Storm Forecast
Valid: Fri 13 Apr 2018 06:00 to Sat 14 Apr 2018 06:00 UTC
Issued: Thu 12 Apr 2018 15:39
Forecaster: PISTOTNIK
(Due to technical problems, the DISCUSSION part got lost in the original outlook. We apologize for the inconvenience.)
DISCUSSION
... N Germany across W and S Poland into E-central and SE Europe ...
A tongue of very warm air is advected from SE Europe towards the North Sea. Sufficiently steep lapse rates and unseasonably rich low-level moisture (2m dewpoints in the 10 to 13C range) allow CAPE between 500 and 1000 J/kg in response to daytime heating.
On large scales, warm air advection will be completed on Friday, and synoptic lift will be dictated by the afore-mentioned short-wave trough. On the mesoscale, however, the complex and disturbed flow suggests that smaller areas with transient but pretty strong lift (both from warm air advection and/or vorticity advection) may unfold on short notice, which makes this forecast particularly challenging. Current model guidance (as of Thu 06 UTC) places the main trough axis from central Germany to Serbia on Friday morning and the cold frontal rain band with residual embedded, elevated and weak convection slightly ahead of it, probably near a line Hamburg - Budapest. It will further weaken in the morning while slowly shifting northward. New storms will fire up from noon onwards along its outflow boundaries, or else where the combination of lift and daytime heating manages to erode and break the cap.
Similar to the previous days, the rear flank of the CAPE body may favorably overlap with a wind maximum, probably creating 0-3 (0-1) km shear on the order of 15 (10) m/s. Expected storm mode is therefore a mixture of multicells and interspersed, if transient, supercells. Large hail is the main hazard in the initial stages, added by a flash flood risk and maybe isolated severe wind gusts later on if one or two large storm clusters evolve. A tornado is not ruled out if a storm can follow a pre-existing convergence line, e.g. an outflow boundary.
High storm coverage may bring quite widespread severe weather into W Poland and NE Germany, where a level 2 is issued. Further west near the tip of the warm air tongue, cloudiness, convective debris and limited daytime heating reduce the severe weather potential, but a level 1 is expanded westward to account for a still remaining risk of heavy rain with an elevated storm cluster. Convection will move NW-ward and will weaken as soon as it moves over cooler surface air, in particular over the Baltic Sea and coastal areas, and in general in the evening and overnight.
E-central and SE Europe are only grazed by the short-wave trough. Convective initiation will largely be tied to the mountains in the afternoon. CAPE on the order of 500 J/kg and slightly increasing shear across deeper layers suggest a risk of isolated large hail. Supported by weak synoptic lift, scattered storms may be sustained over flat terrain and persist into the night.
... SE Germany, Czech Republic, N Austria ...
A SW-erly flow resides behind the short-wave trough and the cold front. Foehn winds will still advect steep lapse rates off the Alps, and a few hundred J/kg of CAPE may form under enhanced vertical wind shear. In conjunction with another strong mid-level vorticity maximum, a surge of cooler W winds will rush eastward in the afternoon and evening. Limited area models, in particular UM and AROME, predict the formation of a bowing convective line in this process that could sweep across N Austria and parts of the Czech Republic with severe wind gusts. Despite some doubts if low-level moisture will suffice to sustain convection, a level 1 covers this scenario.
... S Turkey ...
A few hundred to 1000 J/kg of CAPE may build over parts of S Turkey, where moist sea breezes and upslope flows cut underneath steep lapse rates inland. With NW-erly mid-level flow, vertical wind shear may be enhanced just enough (10 m/s across the lowest 3 km) to allow some storm organization. Scattered afternoon storms along the sea breeze front / dryline at the southern rim of the Anatolian plateau may be accompanied by large hail and possibly isolated downbursts.
