Storm Forecast

Storm Forecast

Valid: Mon 18 May 2015 06:00 to Tue 19 May 2015 06:00 UTC

Issued: Mon 18 May 2015 05:04

Forecaster: VAN DER VELDE

A level 1 was issued for SE UK for a marginal tornado chance.

A level 1 was issued for Belarus/Ukraine/Russia for a marginal tornado chance.

A level 1 was issued for NE Switzerland, S Germany and NW Austria mainly for large hail.

A level 1 was issued for SE Italy and S Balkan mainly for large hail.

A level 1 was issued for N Turkey mainly for large hail.

A level 1 was issued for N Morocco mainly for large hail.



SYNOPSIS



Large low pressure systems are found over northern UK, W Russia and Sicily. Smaller cut-off lows near Sicily and west of Morocco. A warm sector advances over the UK soon followed by the cold front. By Tue 06Z the occlusion lies over S Sweden, the warm front over Poland and the cold front over Germany to SW France. The Russian low sends cool unstable airmass into Belarus and SW Russia, while sending warm unstable air westward over NW Russia. Warm air advection with steep lapse rates also occurs over S Balkan/Italy and Turkey.



DISCUSSION



...southeastern UK...



This area was marked for only slightly elevated tornado chances as models simulate marginal CAPE at the occlusion point moving through southern UK during 09-12Z. ECMWF simulated a small SREH spot and overall 0-1 km shear is above 12 m/s and 0-6 km shear over 25 m/s. The cold front is destabilized by differential advection of theta-e with mid levels cooling earlier than low levels. The development of convection is far from certain, however.



...Belarus-Ukraine-Russia...



Another area with just marginal tornado chances where low-level shear over 10 m/s and weak CAPE occur under the jetstream.

In NW Russia low level shear magnitude and directional shear is also strong, but there appears no shear above that.



...NE Switzerland, S Germany, NW Austria...



In an extension of the surface trough belonging to the British low, weak CAPE near the Alps combines with 10-15 m/s shear, with 200 m²/s² over NW Austria. There is little forcing apart from some local convergence lines.



...Italy-Balkan...



Moderate to strong DLS with maximum of 20 m/s over W Greece and Albania occur in a region with 500-1000 J/kg MLCAPE (MUCAPE over sea). The split level cold front circles the northeast of the upper low, collocated with lift from the PV gradient. Over Albania, 300 m²/s² SREH is simulated by GFS. Supercells with large hail are possible, also in a region stretching to Bulgaria, where lift is weaker. LCL heights are predicted to be moderate around 1500m, in support of the hail threat.



...N Turkey...



Large hail from multi/supercell storms is the main threat. There is not much lift except for orography. The LCL heights are 2000m or higher with CAPE 500-1000 J/kg and moderate 0-6 km shear (10-15 m/s).



...N Morocco...



The circumstances are comparable to those over Turkey, except here a mid level vorticity advection creates some lifting. Main trigger should be the upslope western flow.





