Storm Forecast
Valid: Fri 15 May 2015 06:00 to Sat 16 May 2015 06:00 UTC
Issued: Thu 14 May 2015 21:18
Forecaster: TUSCHY

A level 2 was issued for parts of N-Italy mainly for large hail, severe wind gusts, tornadoes and excessive rain.

A level 2 was issued for extreme NE-Italy and far S-Austria mainly for excessive rain.

A level 1 was issued for the surroundings mainly for a similar risk but with lower probabilities in coverage.

SYNOPSIS

A glance on the maps reveals ongoing disturbed flow pattern over Europe with three dominant vortices. One longwave trough is situated over NE Europe. Circling low amplitude waves cause a marginal eastbound expansion, but overall no serious propagation of this trough is forecast.
Another intense vortex approaches NW Europe during the forecast but won't have any direct implications on the European weather, despite assisting in a bulging ridge downstream, which affects UK and France during the day.
Convection-wise the most interesting feature will be a southbound dropping deep and symmetric cold-core vortex over France and over the W-Mediterranean during the night . This cut-off is a potent one, also visible on IPV maps, which reveal a deep tropopause fold and model cross sections show influx of low-stratospheric air. Yesterday's sounding of Brest offered a nice insight into this vortex with unusually low tropopause levels and a cold mid/upper troposphere. During today's outlook, this feature will interact with the moist and unstable air mass over the N-CNTRL Mediterranean, which will spark numerous organized thunderstorms. This area will be the main focus for organized DMC.

At lower levels, this third vortex will induce downstream pressure fall over N-Italy and a broad and temporarily strengthening LL depression evolves. This feature will push a cold front rapidly east, while assisting in a sharpening warm front over the CNTRL Balkan States. During the overnight hours, a second LL depression evolves over the Adriatic Sea which slows down any frontal displacement.

Elsewhere numerous fronts and LL depressions affect the European weather, but limited access to an unstabel air mass keeps thunderstorm probabilities on a lower end side.

DISCUSSION

... N-/CNTRL-Italy, Croatia and Slovenia ...

A complex forecast is expected, but ingredients for an organized DMC event seem to converge over parts of N-/CNTRL-Italy.

Leisurely southbound moving depression over the W-Ligurian Sea pushes a cold front east, which comes ashore along the coasts of Liguria and Toscana during the forenoon hours. Until 15Z it will reach a line Milan-Bolonga, before slowing down and stalling somewhere to the east of this line. The reason for that is that falling MSLP spreads E/SE with onset of the depression over the Adriatic Sea. This enhances backing of the LL wind field over NE Italy. Attendant shallow WAA enhances surface pressure downstream of the cold front. In addition, steering depression over the Ligurian Sea moves south and weakens. All this should support a stalling boundary somewhere over N-CNTRL Italy during the afternoon hours (could be interesting for a training/excessive rainfall risk).

Given dynamic nature of this mid/upper tropopsheric vortex, a pronounced dry intrusion process is forecast with cross sections offering substantial drying down to 700 hPa. This dry slot overspreads N-Italy and the eastbound moving cold front. In general intense synoptic-scale lift overspreads the same area from W to E, with strongest signals/values over NW Italy. All this should lead to destabilization and by models anticipated ~800 J/kg MLCAPE seem reasonable. Even higher peaks are possible with moisture pooling along the immediate frontal boundary.

Kinematics are impressive with 20-30 m/s 0-6 km bulk shear and augmented LL shear, especially along the eastbound moving cold front itself. Initiation along the front will be early and convection spreads east during the day, crossing the level 2 area from W to E. Forecast soundings show a strongly sheared environment with modest CAPE. Weak capping and low LCLs are forecast, so expect an enhanced tornado risk especially with more discrete storm structures. This risk will also be augmented along the Ligurian coast, where 200 J/kg LLCAPE is forecast. Otherwise, severe wind gusts and large hail will be another hazard with well organized multicells/supercells.

As described, stalling boundary could serve as focus for training convection over N-CNTRL Italy and excessive rain is well possible with flooding problems. Corfidi vectors shrink substantially during the late afternoon and evening hours in this area. Some QPFs indicate 100 l/qm/12 h and more, so kept this threat also in a level 2. QPF plume also includes SE Switzerland, where convectively enhanced excessive rain is likely.

The southern tail of the cold front continues to sag south towards CNTRL-Italy with an ongoing risk of supercells with all kind of severe. We therefore expanded the level 2 far south.

Further east over W-Slovenia and Croatia, a similar setup exists, although initiation is a bit more uncertain due to divergent model signals how far east the dry slot will spread. Nevertheless, a high shear and moderate CAPE setup exists and any more discrete storm could become severe with large hail, severe wind gusts and an isolated tornado event. Due to ongoing uncertainties, we kept those regions in a level 1 but issued a level 2 for convectively induced excessive rain over extreme NE Italy/S-Austria.

We also added Corsica and Sardegna in a level 1, as the cold-core vortex moves atop during the night. A few waterspout events are possible.

...Balkan States ...

Northbound moving warm front stretches a moist and conditional unstable warm sector to its south. Rather warm low-tropospheric readings support strong capping and only weak lift is forecast (weak signals on IPV maps, which cross the warm sector from the SW to the E). Agree with model consensus, that cap should suppress CI with those disturbances. Can't rule out an isolated storm (maybe along mountains) which could pose a temporal large hail risk, but the main risk seems to evolve along the warm front itself, where moisture pooling deepens BL moisture and enhances risk for isolated/scattered CI. This includes Bulgaria, Romania and N-Serbia/Bosnia and Herzegovina. 400-800 J/kg MLCAPE and roughly 25 m/s DLS favor well organized multicells/supercells with large hail and severe wind gusts. Any tornado risk will be maximized along the warm front and especially over E-CNTRL Romania/Bulgaria, where better BL moisture exists. A level 1 should cover that risk due to uncertainties in thunderstorms' coverage. The main activity will occur until midnight, although an isolated storm thereafter can't be ruled out along the warm front.

... E-Europe...

Beneath the long-wave trough, widespread 200-500 J/kg SBCAPE evolve with diurnal diabatic heating. Shear is weak, so expect marginal hail and gusty winds with strongest storms. The activity diminishes after sunset.