Storm Forecast
Valid: Thu 05 Aug 2010 06:00 to Fri 06 Aug 2010 06:00 UTC
Issued: Wed 04 Aug 2010 22:14
Forecaster: TUSCHY

A level 2 was issued for parts of Romania, Bulgaria and NW-Turkey mainly for large to very large hail (significant hail possible), severe wind gusts and locally excessive rainfall amounts.

A level 1 surrounds the level 2 and expands all the way to extreme W-Russia mainly for large hail (an isolated significant hail event possible), strong to severe wind gusts and an isolated tornado event.

A level 1 was issued for parts of Italy and the N-Balkan States mainly for large hail, strong to severe wind gusts and an isolated tornado event.

SYNOPSIS

A potent upper trough is placed over central Europe with another, weaker one over SE Europe, resulting in scattered to widespread shower/thunderstorm activity over most parts of Europe. Stable and very hot conditions continue over the Iberian Peninsula and most parts of W-Russia.

DISCUSSION

... Bulgaria, Romania and NW-Turkey ...

All eyes turned to the western Black Sea, where very moist air resides. SSTs range between 27 to near 30°C (latest RMRI SST data) with widespread BL dewpoints at or above 20°C over parts of E Romania/Bulgaria. There is no reason, why this air mass won't continue to move onshore during the forecast, so BL air mass will be very moist with moist layer thickness increasing during the day. Placed along the outer fringe of an extensive EML, CAPE values soar well above 1000 J/kg MLCAPE (SBCAPE/MUCAPE probably exceeding 2000 J/kg), maximized along a N-S aligned boundary somewhere over central Romania/Bulgaria. Shear is modest with 20 m/s DLS, mainly speed shear with a limited directional component. However, directional shear may increase, if surface depression over NW Turkey grows stronger than currently indicated (GFS 12Z increased the strength of this depression).

In respect of forcing mechanisms, a mixture of strong diurnal heating, initiation along any convergent boundary and some PVA at mid-levels due to a weakening upper trough all indicate scattered thunderstorm initiation over the highlighted area, probably capped in the easternmost parts with strong LL-WAA regime. All this combined hints on an augmented severe hail risk with any thunderstorm, which evolves. Significant hail may occur with hail diameter exceeding 5 cm. Despite more discrete/isolated nature of storm activity, a level 2 was issued to cover the hail threat. Slow storm motion and very moist inflow may also cause excessive rainfall amounts, next to strong, wet downbursts (with thick dry layer atop very moist BL). Thunderstorms continue well into the night, with a gradual decrease in intensity and coverage.

...W-Ukraine to extreme W-Russia...

A quasi-stationary moisture boundary will be the focus for scattered thunderstorm initiation. Numerous short-waves ride atop that boundary from SW to NE and serve as foci for regionally enhanced thunderstorm activity. Moisture pooling along that boundary keeps dewpoints in the upper tens beneath modest mid-level lapse rates, so MLCAPE at or above 1000 J/kg will be present. Shear increases during the passage of each wave, which increases the large hail risk over a fairly large area. An isolated significant hail event can't be ruled out mainly over extreme W-Russia. We went with a level 1, as coverage of storms remains more isolated in nature. Strong to severe wind gusts and locally high rainfall amounts accompany stronger storms. A tornado or two may occur over exrteme W-Russia, where LL wind field indicates best shear component.

... N-CNTRL Italy and N-Balkan States...

A potent upper trough over the W-Alps further amplifies and acquires a slight negative tilt throughout the forecast period. Mid-level CAA atop warm and moist BL creates a broad, unstable sector. BL air mass features only modest surface dewpoints in the mid-tens range, so MLCAPE probably peaks already at 500 - 1000 J/kg. DLS will be around 15-20 m/s, so large hail remains the main hazard with scattered to widespread thunderstorm activity. Risk spreads eastwards during the day and also affect the N-Balkan States during the night with deacreasing instability and hence diminishing severe risk. Right now, a coarse level 1 was issued, as no real swath of significant severe events can be seen. However, an isolated very large hail event is possible over central Italy during peak time heating. Low LCLs, modest LL-CAPE release and probably various outflow boundaries serve as foci for an isolated tornado event. Also, the waterspout risk increases during the evening/night hours over the Ligurian/N-Adriatic Sea, as CAA at lower levels overspreads the warm sea with weak background flow. Storms keep going all night long with a gradual decreasing trend from west to east.

The rest of the highlighted thunderstorm areas indicate a bad overlap of either weak shear or weak instability or both, so nothing organized is expected. Nevertheless, Germany has 15m/s with roughly 500 J/kg SBCAPE and SW Sweden up to 800 J/kg SBCAPE and 15-20 m/s DLS, so an isolated better organized multicell is still possible with large hail and strong wind gusts, but confidence in a concentrated severe risk corridor is too low for any level area. LL-air mass features low T-Td spread and some LL CAPE build-up so a few funnel/isolated tornado reports can't be ruled out.

... NE-Spain to the Balearic Island...

The risk is limited to say at least, but environmental parameters are supportive for rapid storm organization for any storm, which manages to evolve. The area of interest is placed along a NW-SW aligned baroclinic zone with strong NW-erly winds overspreading this boundary during the day. CAPE is forecast along the NE-coast of Spain, where some LL moisture remains in place. An isolated thunderstorm may fire along that boundary and could become supercellular, given very strong shear environment. Large hail and severe wind gusts will be the main hazard, as the activity moves offshore with a weakening trend forecast, as CAPE decays. No level was issued as initiation remains questionable.