Storm Forecast
Valid: Mon 10 Aug 2015 06:00 to Tue 11 Aug 2015 06:00 UTC
Issued: Mon 10 Aug 2015 03:03
Forecaster: VAN DER VELDE
A level 2 was issued for the coasts of NW Italy, W Corsica and Sardinia mainly for tornadoes.
A level 1 was issued for the central Mediterranean area mainly for tornadoes, excessive convective precipitation and large hail.
A level 1 was issued for SE Spain mainly for large hail.
A level 1 was issued for N Turkey mainly for large hail.
A level 1 was issued for W Germany mainly for large hail.
SYNOPSIS
A weak high pressure system over the Baltic Sea was connected to the Azores high, but is being cut off by an Atlantic low moving over the British Isles to Norway and a small low over Germany. A band with warm unstable airmass curls around that low and should produce thunderstorms mainly over western Germany, and also Poland and Ukraine/Russia, where a shortwave upper trough with surface occlusion should activate convection.
The most active system is the upper cut-off low dwelling between southern France and the Tyrrhenean Sea. The cold air bubble is circled by a mid level jet and vorticity advection which creates deep layer shear and destabilization along the Italian west coast. Thermal lows are also found over western Iberia and western Turkey.
DISCUSSION
...NW Italy, Corsica, Sardinia, Tyrrhenian Sea...
Models forecast over 1000 J/kg MLCAPE and over 3000 J/kg SBCAPE. Low-level convergence of the easterly winds along the coastline of NW Italy and lifting at the north and east sides of the PV anomaly will likely favor quite persistent storms which can profit from moderate shear (SREH 100-200 mē/sē, DLS 10 m/s). This may locally produce a multi or supercell storm with chances of large hail. Along the Italy-France coast some 10 m/s 0-1 km shear may enhance the risk of tornadoes, which is already elevated for the waterspout type throughout the level 1. Where the core of the upper low passes, storm motion should decrease below only 3 m/s, which favours both the spin-up of the strong background vertical vorticity into waterspouts as well as local excessive precipitation. The level 2 marks the area with higher confidence in these phenomena than across the larger level 1. Over Sardinia, lifting of a stronger flow around the surface low over orography can trigger storms that may hang around and also produce excessive rain and possibly large hail as the southern edge of the low offers more vertical shear. Enhanced SREH develops also east of Sardinia.
...W Germany...
Cyclogenesis occurs as warm air in SE Germany forms a gradient with cool air over France and Benelux. There may be 1000 J/kg MLCAPE with LCL of 1000-2000m colocated with 100-200 mē/sē 0-3 km SREH or 10-15 m/s deep layer shear. In some WRF models the SREH is much enhanced near Luxemburg, which however seems to reside on the cool, CAPE-starved side of the front. However, large hail is quite likely and also isolated severe wind gusts can occur as evaporational cooling potential is significant with delta-theta-e of 18-22 K in GFS model residing over SW Germany.
...SE Spain...
High-based storms may occur in an area of 20 m/s 0-6 km shear and Mediterranean moisture in low levels leading to 2000 J/kg MLCAPE. These storms can produce large hail.
...N Turkey...
Moderate LCL height and low-to-mid level shear increase chances of large hail with weakly capped CAPE.
