Storm Forecast
Valid: Sun 22 Jul 2012 12:00 to Mon 23 Jul 2012 06:00 UTC
Issued: Sun 22 Jul 2012 12:48
Forecaster: VAN DER VELDE

A level 1 was issued for central Italy and surrounding area mainly for large hail, tornadoes and excessive convective precipitation.

A level 1 was issued for northwestern Romania mainly for large hail.

SYNOPSIS

A large upper trough is in the process of cutting off and installing itself over Italy. High pressure from the Atlantic covers western and central Europe. A stationary frontal zone extends over the Balkan eastward into Ukraine. Moderate 1000-1500 J/kg MLCAPE or MUCAPE is in place south of the front and around the Italian upper low and is in most places overlapped by 10-20 m/s deep layer shear. A very strong northeasterly jet crosses the French/Italian region where more modest CAPE produces some convection in most model output.

DISCUSSION

...Italy, Croatia, Bosnia...

While moderate CAPE is present in models, a significant part of it is a result of shallow surface layers of rich moisture (13-16 g/kg) below very dry air in the layers above. In some areas with a capping inversion. While WRF models predict persistent storms and large precipitation sums along the east coast of Italy, it will probably be constrained to isolated places since upscale growth seems limited by CIN to the south and east where low level flow comes from, and also the dry air layers which reduce precipitation efficiency and enhance downdrafts. On the other hand, predicted storm motion along the eastern coast of Italy is very slow. At 10Z isolated storms appeared to be elevated over the Adriatic Sea. At 12Z several other storms are forming and one is backbuilding over eastern Italy. The vertical wind shear ranges between 10-20 m/s with SREH of 100-200 m2/s2, locally can increase to 300 m2/s2 in the evening, but not all of it will be effective since the convection would most likely become elevated again. The buoyancy and shear environment are conducive to very large hail formation. Isolated microburst (severe wind gusts) can be expected on the basis of the downdraft buoyancy (delta-thetae 20-25K), highest on the west side of Italy. Large vertical vorticity over the Tyrrhenean Sea can give rise to waterspouts or tornadoes (low level shear is lacking, but deep layer shear can support mesocyclones). Along the Adriatic this seems less likely due to the CIN. HiRLAM favors an early morning persistent convective line over the westside of southern Italy in the vorticity advection zone with significant precipitation, but most may fall Monday after 06Z.

...Romania, Ukraine...

In this mostly capped looking area, NW Romania has the most chances to realize storms in a 15-20 m/s 0-6 km shear and 150-250 m2/s2 0-3 km SREH environment supportive of supercells. Large hail is the primary threat.