Storm Forecast
Valid: Fri 19 Jun 2009 06:00 to Sat 20 Jun 2009 06:00 UTC
Issued: Thu 18 Jun 2009 18:19
Forecaster: TUSCHY
A level 1 was issued for N-Italy and the N-Adriatic Sea mainly for large hail and excessive rain amounts.
A level 1 was issued for parts of Austria and the E-Czech Republic mainly for large hail.
A level 1 was issued for S-Poland mainly for tornadoes and large hail.
SYNOPSIS
Numerous upper troughs affect central and eastern Europe. Weak ridging is present over SE-Europe and most parts of the Mediterranean, whereas intense WAA causes hot and stable conditions over Spain and Portugal.
At the surface, a boundary runs from N-Spain to the Alps and then to Belarus. This front will be the focus for thunderstorm activity during the forecast period.
DISCUSSION
... The Alpine region ...
A leisurely southward moving cold front affects the Alps over a prolonged period. Main issue will be the not yet completed boundary layer recovery in the wake of the last cold/dry airmass intrusion over the E-Alps as environment is barely able for good destabilization (visible in yesterday's 12Z sounding data). Best moisture reservoir can be found south of the Alps (N-Italy) and in the immediate environment of the SE-ward moving convergence zone over the E-Alps. Deep WAA is forecast for the complete forecast period, so lapse rates at mid-levels are not particular steep. We think the best CAPE assessment will be MLCAPE release near 1000 J/kg over NW-Italy, gradually decreasing towards NE-Italy and a tongue of 200-500 J/kg MLCAPE over SE/E-Austria. The deep layer shear field is aligned parallel to the boundary/CAPE field, so a confined overlap of better kinematic/thermodynamic parameters exists along the front with 15-20m/s DLS.
Ongoing thunderstorm/showers already affect N/extreme NE-Austria during the morning hours, probably embedded in a stratiform rainshield. However, warm and moist air ought to make its way to the NE over SE-Austria, so SE-ward sliding cold front will get more active as diabatic heating continues and there is a narrow swath over E-central Austria, where strong thunderstorms are possible. Atmosphere is weakly capped, so most likely scenario will be prolonged period of initiation along the southern fringe of the front as thunderstorms move to the NE. Isolated large hail is possible next to strong wind gust. Persistent WAA and strong frontal circulation ought to hold up the re-development of new convection, so regional excessive rainfall amounts are possible.
Further to the west, the cold front will finally reach N-Italy during the afternoon hours, while surface pressure gradually drops over N-CNTRL Italy during the same period. Thunderstorms, which already evolve during the morning hours, will continue during the day. As cold front arrives from the north and a flat surface low pressure channel evolves over parts of N-CNTRL Italy, LL convergence increases significantly and a cluster of thunderstorms will likely evolve over N-Italy, moving to the S/SE. Better instability and augmented LL shear along the front will help thunderstorms to gain organization with supercells possible, mainly over NW-Italy. Large hail, strong wind gusts and an isolated tornado are likely in the level-1 area but the main hazard will be an augmented flash flood risk. Thunderstorms continue southwards during the night with decreasing activity onshore.
A regional level-2 may be needed, if the environment indeed becomes favorable for discrete supercells mainly over NW-Italy.
... E-Czech Republic and S-Poland ...
Limiting factor is slim instability release with 200-500J/kg MLCAPE just ahead of the SE-wad moving cold front. DLS would be supportive for multicells with the favored spot for initiation along the eastward sliding cold front, but we don't think instability will be robust enough for long lived updrafts. However, EZMWF and WRF came up with slightly better conditions, so we expanded the level-1 from Austria all the way to S-Poland, emphasizing the risk of isolated large hail with more discrete thunderstorms. LL speed and directional shear increases over S-Poland during the day, so even an isolated tornado report can't be excluded.
... N-Germany, Denmark, parts of Norway and Sweden ...
Scattered to widespread thunderstorm activity is anticipated beneath cold upper low. Marginal hail and strong wind gusts are possible but weak shear precludes storm organisation. Thunderstorms weaken after sunset.
... E-central Spain and areas just offshore ...
Thunderstorms over the eastern part of Spain will be quite widespread during the afternoon hours, as a short wave approaches from the NW. The combination of incoming forcing and the formation of a cold pool could yield a SE-ward moving line of storms. 15m/s at lowest 3km is enough for some organization with isolated severe wind gusts possible. No risk level will be issued for now but if confidence in a forward propagating MCS and attendant wind event increases, an update may be needed.
