Storm Forecast
Valid: Fri 01 Sep 2017 06:00 to Sat 02 Sep 2017 06:00 UTC
Issued: Thu 31 Aug 2017 21:03
Forecaster: TUSCHY

A level 2 was issued for N-Italy, W-Slovenia and far S-Austria mainly for large hail, severe wind gusts and excessive rain. For N-Italy, an isolated tornado risk exists.

A level 2 was issued for parts of W-CNTRL Italy mainly for excessive rain, large/very large hail, a few severe wind gusts and an isolated tornado.

A level 1 surrounds both level 2 areas mainly for large hail, severe wind gusts and excessive rain.

A level 1 was issued for SE-Spain mainly for large hail, severe wind gusts and isolated excessive rain.

A level 1 was issued for parts of the border between the Ukraine and Russia mainly for excessive rain.

SYNOPSIS

A longwave trough extends from France to Norway/Sweden and drifts east during the forecast. It keeps its positive tilt throughout the forecast. This trough builds far south into the subtropical Atlantic. Downstream WAA assists in rising mid-level heights over Morocco/Algeria, which results in an intensifying mid-/high-level wind field over the W-Mediterranean. Further east, a strong cut-off low sits north of the Black Sea and impacts the E-Ukraine. Its vortex builds down to the surface during the day and a nearly vertically stacked cyclonic vortex evolves through the low to mid troposphere.
Surface pressure distribution is complex over the CNTRL-Mediterranean with numerous transient and weak LL vortices expected. The most dominant one will be a broad and zonally elongated LL depression over N-Italy, fostered by the evolving jet streak over the CNTRL Mediterranean and increasing upper divergence beneath its left exit region. This depression remains in place for most of the forecast period.

An extensive frontal zone extends from far E-Finland/W-Russia to Poland, the Alps and the CNTRL-Mediterranean. This wavy front becomes heavily distored by the Alps, Corsica, Sardinia and the aforementioned LL vortex over N-Italy. A quasi-stationary front over far N-Italy to CNTRL-Austria will be the result with a more mobile cold front extending south to the Tyrrhenian Sea and far N-Africa. This front separates a moist and unstable prefrontal airmass from a much drier and cooler postfrontal one.

DISCUSSION

... Italy, S-Austria, Slovenia, Croatia to N-Serbia ...

The northern part of Italy resides in a favorable environment for continued CI all day long. Ongoing convection from the previous night strengthens during the day and a cluster of organized convection probably shifts east towards Slovenia/S-Austria before noon along a leisurely eastbound moving wind shift line. Degree of instability depends on the morning activity further south (discussed in a separate paragraph), but 400-800 J/kg MLCAPE and 25 m/s 0-6 km DLS indicate a good bet for organized multicells with large hail and a few severe wind gust incidents. A localized flash flood risk accompanies that activity, especially between Genoa and Milan, where a strong confluent onshore flow regime may support in temporal training with excessive rainfall amounts.

Beyond noon, a backed surface flow ahead of the N-Italian depression assists in ongoing moisture advection towards NW-Italy. Eastward fanning W-Alpine EML plume atop this BL moisture features a confined belt of 800-1200 J/kg MLCAPE during the afternoon over far N-Italy. 25 m/s DLS and backed BL flow due to ageostrophic deflection by the LL depression point to a favorable setup of a few afternoon/evening supercells. Forecast soundings show enough BL moisture and LL shear to add an isolated tornado threat with this activity next to large/isolated very large hail and severe wind gusts. This activity weakens during the late evening hours with loss of CAPE but elevated convection continues well into the night. We expanded the level 2 all the way to W-Slovenia, S-Austria to account for the influx of moist air from the N-Adriatic Sea and hence a similar CAPE/shear space. It is interesting that most convective allowing models (CAMs) backed off the risk of supercells a bit. The main problem could be abundant synoptic-scale lift and repeated CI, which lessens the risk of discrete storms a bit. However ingredients remain in place for organized DMC and any more discrete storm could become severe.

The risk of a few organized thunderstorms extends all the way to E-Croatia, N-Bosnia and Herzegovina and N-Serbia. Lower CAPE and shear (800 J/kg and 15 m/s) still assist in a few large hail and excessive rainfall events. A cluster may evolve during the afternoon hours, spreading east with a regionalized enhanced severe wind gust threat. We kept this part of the forecast in a level 1 for now.

Elevated overnight convection affects a broad area south of the Alps and hence we expanded the lightning area far E.

For CNTRL- and S-Italy, pinpointing the area of highest risk becomes more difficult. Latest PWAT analysis of CIMMS indicates rather dry mid-level air atop a very moist BL air mass (with MIMIC-TPWS exceeding 40 mm over the W-Mediterranean). Ongoing WAA regime advects an EML over the moist marine layer and pushes the capped CAPE in the strong region (1500 - 2000 J/kg MUCAPE). Not much synoptic forcing affects the area over CNTRL-Italy and the Tyrrhenian Sea during the day with a substantial increase of subsidence beyond 00 UTC. Hence the main focus for CI will be the SE-ward drifting cold front and its interaction with the coast of W-CNTRL Italy (frictionally induced convergence). A sharp low-/mid-tropospheric temperature and cap gradient accompanies that front although this gradient weakens betimes.

Confluence in the lee of Corsica in combination with the front itself probably spark a cluster between Corsica and Rome before noon. DLS remains modest in the 15 m/s range, but this activity can tap into the very moist marine air mass over the Tyrrhenian Sea with strong CAPE (offsetting modest shear). In addition, a sharp convergence zone aligned normal to the coast may support a V-shaped cluster with training convection. Progressive nature of that cluster probably limits the risk of extreme rainfall amounts, but still excessive rain with a tornado and large (isolated very large) hail risk exists. Strong to severe wind gust are possible as well but more isolated due to a weaker 0-3 km flow. This cluster sags south betimes and with loss of synoptic-scale lift and a dissolving front, the cluster should tend to weaken betimes despite high CAPE and modest shear. We highlighted those uncertainties with a broad level 1 and 15% lightning area far south. Any storm, which stays alive could produce large hail, excessive rain and an isolated tornado event (along the coast).
Much less activity occurs towards the Adriatic Sea with lower CAPE and cloud debris from convection further to the west but an overall increase along the E-coast during the night is anticipated.

... France and Benelux into W-Germany ...

Scattered diurnally driven thunderstorms with peak coverage and intensity during the afternoon are forecast beneath the upper trough. Weak shear but 300-600 J/kg weakly capped CAPE support a few strong pulsating storms with isolated hail and gusty winds. An isolated short lived cold-core funnel/tornado event can't be ruled out either. This activity diminishes after sunset.

... SE-Spain ...

QPF remains divergent for that area but with weak onshore flow and onshore spreading marine air mass, 400-800 J/kg MLCAPE evolve during the afternoon hours with diabatic heating. CI is forecast over the mountains and 25 m/s DLS results in organized updrafts with large hail, a few severe wind gusts and heavy rain. This activity may spread a bit towards the coast before weakening.

... N-Ukraine ...

Wrap-around moisture affects the area as a plume of unstable air circles the depression. The main risk will be heavy rain with an augmented flash flood risk. Elevated nature of convection precludes other risks. This activity shifts east during the night and out of our forecast domain..