Storm Forecast
Valid: Sun 15 Sep 2013 06:00 to Mon 16 Sep 2013 06:00 UTC
Issued: Sun 15 Sep 2013 10:20
Forecaster: PUCIK

A level 2 was issued for Sicily and Southwestern Italy mainly for excessive precipitation, tornadoes, severe wind gusts and large hail.

A level 1 was issued for Sardegna, Corsica and Western Italy mainly for excessive precipitation and to the lesser degree for tornadoes and severe wind gusts.

A level 1 was issued for North Tunisia mainly for large hail and severe wind gusts.

SYNOPSIS

Upon a brief glance on the satellite imagery loops, one might easily notice three main features on the synoptic scale. The first one being a large and deep cyclonic vortex residing over the Northern Atlantic, moving steadily southeastwards. More to the south, a prominent short-wave trough has just crossed Southern France and is heading for Italy. And finally, towards the east, decaying cyclonic vortex is centered over Southern Ukraine. Generally speaking, no major overlap of sufficient low level moisture and steep lapse rates is forecast over much of Europe - therefore, continental part is not going to see any DMC activity during this forecast period. On the other hand, western Mediterranean, under the influence of the short-wave trough, will likely see widespread activity especially during the overnight hours.

DISCUSSION

... Sardegna, Corsica towards Italy and N Tunisia ...

Current observations show very moist airmass especially around the Tunisian coastline with dewpoints well above 20 °C with moisture decreasing towards the north. 00 UTC Trappani sounding shows PW values at 36 mm, which should even improve towards the evening as southerly flow advects moister air northwards. On the other hand, north of Sicily, forecast wind vectors do not reveal any significant southerly low-level jet. With west-southwesterly flow aloft, a plume of steep mid-level lapse rates might spread from Tunisia towards Sicily, but not much further. Here, high values of CAPE are forecast, easily exceeding 2000 J/kg of MUCAPE with PW values over 40 mm. Lower values are forecast towards the north.

Impressive forcing and very favorable kinematic environment is forecast, especially during the night hours. With 500 hPa flow exceeding 30 and 300 hPa flow exceeding 40 m/s, unseasonably strong deep layer shear values are simulated, over 30 m/s in the 0-6 km layer, especially over the southern half of the region. Around the coastal areas and most importantly over Sicily, high values of SREH are well possible between 00 and 06 UTC.

Once the short-wave trough crosses a tongue of enhanced latent instability, explosive growth of convection is expected, possibly quickly yielding one or more MCS building south towards the more unstable airmass. Possibly the most significant MCS will form around Sicily by 03 UTC. Although models diverge on the exact paths of the individual storm systems, southern part of Italy and Sicily might be up to a potent situation for severe storms with full facet of threats. While excessive precipitation should be the most significant one, environment supporting supercellular DMC might result also in tornadoes, severe wind gusts and even large hail. The best conditions will exist over the western and southern coastline of Sicily between 00 and 06 UTC time frame. Towards the north, the primary threat should remain in the form excessive precipitation with a likelihood of other threats decreasing.