Storm Forecast
Valid: Fri 05 Aug 2016 06:00 to Sat 06 Aug 2016 06:00 UTC
Issued: Thu 04 Aug 2016 21:51
Forecaster: TUSCHY

A level 2 was issued for NE Italy and far S-Austria mainly for excessive rain ... for S-Austria also for large hail, severe wind gusts and an isolated tornado event.

A level 2 was issued for parts of CNTRL Italy mainly for large / very large hail, severe wind gusts and excessive rain.

A level 1 was issued for part of Poland and the Baltic States mainly for an isolated tornado, large hail and severe wind gust event.

A level 1 was issued for the E-Czech Republic and far W-Slovakia for an isolated large hail event.

A level 1 was issued for parts of Norway/Sweden mainly for heavy rain and isolated large hail.

A level 1 was issued for SW/W-Turkey mainly for large hail and severe wind gusts.

SYNOPSIS

A strong upper trough shifts east and its tip starts to drip off over the Alps and far N-Italy. Pronounced ridge builds in from the SW towards the Iberian Peninsula and keeps this part of the forecast area hot and stable. An elongated but weak upper low is situated over SE Europe and supports unsettled conditions with showers and thunderstorms.

At lower levels, a wavy frontal boundary extends from the CNTRL Mediterranean to Poland, far W-Russia and all the way to NE-Finland. This boundary will be the main focus for organized thunderstorm activity.

DISCUSSION

... Parts of Italy, parts of Austria and Slovenia ...

Southeastbound moving upper trough crosses a moist BL air mass south of the Alps with dewpoints in the lower 20s, Moderate mid-level lapse rates atop offer 500-1000 J/kg MLCAPE - regionally enhanced with values in excess of 1 kJ/kg.
Shear weakens beneath the upper low but increases to 15-25 m/s along its fringes - especially over CNTRL-Italy as mid-level speed max approaches from the NW.

Ongoing cluster over the W-Alps builds east during the day, having an impact on N-Italy and S-Austria. Forecast soundings show rather unidirectional shear profiles with deep moisture and strong advection towards the Alps, so main risk will be excessive rain. This risk may be enhanced along the S-Alps due to upslope flow. Due to the progressive nature of that trough, this cluster tends to move east, affecting Slovenia during the overnight hours while weakening. Far S-Austria may see CI ahead of that cluster in a favorable environment for organized convection. All kind of severe is possible.

Forecast soundings show a strongly sheared environment over CNTRL Italy with veered profiles but with a rather pronounced cap. Both, cooling of the cap's layer and approaching forcing should support scattered thunderstorms. This activity grows upscale into numerous well organized multicells and a few supercells. A rather dry and well mixed subcloud layer indicates a chance for severe wind gusts next to large / very large hail with mature supercells. Despite the warm LL with ongoing but weakening cap, an isolated tornado event is possible.
This activity may be overtaken by the cluster of thunderstorms from N-Italy, which tends to build south during the late afternoon/evening hours. Models diverge with that scenario (how far south the N-Italian cluster will move). Anyhow, excessive rain and strong/isolated severe wind gusts would be the main hazard.

... Poland and the Baltic States ...

A weak disturbance is forecast to evolve along the frontal boundary, which bisects Poland in the middle from N to S. Model uncertainties remain large with that feature with some models indicating a depression while others showing a pronounced wave. This has a major impact on how far N/NW the warm front will be advected over N-Poland, Lithuania and Latvia and how strong the LL shear will be. Most models however indicate favorable conditions for organized convection along the NW-ward moving warm front.

CI will occur early - probably before noon - with thunderstorms increasing in strength during the day. Modest DLS with 15 m/s supports organized multicells with large hail and isolated severe wind gusts. During the evening hours, lowering LCLs and strengthening 0-1 km shear with enhanced LL backing support an isolated tornado risk with ongoing convection. Tornado probabilities will probably be maximized along and just to the south of the warm front. The final tornado risk however depends on the strength of the LL depression which has to be monitored closely!
Betimes, clustering thunderstorms will pose an heavy rainfall risk - most likely for the Baltic States.

Further south, an isolated large hail risk exists over far E-Czech Republic and W-Slovakia with initiating storms in 15 m/s DLS.

... Norway, Sweden and Finland ...

Scattered thunderstorms (diurnal driven) beneath cold mid-levels develop before noon and weaken during the overnight hours. Slow storm motions and 400-800 J/kg SBCAPE should result in strong / isolated severe thunderstorms with marginal / isolated large hail and heavy / isolated excessive rain. Towards Finland, DLS increases to 15 m/s, which could increase the hail risk a bit, but warmer mid-levels could at least partially offset that enhanced risk and therefore no level was issued.
For Norway/Sweden, a level 1 is issued, where highest thunderstorm coverage is forecast. The activity weakens after sunset but isolated non-severe thunderstorms can persist all night long.

... SW-Turkey ...

Scattered to widespread thunderstorm activity occurs beneath the upper low. Yesterday's 12 UTC soundings also sampled that environment with steep mid-level lapse rates and inverted-V profiles along the S-coast of Turkey. Coastal regions see surface dewpoint well into the 20s, which offers 1-2 kJ/kg MLCAPE - decreasing to the north. Despite marginal shear, expected CAPE distribution (fat mid-level profiles) and LCLs in excess of 2 km point to an augmented chance for severe downbursts and large hail - even very large hail can't be ruled out. Slow moving storms will also pose an heavy rainfall risk. Main reasoning for not issuing a level 2 was missing shear and scattered nature of expected thunderstorms. However, an extreme hail event is possible and therefore this level 1 will be an high-end one.