Storm Forecast
Valid: Fri 07 Jun 2013 06:00 to Sat 08 Jun 2013 06:00 UTC
Issued: Thu 06 Jun 2013 19:52
Forecaster: TUSCHY

A level 1 was issued for N-C Spain and SW/W France mainly for large hail, a few severe downburst events and heavy rain.

A level 1 was issued for NW Spain and extreme N-Portugal mainly for an isolated non-supercell tornado risk.

A level 1 was issued for W-Finland mainly for heavy rainfall.

A level 1 was issued for Estonia to NE-Poland mainly for large hail.

A level 1 was issued for NE Romania, Moldova and S-Ukraine mainly for heavy rainfall amounts.

SYNOPSIS

Steering mechanisms over Europe remain ill defined. A strong cut-off low moves from Portugal to Spain but otherwise, a weak geopotential height gradient regime continues over C/SE Europe. Two slow moving and ill defined vortices affect parts of Scandinavia but they have no significant impact on today's thunderstorm forecast.

DISCUSSION

... C/SE/E/N-Europe ...

A weak shear/modest CAPE environment once again supports scattered to widespread thunderstorm development over a broad area. A belt of at least modest CAPE extends from N-Italy to Hungary to Belarus to Estonia with MLCAPE increasing from 500 to 1000 J/kg from west to northeast. With no shear to work with, main focus for storms to become severe at least on a preliminary basis will be the initiation phase. Strong pulsating storms may yield an isolated large hail and strong downburst risk. Betimes, storms tend to grow upscale with a localized flash flood risk on the increase. Regionally enhanced cold pool developments could once again support numerous concentrated swaths of strong wind gusts, but this hazard remains too diffuse/storm motion will be too erratic to highlight that risk.
Best set-up for isolated non-supercell tornado development probably evolves over N Italy, Slovenia, S-Austria, Latvia, Lithuania, Estonia and Montenegro to Macedonia , where LL CAPE will be maximized. This risk however remains a low-end threat at best.

It has to be emphasized that CAPE further north (e.g. E-C France, C-Germany, Czech Republic into Poland) will be lower, but up to 20 kt 1-8 km shear exists, which may compensate for meager CAPE signals. Hence, I would not be surprised to see a few pulsating storms similar in strength compared to the aforementioned regions. Also, isolated CI over E-C France may produce a few strong and potentially isolated severe downburst events, given well mixed sublcoud profiles and strong theta-e difference but limited coverage kept the proabilities below a level-1 threshold for now.

Still, two areas have to be discussed a bit more in detail:

The first area extends from Estonia to NE Poland, where 20-30 kt 1-8 km shear and CAPE in excess of 1000 J/kg may result in a few well organized multicells with a distinct large hail risk. We therefore upgraded that part to a level 1.
The second area of concern regarding enhanced flash flooding extends from NE Romania to S-Ukraine, where a weakness in the upper flow regime evolves along the W-fringe of an upper low over the W-Black Sea. This ends in a complete break down of the steering flow and very slow moving thunderstorms are forecast. PW in excess of 30 mm, 0-1 km mean mixing ratios of 9-10 g/kg and LCLs at or below 1 km point to an augmented collision-coalescence process (although warm cloud depth is not that deep) with little subcloud evaporative cooling, so high rainfall rates with slow moving storms may cause flash flood problems. Hence a level 1 was added.

Thunderstorms gradually decay until midnight, but a few storms may keep going all night long ... especially in the high-energetic air mass from Estonia - E-Poland - Ukraine- Bulgaria.

... W-Finland ...

The frontal boundary, which shifted west as a warm front a few days ago now approaches the area of interest from the west as a cold front. Slow progression of the front, near and very weak front-parallel flow, rich prefrontal moisture and increasing mid/upper diffluence may cause repeatedly developing thunderstorms, which undergo rapid upscale growth into a leisurely eastward shifting cluster of storms. The current set-up also supports some training activity while PW approaches 20 mm, so we could see some flash flood problems with those storms. A low-end level 1 was issued to cover that rainfall risk. Marginal hail and strong wind gusts accompany stronger storms,too. Thunderstorms decay until midnight.

... N-Sweden ...

A slowly eastward moving cold-core low starts to open and re-connect with the westerlies to its north. During this process, this feature acquires a more negative tilt, which may induce a weak surface low somewhere over the N-Baltic Sea during the late afternoon/evening hours (although models like GFS tend to be later ... more during the overnight hours). Nevertheless, 500-800 J/kg SBCAPE and 10-15 m/s DLS overlap could result in a few healthy thunderstorms with marginal hail and strong wind gusts around peak time heating. The thunderstorm coverage rapidly decreases thereafter.

... N-Spain and SW/W France ...

A more robust severe risk arises as a strong cut-off low impacts most of the Iberian Peninsula. Cut-off lows in general have a tight shear gradient, so the current concern is not an ample shear/CAPE overlap over a broad area but more a confined region over N-C Spain, the W-coast of France and offshore over the Bay of Biscay, where organized storms are possible.

Timing of thunderstorm initiaiton will be manifold as thunderstorms already evolve over NW France during the early morning hours with more initiation followed more to the SE over C-France. This convection is mainly driven by the exceedance of the convective temperatures and coastal convergence foci.

Around noon, highly diffluent mid/upper level flow and placement beneath the left exit region of a 30 m/s 500 hPa jet core cause rapid and scattered CI over the rough topography of N-Spain. Storms grow upscale and eventually become detached from the mountains with strengthening cold pools. Numerous clusters of storms then shift N/NW-ward over the Bay of Biscay and also towards SW France/W-coast of France during the evening and overnight hours. Forcing by the approaching cut-off only increases during the night, so continued re-development of thunderstorms is expected in those areas probably all night long.

Either CAPE in excess of 1000 J/kg over NW/W/SW France but weak shear or modest CAPE of 500 J/kg but 25 m/s DLS over N-C Spain support well organized pulsating storms (north) and multicells (south) with a large hail risk. Strong to severe downbursts are also possible mainly over W/NW France where enhanced delta theta-e is expected. Slow moving and repeatedly developing thunderstorm clusters may also increase the flash flood risk over SW France especailly during the evening and overnight hours, when PWs exceed 25 mm.
The regions Poitou-Charentes, Limousin and N-Aquitaine may see some temporal overlap of 15 m/s DLS and MLCAPE in excess of 1 kJ/kg during peak heating, which may cause an isolated hail event in excess of 3cm, but given trend of rapid clustering, no higher probabilities were iussed.

Scattered cold-core convection occurs beneath the cut-off's center over N-Portugal and NW Spain. Marginal hail and gusty winds are forecast next to an isolated non-supercell tornado risk given LCLs below 800 m and increasing LLCAPE until sunset. This thunderstorm risk rapidly diminishes after sunset. The level 1 was expanded far west for that reason.