Storm Forecast
Valid: Fri 26 Jun 2009 06:00 to Sat 27 Jun 2009 06:00 UTC
Issued: Thu 25 Jun 2009 17:26
Forecaster: TUSCHY

A level 2 was issued for W-Ukraine mainly for large hail and severe wind gusts.

A level 2 was issued for S-Ukraine mainly for large hail and severe wind gusts.

A level 1 was issued for the surrounding regions, Ukraine and S-Belarus, mainly for excessive rainfall, tornadoes, large hail and severe wind gusts.

A level 1 was issued for S/SW-UK and NE-France mainly for excessive rainfall.

A level 1 was issued for parts of Germany, the Czech Republic, S-Poland and Slovakia mainly for excessive rainfall and tornadoes.

A level 1 was issued for parts of Hungary, Croatia and Slovenia mainly for tornadoes.

A level 1 was issued for parts of Itlay and the Adriatic Sea mainly for excessive rainfall.

A level 1 was issued for Romania, Moldova mainly for large hail and severe wind gusts.

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

SYNOPSIS

Weak pressure gradients at all levels of the troposphere affect most parts of Europe. The stratification of a somewhat cooler mid-level airmass atop of a warm/moist boundary layer yields widespread moderate to strong instability release in a weakly capped environment. Thunderstorms are forecast over most part of W/CNTRL and E-Europe. The SW-Mediterranean remains capped. Cold and stable conditions are present over extreme NE-Europe, which suppresses deep convection.

DISCUSSION

Broadly speaken, thunderstorms are forecast from Ireland to the Black Sea and from Poland to Sicily. We don't want to keep the outlook that short and hence focus on regions, where thunderstorm coverage and attendant flash flood risk ought to be enhanced. It also makes no sense to concentrate on the paths of the 'omnious blobs' in the QPF field of GFS, the result of a failure of the convective parameterization scheme, which does not handle the release of latent heat in a correct way, producing artificial storm clusters, which also affect the mass flux and therefore causes shear and QPF maxima. As far as possible we basically stick with the WRF solution to avoid that problem but keep their calculations in mind, which occured mainly over E-Europe. For the rest of the outlook we once again split the huge forecast area into smaller regions, to keep an overview:

... SW-UK, CNTRL/N/NE-France and extreme SW-Germany...

The main story at mid/upper levels is a gradually northward sliding trough, weakening during the forecast and forcing a weak front (most prominent in moisture fields) to the east/northeast. The front itself loses strength, so its circulation ought to weaken betimes. Nevertheless it will be the focus for scattered to widespread initiation as atmosphere is weakly capped and humid with surface dewpoints in the mid-tens. The shape of the W-E aligned upper trough favors some mid-/upper divergence due to a departing speed max over Ireland, so current model solution of ongoing thunderstorm activity over the English Channel and SW-UK during the morning hours is not unrealistic. This cluster ought to get fed by moist and warm air, so it will probably keep its strength over SW-UK with heavy rain/flash flooding being the main risk. Over NE-France, thunderstorms evolve during the morning hours in a very weakly sheared environment, so slow storm motion could result in locally heavy rain rates. Any cluster will slowly drift northwestwards and affect parts of S-UK during the afternoon/evening hours.
A level-1 was introduced, where excessive rainfall is possible, probably maximized over SW-UK.

... N-CNTRL/E-Germany, the Czech Republic, Slovakia and S-Poland ...

GFS/WRF and EZMWF drop a weak moisture convergence zone to the south with different results in the final position. Nevertheless, this boundary serves as focus for the most active thunderstorm swath due to an increase in the boundary layer moisture depth next to the convergence zone, some forcing and abundant CAPE release in a weakly capped environment. The atmosphere allows 700-1000 J/kg MLCAPE to develop along this boundary over Germany and the Czech Republic with even higher values over Poland. Yesterday, the soundings in this airmass and also forecast soundings reveal 'skinny' CAPE profiles with deep updrafts (EL temperature forecast area-wide below -50°C) and hence a prolonged period for collision-coalescence and warm rain processes (soundings also indicate a deep, and warm/moist cloud layer). In addition, DLS remains weak, roughly 10m/s, so entrainment processes may play a subordinate role. Torrential and potential flash flood producing rain amounts are possible beneath strongest convection over N/CNTRL/E-Germany.

