Valid: Sun 31 Aug 2014 06:00 to Mon 01 Sep 2014 06:00 UTC
Issued: Sun 31 Aug 2014 07:03
Forecaster: VAN DER VELDE
A level 2 was issued for northern Italy, Slovenia and Croatia mainly for excessive convective rainfall, large hail, and isolated tornado chances.
A level 1 was issued for Austria, Czechia and southwestern Poland mainly for large hail.
A level 1 was issued for Turkey mainly for large hail and severe wind gusts.
A level 1 was issued for northern Netherlands and Germany, Denmark and adjacent sea mainly for spout-type tornadoes.
A level 1 was issued for northern parts of Morocco, Algeria and Tunesia mainly for severe wind gusts and large hail.
A level 1 was issued for eastern Belarus mainly for large hail.
An upper level low will move from the North Sea across Germany towards the northern Mediterranean where lee cyclogenesis will occur across N Italy. A strong surface cold front will make its way across Germany towards the Adriatic sea, where it will trigger thunderstorms with a major threat of excessive precipitation during evening and night. A weaker mid level cold front passes Spain from north to south.
Another upper low with associated slight instability resides over Belarus. Another mid level trough slowly passes over western Turkey.
...northern Italy, Slovenia and Croatia...
At 06Z 2m dewpoint values are already 2 degrees higher than predicted by GFS run of 00Z. The predicted mixed layer CAPE stays just under 1000 J/kg but can be expected to reach 1500-2000 J/kg as a result, also given the typical underestimation by GFS in similar situations in the past. The current situation is well below extreme in terms of vertical shear and storm-relative helicity: the strongest shear occurs under the jet running over northwestern Italy, with 15-25 m/s 0-6 km shear, before the cold front removes the warm airmass after 18Z. These values clearly support supercells (typically with large hail and possibly severe wind gusts). Northeastern Italy and the western Balkan is under 10-15 m/s 0-6 km shear magnitude which also can support some supercells, but large scale lifting will take effect after 18Z, transforming isolated storms into clusters and mesoscale convective systems. Upper divergence signals are very strong over northeastern Italy, southern Austria and Slovenia. With slow system motion, long-lived storm clusters are anticipated which can cause serious precipitation threats in this region. Other threats are large hail. Tornadoes are not helped by the predicted generally weak 0-1 km shear but with strong updrafts over the warrm sea, waterspouts can spin up.
With relatively dry low levels, high-based storms (1500-2500m) with more than 1000 J/kg MLCAPE can form in an environment with 10-15 m/s shear, supportive of strong, sometimes rotating updrafts, with large hail and severe wind gusts. The latter are helped by delta theta-e over 20 degrees. Similar conditions prevail over northern Africa.
...eastern Austria, Czechia, Poland...
Weak CAPE <500 J/kg combines with good lift and reasonable vertical shear (10 m/s 0-6 km). Orography can often locally enhance it so rotating thunderstorm updrafts may form. An isolated large hail event is possible.
GFS indicates a well-defined convergence zone and mid level lifting with weak CAPE <500 J/kg combining with 15 m/s 0-6 km shear, supportive of an isolated supercell with large hail.
...Denmark and surroundings...
Good low-level buoyancy is indicated by 00Z soundings from De Bilt, Norderney and Greifswald. Very slow cell motion and weak shear support spin up of funnel clouds/tornadoes. The main mid level vorticity maximum passes over The Netherlands.