Storm Forecast
Valid: Sat 14 Aug 2010 06:00 to Sun 15 Aug 2010 06:00 UTC
Issued: Sat 14 Aug 2010 05:53
Forecaster: VAN DER VELDE

A level 2 was issued for N Italy, S Austria and Slovenia mainly for excessive convective rain, as well as severe wind gusts and tornadoes.

A level 2 was issued for W Poland and level 1 for parts of Lithuania, Czechia, Slovakia, Hungary, Croatia and Serbia mainly for large hail and severe convective wind gusts.

A level 1 was issued for England and NW France for water/landspouts.

A level 1 was issued for northern Scandinavia mainly for severe convective wind gusts and enhanced tornado potential.

SYNOPSIS

With a large upper low over western Europe, centered over the English Channel, a warm humid airmass is transported northward from the Mediterranean to central Europe. During this period it will cause rainy and thundery conditions around the surface low over northern Italy and the south side of the Alps. Very unstable air is in place in central Europe. Weaker unstable air is present under the upper trough over France and UK, and over Scandinavia.

DISCUSSION

...N Italy, Slovenia...

Increasingly unstable air is transported into the region. First affected will be Italy's west coast with large QG forcing where storms can stick around for a while and produce flash floods. Moderate deep layer shear and storm-relative helicity (up to 250 m²/s²) will favor storm rotation while also low level shear (0-1 and 0-3 km) is high enough (>10 m/s) to mention a chance of tornadoes. On the south side flow is stronger, LCL higher and severe wind gusts >25 m/s are likely with storms. During the night the focus shifts to the northeast of Italy and Slovenia with large precipiation sums likely and the other mentioned threats also present to some extent.

...W Poland, central Europe...

MLCAPE on the order of 1000-1800 J/kg (see Szeged 00Z sounding) combined with deep shear of 10-15 m/s and SREH around 150 m²/s² can yield intense multicells and supercells with large hail. Steep lapse rates, LCL around 2000 m and dry midlevels further stimulate local severe wind gusts. Triggering by low-level convergence zones is most likely in western Poland.

...N Scandinavia...

Slightly unstable low-LCL airmass embedded in quite strong flow and low level shear >12 m/s (in GFS) can produce thunderstorms with chance of severe wind gusts, while a tornado is not excluded.

...England and NW France...

The upper trough and weak surface winds provide convergence zones with high vertical vorticity which can be spun up by convective updrafts in the humid airmass (e.g. 00Z Herstmonceux, Trappes, Camborne, De Bilt soundings) while not being disturbed by vertical shear or mixing/entrainment of the superadiabatic surface layer. Land/waterspouts and funnel clouds are therefore likely.