Storm Forecast
Valid: Wed 22 May 2013 06:00 to Thu 23 May 2013 06:00 UTC
Issued: Wed 22 May 2013 09:10
Forecaster: PUCIK

A level 2 was issued for Southern to Central Italy mainly for large hail, severe wind gusts, tornadoes and excessive precipitation.

A level 1 was issued for the Northern Ionian Sea, the Adriatic Sea and Western Balkans coastline mainly for excessive precipitation and tornadoes.

A level 2 was issued for Serbia, Southwestern Romania and Northwestern Bulgaria mainly for (very) large hail and severe wind gusts.

A level 1 was issued for Croatia, Macedonia, Bulgaria, Romania, Moldova, Ukraine and Russia mainly for large hail and severe wind gusts.

A level 1 was issued for Poland mainly for lage hail.

SYNOPSIS

A high-amplitude flow pattern persists over Europe with large, complex trough stagnating over most of Western and Central Europe. Well defined cyclonic vortex will push from Atlantic towards the Northern Sea while over the Mediterranean, a powerful short-wave trough will translate towards Italy and Balkans area. Another short-wave will stretch from Poland towards Slovakia, Hungary and Romania with northeastward movement. The main focus for (severe) thunderstorm development should be in the area of Central Mediterranean and Southeastern Europe, under the broad and strong southwesterly flow disturbed by several "vorticity maxima". This flow is advecting steeper lapse rates towards northeast, contributing to the buildup of low to moderate degree of latent instability. Another area with scattered thunderstorm activity will be eastern half of Central Europe with cold mid-level temperatures, on the forward flank of the trough.

DISCUSSION

... Southern to Central Italy ...

A perhaps unseasonably well developed low pressure system is forecast to affect the area, with surface pressure plunging down to 990 hPa as the low approaches southern Italy by the evening hours. Deepening is forecast thanks to the strong PVA and WAA ahead of the approaching sharp short-wave trough, that should cross Italy in the evening to night hours. At its forward flank, 20 m/s southwesterly flow around 500 hPa is forecast over the area. With southerly to easterly low level flow ahead of the surface low, very favourable vertical wind shear profiles are forecast in the afternoon hours. DLS should reach 20-25 m/s while veering with height in the lower levels should lead to moderate SREH values (locally above 250 m2/s2). With moderate CAPE values around 1000 J/kg thanks to the steep mid-level lapse rates, environment should be conducive to supercells and well organised multicells capable of (very) large hail and severe wind gusts. Tornadoes might occur as well with supercells, especially later on, as low level shear increases and LCLs decrease. Storm coverage could be somehow limited at this afternoon stage due to the enhanced CIN values.

Towards the evening, strong low level convergence and moist low level jet towards the area will likely contribute to the rapid development of a large MCS, capable of producing large swath of very heavy precipitation. A solid Level 2 seems to be warranted for this area.

... Northern Ionian, Adriatic Sea, Western Balkans coastline ...

By the evening and night hours, as the trough approaches area, strong low level jet over the sea should rapidly advect moisture and "feed" large MCS moving from Italy towards northeast. Albeit excessive precipitation should be the most likely threat in this area, strong DLS, LLS and abundant SREH point to the possibility of a tornado or severe wind gusts, especially in case that boundary layer rooted isolated supercells manage to develop. In such case, isolated large hail would not be ruled out either.

... Serbia, Southwestern Romania and Northwestern Bulgaria and surrounding areas ...

Moderate to strong southwesterly flow should contribute to the strong DLS values, between 15 - 25 m/s, values quite conducive to well-organised convection, including supercells. In the Level 2 area, enhanced SREH will exist as the low level backs due to the falling surface pressure. CAPE values will strongly depend on the availability of low level moisture and that could be enhanced along the local convergence zones, which should act as a foci for scattered thunderstorm development. From the current observations, dew points around 15 °C in the area, which should be easily enough for 500 - 1000 J/kg of MLCAPE. With these facts in mind, large hail and severe wind gusts would be likely threats with any developing supercells, not ruling out an isolated report of very large hail thanks to the steep mid-level lapse rates. Level 2 is drawn for the area with the highest threat of isolated extremely severe event.

... Romania, Moldova, Ukraine and further into Russia ...

A favourable overlap of moderate mid-level flow with marginal to moderate CAPE in the area (highest CAPE values perhaps over eastern Romania with the best low-level moisture) could sustain a few well organised multi- or even supercells capable of large hail and severe wind gusts. Passage of the short-wave troughs along with local convergence zones should be the primary factors contributing to the thunderstorm initiation.

... Poland ...

Even though weak shear regime is forecast for the area, very cold mid-level temperatures contribute to the rather steep lapse rates and low freezing levels. Passage of the short-wave trough could help storm organise a bit and increase the coverage. Especially in the areas with mesoscale enhancement of either low level moisture or vertical wind shear there will be a possibility of few strong multicells capable of marginally large hail.