTC
Issued: Fri 11 Aug 2017 16:06
A zone of convergent surface winds has established over western Poland near a line from Koszalin to Wroclaw. east of this line, the humid boundary layer has warmed with 2m temperatures of 30°C and higher. The dew point temperature is near 20°C. Combined with the steep lapse rates observed in the 12 UTC Prostejov sounding, this yields a very unstable air-mass with CAPE values upto 3000 J/kg.
Convection has initated near the S and SW border of the MD area. Aided by low-level frontogenesis over the MD area, storm coverage is expected to expand northward. Deep-layer shear is strong, just like shear in the 0-3 layer, which NWP guidance predicts to strengthen to near 20 m/s.
The combination of strong CAPE and strong 0-3 km shear and the convergence creates favorable preconditions for a northward progressing bow-echo to develop during the coming hours. In addition, initially a few supercells with large to very large hail will be possible too. A tornado or two are not ruled out either, since 10-15 m/s of 0-1 km shear should be supportive of strong low-level rotation. The anticipated rapid evolution to bow-echo mode reduces the risk somewhat.
To the east of the convergence line, more scattered to isolated storm development is possible as well, although it is difficult to know how far east the cap will be weak enough to sustain convection. Very large hail, severe wind gusts and possibly a tornado will be possible there as well.