Over the Czech Republic, Slovakia and S-Poland, 0-3km shear increases to 10-15m/s during the day with a near parallel component to the convergence zone throughout the forecast. Missing veering to southerly directions in lowest 2km may limit persistent moisture transport to the convergence zone, but shear is adequate for the development of numerous small MCSs with strong wind gusts and marginal hail. However, given history of impressive rainfall over parts of S-Poland/E-Czech Republic and the prolonged period of E-W moving convection, flash flooding will be a distinct hazard.

For all highlighted regions from N-Germany to Poland, the tornado risk increases during the afternoon and evening hours as enhanced LL CAPE and an increase in speed/directional shear at the lowest 1000m overlap, before decreasing during the night hours due to a weakening LL shear field.


... Romania and most parts of the Ukraine ....

Regarding ingredient based forecast methodology, the risk for severe thunderstorms increases over Ukraine, Moldova, parts of Romania and probably even Belarus. Steep mid-level lapse rates overspread a moist and warm boundary layer with dewpoints in general in the mid to upper tens and strong diabatic heating is forecast. This yields modest MLCAPE release with 500-2000 J/kg, dependant on the depth of the moist surface layer and degree of diabatic heating but probably maximized over W-Ukraine, extreme SE-Poland and E-Slovakia. The airmass is somewhat better capped over most parts of the Ukraine due to warmer lower levels but initiation will start during the noon/early afternoon hours probably first over Romania/W-Ukraine and Belarus due to weaker capping. Dependant on the local degree of CAPE release, strong to explosive thunderstorm development is expected in an environment with 15m/s DLS, 15m/s 0-3km shear and some low to mid-level directional shear. During the initiation phase, when storms remain more discrete, large to very large hail and severe wind gusts due to a large delta theta-e spread and a well mixed subcloud layer will be the main hazard, mainly in the level-2 area. Cold pool driven MCSs could locally result in a concentrated swath of severe wind gusts. During the evening and night hours, those thunderstorm clusters also affect Belarus from the south and despite an overal decrease in severe, excessive rain is still possible. MUCAPE greater than 500J/kg indicates that storms will keep going probably until the ealry morning hours. This level-2 was issued due to the expected high coverage of large hail and severe wind gust reports.

Further east, very unstable conditions develop during the day with 15-20m/s DLS and some SRH at the lowest 3km. Large to very large hail and severe wind gusts are the main hazard and hence a level-2 was issued for that area.

... W-Turkey ...

Numerous short-waves round a flat upper trough, which is placed over SE-Europe, so early initiation and a prolonged period of activity is forecast over NW-Turkey. Latest synop data indicate dewpoints in the mid tens along the coastal areas, and that's the region, where best instability release is forecast as lapse rates remain steep at mid-levels. 15-20m/s DLS and some directional shear ought to result in multicells/isolated supercells with a large hail and severe wind gust threat. Strongest activity likely occurs until 14Z, before instability weakens slightly and strongest shear departs to the north/east. We decided to stick with a level-1 due to the limited time frame of thunderstorm activity and detoriating shear/instability conditions. More storms will evolve further towards the west during the afternoon/evening hours, but weak shear/instability preclude organized convection.

... Rest of the highlighted areas ...

For the not mentioned level-1 areas, the main issue will be an augmented heavy rainfall risk due to slow storm motion and/or training storms, like Italy but also SE-Germany/N and NE Austria, where history of rain for days provided for a saturated soil, so flash flooding is possible with slow moving storms. Slovenia, Croatia and parts of Hungary got a marginal level-1 due to enhanced LL CAPE release and hence a low-end tornado chance.

For the highlighted thunderstorm regions without any levels, either shear or instability or both will be too weak for organized storms.