Storm Forecast
Valid: Fri 04 Apr 2014 10:00 to Sat 05 Apr 2014 06:00 UTC
Issued: Fri 04 Apr 2014 08:33
Forecaster: PUCIK

A level 1 was issued for Central Italy towards the western Balkans coast mainly for marginally large hail and severe wind gusts.

SYNOPSIS

A rather complex synoptic-scale scenario unfolds over Europe - satellite loops reveal pronounced cyclonic vortex at mid to upper troposphere with its center close to the Balearic Isles this morning. Vortex will continue moving eastwards during the day with strong southerly to southwesterly flow ahead of it. At the same time, a ridge will be observed to the north, covering much of France and Germany, stretching to southwestern Scandinavia. To the east, a deep trough filled with arctic airmass will translate across Russia towards southeast.

Closer to the surface, the Mediterranean low has induced a WAA ahead a to the north of it, covering Southeastern Italy, the Adriatics region and much of Central Europe, with a weak frontal boundary running from France towards Italy. On the forward flank of a high pressure system over Scandinavia, another outbreak of arctic airmass will spill over the Russian plains during the forecast period.

DISCUSSION

... Eastern France, Central Europe ...

00 UTC soundings (e.g. Idar, Kuemmersbruck and Prague) show steep lapse rates at lower troposphere (up to 650 hPa) while remaining rather dry at these levels and 2 m dewpoints generally below 10 deg C. The only region with slightly more low level moisture will be Eastern France, especially after the overnight rain. Based on the soundings it is apparent that with lack of moisture advection and with the lack of mid to upper tropospheric forcing, abundant surface heating will be necessary for at least marginal CAPE build-up. However, this might be inhibited locally by a very thick, dust containing, cirrus and altostratus plume that now covers much of Eastern France, Western Germany and runs through Northern Austria towards Hungary. Due to this negative factor, only low probability line of lightning is introduced over the whole area. DMC will be tied either to the local, topohrapically induced circulations, or to the weak frontal boundary lingerning over France. Marginal CAPE values along with weak vertical wind shear will likely limit the severe weather threat to minimum. Marginally large hail and some downbursts might occur with stronger, chaotically organised, multicells, but overall threat seems to be too low to warrant a Level 1.

... Central Italy to the Adriatics region ...

A plume of steep mid-level lapse rates has advected form Sahara towards the region of interest ahead of the cyclonic vortex in 20 m/s+ southwesterly flow at 500 hPa. Surface observations reveal dewpoints of 10 to 15 °C in the southerly low level flow in front of the slowly advancing frontal boundary. CAPE on the orders of few hundreds J/kg is simulated per GFS and ECMWF, spreading eastwards during the day. An overlap will exist between the unstable area and the belt of moderate to strong mid tropospheric flow, yielding more than 20 m/s of DLS. Such values would already be sufficient for some well organised multicells or supercells, capable of marginally large hail and severe wind gusts. Threat will shift from Italy in the noon to afternoon hours towards the Adriatic Sea and the Balkans coastline in the overnight hours. A level 1 seems to be warranted.