Mesoscale Discussion
Valid: Mon 22 Sep 2014 19:00 to Tue 23 Sep 2014 00:00 UTC
Issued: Mon 22 Sep 2014 19:44
Forecaster: PISTOTNIK

A very interesting and complex setup has evolved in Italy with the passage of a cold front and a strong short-wave trough aloft. With easterly (Bora) winds over the Northern Adriatic Sea, the remaining moisture is squeezed against the Italian east coast. Ferrara and Cervia still recorded temperature/dewpoint values of 22/20°C and 21/21°C at 18 UTC, recently.
In contrast, the cold front has crossed the Alps in form of North Foehn. The strong subsidence resulted in similar temperatures but significantly lower dewpoints barely above 0°C in large parts of the Po Valley. Brescia recorded a dewpoint of even -5°C at 18 UTC, which is 24K lower than in the late morning!
The boundary between these two air masses has hence attained the charateristics of a dryline. While no CAPE can be realized any more in the dry downslope flow (refer to the 12 UTC Milano sounding), robust CAPE in excess of 1000 J/kg and possibly even up to 2000 J/kg is still available in the moist air ahead of it (refer to the 12 UTC Udine sounding).

Thunderstorm activity is in full swing in the vicinity of this boundary, where a large cluster has formed in Friuli (NE Italy). With strong forcing ahead of the upper-level trough and deep-layer shear boosted to 30 m/s, embedded supercells are likely and can bring large hail and severe wind gusts. Besides, flash floods are possible as long as this storm cluster is still rather stationary.

In the next couple of hours, strong pressure rise from the North will likely result in a surge of cool Northerly winds which races southward along the Italian east coast, trapped by the Appennin mountains. Due to the unclear and probably variable depth of the remaining near-surface moisture, convection may struggle to survive and may easily be outrun by the wind shift line. However, those storms which do persist may still organize well and bring further severe weather. Besides, wind gusts may exceed 25 m/s even in the absence of convection with the passage of the wind shift line.