Storm Forecast
Valid: Sun 17 Aug 2014 06:00 to Mon 18 Aug 2014 06:00 UTC
Issued: Sun 17 Aug 2014 05:38
Forecaster: VAN DER VELDE
A level 2 was issued for eastern Greece, eastern Bulgaria and Black Sea mainly for excessive convective rainfall and isolated large hail.
A level 1 was issued for parts of North Sea, Denmark, Norway, Netherlands, Germany and Finland mainly for isolated excessive precipitation and tornadoes.
SYNOPSIS
A large low pressure of under 990 hPa deep enters the North Sea. Unseasonably strong pressure gradients surround the low over the British Isles, Benelux, Germany, Denmark, Poland and Baltic Sea. The advection of cold upper air over relatively warm sea surfaces results in MLCAPE values of several hundred J/kg. The unstable air resides behind a mid-level cold front which slowly will move over northern Netherlands, Germany and Denmark. Over Russia the remnants of an upper trough still trigger widespread convection. A much more active trough passes over Greece into the Black Sea with strong QG lifting. The very moist low-level flow over the Black Sea is northeasterly, directly opposite to the winds at 500 hPa, resulting in strong vertical wind shear.
Over the Iberian Peninsula the Saharan Air Layer (dry, steep lapse rate mid level airmass) returns in response to advection from the south in advance of a large amplitude mid level trough coming from the Atlantic. The predicted GFS CAPE appears to remain mostly capped until Monday afternoon, but isolated showers from elevated parcels could occur.
DISCUSSION
...Greece, Bulgaria, Turkey, Black Sea...
Very rich moisture (13 g/kg) meets strong lifting (PV/QG) during the afternoon along the west coast of the Black Sea. With almost neutral lapse rate saturated profiles hundreds to more than 1000 J/kg MLCAPE is predicted, suggesting a large chance of excessive convective precipitation, helped by slow Corfidi storm motion vectors. The strong wind shear (25 m/s 0.6 km) and storm-relative helicity (150 mē/sē) imply a risk of supercells, which may also produce large hail. The chance of this should be better to the east where the lapse rates are much better.
...eastern North Sea, northern Baltic Sea...
Onshore advection of unstable airmass will occur in western Denmark and southwestern Finland, where at night a coastal convergence line may organize linear clusters of storms bringing locally excessive rain. This threat seems to stay offshore of Netherlands and Germany within this period (but affects it possibly after). The northeastern North Sea will have relatively weak pressure gradients and slow storm movement which can favour waterspouts. The passage of mid-level cold front over the southeastern North Sea and Denmark may be associated with slight instability and overlap with strong low-level wind shear (15 m/s 0-1 km), and lies under the jet stream increasing the deep layer shear as well (25 m/s 0-6 km). Together with very low LCLs the chance of mesocyclonic tornadoes therefore is increased, if there is indeed deep convection at this front.
