Storm Forecast
Valid: Fri 31 Aug 2007 06:00 to Sat 01 Sep 2007 06:00 UTC
Issued: Thu 30 Aug 2007 17:21
Forecaster: TUSCHY

SYNOPSIS

Strong split flow regime is currently present over Europe and will persist throughout the forecast period. The well defined branch of the polar vortex is still very active and numerous disturbances rotate around this feature.
A broad upper-level trough over central Europe will shift eastwards, while weakening.
Hot and dry conditions can be found over parts of Spain, while very hot conditions prevail over SE Europe.

DISCUSSION

...Romania,parts of Moldova and extreme southern Ukraine...

Overall set-up contains a few uncertainties regarding final evolution but nevertheless a considerable severe thunderstorm threat exists over this region !

Inspection of the latest WV images show a well defined dark stripe on the downstream periphery of the positively tilted upper-level trough, running from NE Spain all the way to Romania. Right under this area, a wavy formed frontal zone at lower levels can be found and marks a transition zone from cool and dry air on its north side to a warm and humid airmass on its south side. This zone was placed from central Croatia to north-central Serbia and southern Romania (yesterday, 15Z) , which nicely coincides with latest GFS output. Quite diffuse surface wind field and strongly weakened dewpoint gradient indicate that this boundary has weakened and is nearly stationary ( also supported by parallel alignment towards the background flow ).
The main concern right now is that this front could alter due to ongoing/developing storm clusters and could shift further towards the south than currently anticipated. This possibility was reflected in the southwards expansion of the level areas.

Conditions look fine for repeated thunderstorm evolution over the area of interest. Numerous short-wave troughs are forecast to travel over Romania from the west although the main player will be a rapidly eastward racing short-wave trough, which should help to spark thunderstorms during the late evening hours ( W/S-Romania ) and evening hours ( eastern Romania and Moldova ).
Airmass south of aforementioned upper/lower-level boundary is characterized by steep mid-level lapse rates and a hot and moist boundary layer ( dewpoints between 15 and 20°C ) which should offer up to 1500 J/kg MLCAPE.

Deep layer shear is quite strong and modified forecast soundings reveal mainly unidirectional hodographs although somewhat better veering could develop over eastern Romania during the evening hours, as 850hPa wind tends to veer more towards the south.

GFS brings the cluster of storms, which affected N-Italy yesterday, in its decaying stage towards northwestern/northern Romania and we would not be surprised to see some activity (showers and weak thunderstorms )over this part of Romania during the morning hours. A weakly capped environment and strong diabatic heating over southern Romania will help to spark numerous storms over central Romania ( mainly along the rough terrain ), while eastern Romania and Moldova should stay capped until the mid-afternoon hours. Betimes, thunderstorms should also evolve further towards the east, as convectively altered front mixes eastwards, upper-level impulse approaches and a possibly developed storm cluster ( please refer to the next paragraph! ) enters Romania from the west. Shear and instability favor rapid storm organisation with an enhanced severe wind gust and large hail risk ( mainly over eastern Romania and Moldova, where more discrete thunderstorm evolution is expected).
The tornado threat will be enhanced over eastern Romania/Moldova, where dewpoints are higher and LL shear stronger.
Storms are forecast to cluster and this feature will shift towards the NE, spreading the severe weather threat as far east as SW Ukraine.


...Serbia, Bulgaria, Bosnia-Herzegovina and Croatia...

An ill-defined upper-level trough approaches Bosnia-Herzegovina and Croatia from the west and attendant PVA spreads rapidly eastwards. There is a distinct possibility that a cluster of storms will develop over aforementioned areas, which will shift eastwards and could reach Romania during the later afternoon / evening hours.
More thunderstorms will also develop further towards the south, as 850hPa temperatures cool down during over Serbia and Bulagria.
Moderate instability release and strong shear again favor strong to severe thunderstorms with an attendant severe wind gust and large hail risk.

...Central Italy...

Eastward shifting upper-level trough will help to release low-end to moderate instability. Best shear and strongest instability will be separated, but we included a small level-1 area, where confidence is higher that a few severe thunderstorms can develop. An isolated large hail event will be the main threat.
Although not perfect, conditions will become more favorable for a few spout events over central Italy, as 0-3km CAPE increases, shear relaxes and LCLs stay low.

... N-Germany and Denmark...

A strong vort max will cross the area from the NW and should support a limited time-frame (starting at 9Z over NW Germany , ending at about 16Z over NE Germany /NW Poland) for a somewhat enhanced thunderstorm threat. Constantly cooling mid-levels, ELs, which cool down to -30 to -40°C and some diabatic heating will support a few thunderstorms, mainly along the coastal areas. The questionable parameter will be the amount of instability and latest thinking is that only traces will be released. Limited diabatic heating should also help to keep chances for surface based storms down and hence it is not anticipated that storms can cash in on enhanced LL shear.
No severe weather threat is expected although gusty winds and small hail can accompany storms.