Storm Forecast
Valid: Mon 19 Sep 2011 06:00 to Tue 20 Sep 2011 06:00 UTC
Issued: Sun 18 Sep 2011 22:54
Forecaster: TUSCHY

A level 2 was issued for the coast of the NE Adriatic Sea, Slovenia and extreme S-Austria mainly for excessive rainfall.

A level 2 was issued for parts of Sicily and S-Italy mainly for large to very large hail, severe wind gusts and tornadoes.

A level 1 was issued for Croatia and W-Hungary mainly for an isolated tornado/severe wind gust and large hail event.

A level 1 was issued for the Ionian Sea and SE-Adriatic Sea mainly for excessive rainfall and severe wind gusts.

A level 1 was issued for most parts of the central Mediterranean mainly for a few tornado/waterspout events and marginal/isolated large hail.


SYNOPSIS

A piece of the powerful upper trough drips off over the central Mediterranean and slowly moves to the SE. At lower levels, a complex LL depression evolves south of the Alps. This configuration is supportive for very heavy rainfall along parts of the SE-Alps and a broad area where organized thunderstorms can be expected.
Otherwise no severe risk is forecast over Europe.

DISCUSSION

... Sicily, S-Italy and the Ionian Sea ...

As a part of the upper trough constricts over the central Mediterranean, its slows down somewhat. In contrast, the cold front keeps going towards the SW, crosses Sicily during the morning hours and affects the Ionian Sea after noon. Due to the progressive nature of this boundary, it gradually moves into an environment with warmer temperatures at mid-levels and also away from the best forcing. Also, increasingly dry air at mid-levels will be advected ahead the SE-ward moving cold front (sampled by latest N-African soundings). All that indicates that the cold front passage will probably be only marked by isolated to scattered thunderstorms with the highest coverage in the Tyrrhenian Sea and S-Italy.

As the cold front advances southeastwards towards the Ionian Sea, conditions become supportive for numerous large clusters to form as the inflow of a very moist SW-erly flow sets-up. Repeated thunderstorm development is expected along the SE-ward moving boundary, which slows down a bit, so a prolonged period of activity is forecast over the Ionian Sea all the way into the S-Adriatic Sea.

For all areas, kinematics will be favorable for organized thunderstorms as a 25-30 m/s mid-level jet moves in from the NW. 0-6 km bulk shear along the boundary varies between 15-25 m/s and similar values are present in the lowest 3 km. The wind below 850 hPa is a bit weak with no clear jet streak seen in model data whereas LCL heights remain AOA 1 km. However, we upgraded to a level 2 mainly due to the intense jets at 700/500 hPa, assisting in potential discrete supercells with large/very large hail, severe wind gusts and tornadoes. Over the Ionian Sea, the more dominant risk gradually becomes the excessive rainfall risk due to rapid clustering and a very moist inflow from the SW. This risk extends all the way to the S-Adriatic Sea and affects the coastal areas from Albania to W-Greece during the night .

... N/E-Adriatic Sea ...

A potentially dangerous flash flood/flood event is unfolding/ongoing over the NE-Adriatic Sea northwards. At the surface, a broad LL depression is in progress over NE-Italy and continues to slowly move to the E/NE. However, models like WRF also hint on a re-development of another LL vortex over N-CNTRL Italy/Adriatic Sea, which drifts to the SE later the day. In that case, a prolonged period of onshore flow exists for the E-coast of the Adriatic Sea, with the QPF maximum only leisurely shifting eastwards. Despite ongoing uncertainties how the LL depression really acts, a combination of an increasing diffluent flow aloft, rich BL moisture and orographic lift all point to excessive rainfall over a broad area. Some MUCAPE is present, so embedded thunderstorms are forecast, locally increasing the RR significantly. A level 2 was issued for the region with highest QPF amounts, surrounded by a broad level 1 area.

Another hazard will be a tornado risk over Croatia and W-Hungary between 12-18 Z. Some low-end instability builds up in an environment with 12-15 m/s LL shear and augmented SRH-1. Nex to that risk, a few severe wind gusts and marginal hail are also expected. After sunset, storms become elevated with a decreasing severe risk.

... Ligurian Sea, Tyrrhenian Sea, Italy, Adriatic Sea ...

During the day, the cold core of the upper low approaches and widespread ~500 J/kg MLCAPE are forecast. DLS remains weak, but augmented LL-CAPE atop 22-26°C SSTs may assist in a tornado/waterspout risk. At the current track of the upper low, we're also concerned about an heavy rainfall risk over NE-CNTRL Italy (e.g. between San Marino and Venice) and therefore we issued a large-scale level 1 to cover al those various risks.