Storm Forecast
Valid: Sun 14 Sep 2014 06:00 to Mon 15 Sep 2014 06:00 UTC
Issued: Sun 14 Sep 2014 08:05
Forecaster: VAN DER VELDE
A level 2 was issued for northeastern Spain mainly for large hail.
A level 1 was issued for northern Spain for large hail and severe wind gusts.
A level 1 was issued ofr parts of Hungary, Croatia, Serbia and Bosnia-Herzegovina mainly for excessive convective rainfall and large hail.
A level 1 was issued for northwestern Turkey mainly for large hail and spout-type tornadoes.
SYNOPSIS
An extensive blocking high pressure system is situated between Ireland and Russia, centered over Scandinavia. An Atlantic low lies in front of Portugal, with a thermal low over the Iberian Peninsula. Three upper cut-off lows are found near Denmark, Croatia and Russia.
DISCUSSION
...Iberian Peninsula...
Very moist Mediterranean air (13-14 g/kg) accumulates along the east coast. With diurnal heating the thermal low activates and moist air gets advected inland mainly through the Ebro Valley. In mid and upper levels a cold front passes from the southwest, crossing northeastern Spain during afternoon and evening, helping destabilize the mid levels where soundings indicated some inversions. A SW-NE oriented PV gradient is advected parallel to the flow which can add upward destabilizing motions. MLCAPE will range from 1000 to 1500, maybe locally up to 2000 J/kg. Vertical wind shear is enhanced to double the values present last week: around 20 m/s 0-6 km vectors and 30 m/s BL-8 km vectors. These should support supercells as the main mode, but low-level end of the hodographs in GFS do not show much curvature, in fact 0-1 km shear is weak and streamwise vorticity (storm-relative helicity) remains below 100 mē/sē. Some split-cell behaviour will likely be observed. The storms will easily produce large to very large hail (5 cm or larger), helped by broad updrafts with cloud bases at 1500-2500m. Isolated severe wind gusts also are likely.
As CAPE should decrease toward the north of Spain, but low-level hodographs improve, the level 2 spans also this area where supercellular storms can occur. To the east, parts of Catalonia will remain capped. Northern Aragon may see an MCS developing during evening into the night with continued chances of large hail and severe wind gusts.
...Hungary, northern Balkan...
Along the east side of the PV anomaly associated with the cold pool, 500-1000 J/kg MUCAPE and lifting coincide, with 10-15 m/s deep layer shear. This may be sufficient for some supercells, but rather than large hail the low cloud bases and slow storm motion (possibly training over Hungary) poses a larger threat of locally excessive rain events.
...Turkey...
10-15 m/s deep layer shear and 500 J/kg CAPE per GFS model could organize into some multi and supercells, with some chance of large hail. Over the Black Sea, slow storm motion and strong low-level buoyancy contribute to waterspout chances, which will converge near the coast in northeasterly winds.
...northern Germany...
Modest CAPE will be available mainly under the cold mid level bubble, whereas 15 m/s DLS, 10 m/s 0-1 km shear and SREH over 100 mē/sē surround this - with a possible overlapping region. An isolated tornado might then be possible. Nowcasting should focus on the CAPE-shear overlap.
