Storm Forecast
Valid: Sun 28 Jun 2009 06:00 to Mon 29 Jun 2009 06:00 UTC
Issued: Sat 27 Jun 2009 19:33
Forecaster: TUSCHY
A level 1 was issued for parts of Hungary, Slovakia, Poland, the Czech Republic and E-Austria mainly for excessive rain.
A level 1 was issued for Latvia, Lithuania, Belarus and W-Russland mainly for excessive rain, large hail and severe wind gusts.
A level 1 was issued for W-Germany, E-Belgium, Luxembourg and extreme E-France mainly for excessive rain.
A level 1 was issued for the area S of Greece mainly for large hail.
SYNOPSIS
No change on the European weather map, with weak pressure gradients present. Widespread thunderstorm initiation is forecast over most parts of Europe. Cold and stable conditions continue over extreme NE-Europe.
DISCUSSION
...Central/E/S-Poland, E-Czech Republic, W-Slovakia, E-Austria and W-Hungary...
Numerous short-waves continue to round a quite disorganized upper vortex, centered over SE-Europe and result in the development of numerous MCSs. They are barely captured by global models and cause major uncertainties for today's outlook due to the independent existence of those MCSs. Those features could have some impacts on the final position of a developing convergence zone over parts of central Europe.
For now, global models agree reasonable well in the development of an elongated convergence zone, which runs from Croatia to Poland and then to extreme W-Russia, e.g. Moskow. Strong moisture pooling along this convergence zone causes a regional increase in the depth of the moist boundary layer. Despite that backing mechanism, we go with the median of EZMWF/GFS CAPE forecasts as atmosphere warms up slightly and lapse rates remain moderate at best. Roughly 500-1000J/kg MLCAPE will evolve in an environment with high PWAT values, narrow but elongated CAPE and very weak wind profiles throughout the atmosphere. Beside the convergence zone, a weak high-level depression causes winds above 500hPa to increase over E-Austria, Hungary and Croatia, which ought to assist in regional upper divergence and enhanced thunderstorm activity. Slow to near stationary storm motion, training storms, and developing clusters all pose a significant flash flood risk. If this zone of LL convergence finally sets up further west, e.g. E-Austria /E-Czech Republic, a serious flash flood threat could arise given background of copious amounts of rain during the past few days. In addition, as past 2 days proved, isolated large hail can't be ruled out f.ex. due to hail cycling in feeder cells despite weak shear/moderate instability release. Also an isolated funnel/tornado event can't be ruled out along the convergence zone mainly during the afternoon hours, when LL CAPE release is maximized.
... Lithuania, Latvia, Belarus and parts of W-Russia...
The main focus remains the convergence zone, however LL moisture content increases and the same with the mid-level lapse rates, so MLCAPE likely exceeds 1000J/kg , probably approaching locally 2000 J/kg. DLS increases slightly to 10m/s and placed in the right entrance of a mid/upper streak, scattered to widespread thunderstorms are forecast in the level-1 area. The main hazard is a flash flood risk, given persistent coupling of strong LL convergence and upper divergence in a weakly sheared and moist airmass. However, an augmented delta theta-e spread points to more robust cold pool development with the evolution of an MCS with an attendant strong to severe wind gust risk, probably concentrated in a narrow swath along the convergence zone. Finally, large hail and an isolated tornado are possible, too, especially beneath more discrete thunderstorms, given abundant LL/mid-level CAPE release.
... W-Germany, extreme NE-France, Luxembourg and E-Belgium...
Surface dewpoints in the upper tens feature a very moist BL. Still some convergence signals are present for today in an environment with very weak background flow. Slow moving storms pose a flash flood risk.
... S-Balkans and N/CNTRL Italy ...
A low-end tornado risk is present, given strong LL CAPE release and scattered initiation. Slow storm motion gives rise to isolated flash flood concerns.
... Portugal and NW-Spain...
A nearly stationary N-S aligned boundary over that region serves as focus for isolated to scattered thunderstorm initiation during the afternoon hours as weak short-waves cross the area from SW to NE. Limited instability ought to suppress a severe risk, so marginal hail and strong wind gusts are the main hazard with those storms. The activity diminishes after sunset.
... The rest of the highlighted regions...
Shear/instability remain too weak for an appreciable severe thunderstorm risk. A few hail events, even an isolated large one, are possible over central Finland, but limited thunderstorm coverage precludes a level-1 for now. At least a low-end thunderstorm risk exists south of Greece, with strong instability release. Any developing thunderstorm could bring strong to severe wind gusts and large hail. Despite no model indicating convective precipitation in their forecasts, we went ahead and issued a marginal level-1 as strong forcing and weakly capped atmosphere could support isolated thunderstorm development. Airmass is quite dry, but given intense CAPE release, strong updrafts ought to be able to survive enhanced entrainment.
