Storm Forecast
Valid: Wed 10 Sep 2008 06:00 to Thu 11 Sep 2008 06:00 UTC
Issued: Tue 09 Sep 2008 21:59
Forecaster: PUCIK
SYNOPSIS
A complex pattern shapes up across Europe. Dominant feature of the day should be extensive ridge of high pressure, which has developed over N. Africa, now stretching across much Medditerranean into Middle Europe. EML will persist across the given region, yielding unstable conditions, especially across the western part, although mostly with strong, capping layer. E.g. Palma de Mallorca 12Z from yesterday sounding revealed more than 3000J/kg of MLCAPE. On the fringe of the ridge, several short wave troughs will be spreading eastwards, with the deepest one affecting British Isles. Underneath it, deep low-level cyclone with strong flow both at low and mid levels will slow down with center placed to the N. of Ireland and start to fill in. A high pressure system should also affect N/NE. Europe, bringing stable conditions there. On the other hand a shallow low persists over continental part of Europe, centered over W.Russia and Ukraine.
DISCUSSION
...Morocco, NW.Algeria...
EML combined with strong surface heating should yield moderate to strong instability development. A shallow short wave, affecting SE. Atlantic region and Iberia, will strenghten mid and upper level flow with increase in DLS values, which should be between 15-20 m/s. Also, as predicted by GFS, some SREH (locally over 100J/kg) should be available to developing storms. Current thinking is, that well organised multicellular or even supercellular storms might develop. Large hail may accompany any stronger cell and severe wind gust threat should be enhanced with dry EML present above the lower, moist layer. Gusts will be especially prominent threat in case supercells manage to develop. Due to the uncertain initiation ( weak convergence /precipitation signals and almost absent upper level support) over the region with the most favourable conditions, we will stay with Level 1 for this moment, though update might be required, if initiation over NW. Algeria proves true.
...N.Spain,S.France...
MLCAPE values might exceed 1500 J/kg as boundary layer warming establishes during the day. A partially strenghtened flow should bring DLS values around 15 m/s ( locally up to 20 m/s). A weak trough as revealed by model, will induce at least some SREH around Pyrenees. Therefore, we have to count with well organised storms, mostly multicells, but supercell is not ruled out either. Large hail should accompany stronger cells. Dry mid-level air will promote enhanced evaporation and increase downdraft strenght, so severe wind gusts in downbursts are also possible. This threat should rapidly dimnish after sunset, with boundary layer cooling. Considering overall threat, level 1 seems warranted for this region.
....Rest of Spain....
Despite the fact, that moderate instability development is expected, especially over S.Spain, only weak wind shear conditions should dominate region. DLS values will stay around 10 m/s. GFS promotes SREH over 200 J/kg within shallow trough, but highest values should stay well away from regions with convection. Current thinking is, that some downburst potential should exist and few marginal hail events migh occur as well. However, overall threat is currently too low to issue a Level 1. Due to the slow storm movement, flash flooding will be possible , especially over mountaineous region.
...E.Ukraine/SW.Russia...
MLCAPEs above 1000 J/kg are possible as high mid-level lapse rate airmass overspreads the region of moist air, converging ahead of the cold front. Both cold front and short wave trough travelling SE-wards should serve as a trigger for convection. Storms should form in weak wind shear , being around 10 m/s in magnitude, but frontal trough should be able to enhance SREH values, locally climbing to 200 J/kg. Therefore, we assume some organised storms might develop, with few isolated large hail reports. Possibility of marginally severe wind gusts will also exist, as dry EML might aid in the downdraft enhancement.
