Storm Forecast
Valid: Tue 05 Oct 2010 06:00 to Wed 06 Oct 2010 06:00 UTC
Issued: Mon 04 Oct 2010 23:40
Forecaster: KOROSEC

A level 1 was issed for NE Italy, N-CNTRL Adriatic sea, western Slovenia and parts of western Balkans mainly for excessive rainfalls.

A level 1 was issued for south-central Italy, Adriatic sea and SW Balkans mainly for excessive rainfalls, strong wind gusts, a couple of tornadoes or waterspouts and lesser extent for large hail.

SYNOPSIS

A large and deep trough persists over NW Atlantic sea with a slowly eastwards progressing cold front extending SW-wards from the accompanied surface low NW of British Isles. This front will bring high rainfall accumulations into NW parts of Iberia, but will mostly be non-convective precipitation.

Another features of interest will be two upper cut-off lows. One moving across Italy from the NW Italy into the southern Adriatic sea and another one shifting eastwards from the northern parts of Black sea/southern Ukraine into western Russia. A well defined cold front accompanied by the northern Mediterranean upper low moves eastwards during the day and will be a focus for widespread convective activity.

A large upper ridge with extensive high surface pressure area persists over NNE Europe and western Russia.

DISCUSSION

... parts of Italy and western Dynaric Alps including western Slovenia, NE Italy and western Croatia ...

Ahead of an approaching low/front, a very warm and moist airmass will be present. Monday's afternoon soundings and SFC observations revealed impressive mixing ratio values and near SFC dewpoints in range between 18 and 20°C around north- central Adriatic sea. Moderate SSW-erly flow in the front side of the upper low will maintain an environment for strong orographic precipitation from NE Italy and along the western side of the Dynaric Alps towards western Balkans. There, high rainfall accumulations are expected. During the afternoon, an upper low moves further SE towards central Italy and steepening lapse rates will result in marginal instability. However, the main convective activity will be mostly aligned to the front itself, but storms may also form slightly ahead of it during the day. In moderate 15-20m/s deep layer shear, organized storms will bring threat for intense rainfall and locally strong winds.

Rather slow moving system/upper low and persisting mid-level flow could also favor training effect of the convective cells, where slow moving clusters are possible. Those could locally bring high amounts of rain and threat for flash floods.

... south-central Italy, Adriatic sea and SW Balkans ...

The coverage of the storms will be lower of southern portions of the level 1 given the higher capping, but instability should be more robust there with warmer SSTs. Activity should continue overnight on Wednesday there as well. Moderate shear will be enough supportive for organized severe storms, where some of them could bring strong winds and large hail threat, as well as a couple of tornadoes. This threat will be higher over Thyrrenian and Adriatic eastern coast where LL shear seems to be the most favorable for rotating updrafts.

However, a large area has been placed into the threat level 1, although the western parts of southern Italy and parts of (coastal) western Balkans will see the highest threat for the most convective activity given the good overlaping of instability and shear, so a high-end level 1 threat seems warranted there.