Storm Forecast
Valid: Sun 30 Sep 2012 06:00 to Mon 01 Oct 2012 06:00 UTC
Issued: Sat 29 Sep 2012 21:25
Forecaster: TUSCHY

A level 2 was issued for parts of the Tyrrhenian Sea, SE Sardinia and parts of C-Italy mainly for excessive rainfall amounts, large to isolated very large hail, strong to severe wind gusts and an isolated tornado risk.

A level 1 surrounds the level 2 for a similar risk, but less intense/widespread.

A level 1 was issued for N-Italy mainly for locally heavy rain, marginal hail (isolated large hail) and strong wind gusts.

SYNOPSIS

The upper low, which caused a prolonged period of severe weather over the W-Mediterranean becomes included into the westerlies during the forecast period. Attendant surface front is placed over the Tyrrhenian Sea and continues to weaken. Nevertheless, another round of active thunderstorm development is forecast for parts of the C-Mediterranean. Strong westerlies with numerous well structured troughs still affect most parts of N-Europe with unsettled and cool conditions.

DISCUSSION

... Tyrrhenian Sea, parts of Italy and Sardinia ...

A synoptic-type boundary is present atop the Tyrrhenian Sea although models diverge, which type of boundary it might be. Most models show an occlusion or cold front, moving slowly SE-wards. However models like HIRLAM want the front to stall somewhere over the N-C Tyrrhenian Sea due to some weak LL cyclogenesis (either induced by the orography of Sardinia or due to some type of coupled jet configuration). This uncertainty however only influences how far SE the level areas will be drawn. Given the tendency of the convection from the overnight hours (29th to 30th September) to form way to the south to what models forecast, we expanded the level areas far to the SE with the level 2 even approaching Sicily.

The environmental conditions favor well organized multicells and supercells due to 20-25 m/s 3 and 6 km bulk shear and somewhat less enhanced LL shear (with augmented directional shear along the boundary or any evolving weak depression forecast). With dewpoints in the low to mid-twenties beneath the NE-ward advecting EML, 1500 to 2500 J/kg MLCAPE and even higher SBCAPE will likely evolve with peak magnitude probably evolving between Sardinia and Sicily.

Numerous more or less amplified short waves, embedded in this brisk SW-erly flow cross the area of interest during the daytime hours and result in a prolonged period of lift and convective initiation.

Most models try to blow up numerous convective clusters along that boundary during the forecast period. The exact placement of the boundary will be important for Sardinia, which may see a prolonged period of thunderstorm activity in case of a close boundary position. Especially during the more discrete stage, storms pose an all around severe threat with large hail (an isolated very large hail event possible), severe wind gusts and heavy to locally excessive rainfall amounts. HIRLAM and GFS both agree in quite favorable conditions for tornado development, which is certainly true for any storm, which rides along the boundary to the NE. Betimes, thunderstorms tend to cluster. Despite similar shear/CAPE values compared to yesterday (where only a level 1 was issued), a level 2 was added for this forecast period due to an existing focus for a more widespread severe risk (the frontal boundary). The level 2 was expanded far to the NE, as repeatedly onshore moving thunderstorm clusters may pose an excessive rainfall risk for parts of C-Italy (beside aforementioned other hazards).

The N-Adriatic Sea may see significant obstruction of CI due to thick anvil cirrus clouds from the DMC further upstream. Despite a favorable CAPE/shear overlap, only isolated DMC is forecast (mainly confined to the N/E-coast of the Adriatic Sea). A few strong to isolated severe storms are possible with heavy rain, isolated large hail/an isolated tornado and strong wind gusts. With a gradually cooling lower troposphere, conditions for isolated waterspout development improve over the far N-Adriatic Sea during the night. For most areas, the thunderstorm activity gradually diminishes after sunset. This is not true for Sicily, where the approaching frontal boundary probably helps to trigger a few thunderstorms, which likely acquire organization. Large hail, heavy rain, strong to severe wind gusts and an isolated tornado are possible with that activity.

... N-Italy ...

Scattered to widespread thunderstorm development occurs beneath the onshore moving cold-core low. Dewpoints in the mid-tens assist in moderate CAPE build-up and 15 m/s DLS affect at least parts of that CAPE plume, so a few strong to isolated severe thunderstorms are expected. Isolated large hail and strong wind gusts will be the main risk. Any sporadic tornado risk will be driven by locally enhanced diabatic heating/mesoscale boundaries and will be a nowcast subject. During the evening and overnight hours, large-scale lift increases and may result in the development of a broad but ill defined cluster of showers and thunderstorms over N/NE Italy. Any embedded thunderstorm activity however will be sub-severe (despite heavy rain). Some models show heavy rain spreading all the way to E-Austria, but confidence in this scenario is very low.

... Estonia, Latvia and Lithuania ...

A potent trough crosses the area of interest during the forecast period from west ot east. A plume of somewhat better BL moisture from the Norh Sea advects east and beneath the base of the upper trough. Hence, locally up to 500 J/kg SBCAPE evolve during the day with scattered showers and thunderstorms forecast. Strongest storms may bring marginal hail (an isolated large hail event not ruled out) and strong wind gusts. The thunderstorm risk decreases after sunset.