Storm Forecast
Valid: Fri 13 Aug 2010 06:00 to Sat 14 Aug 2010 06:00 UTC
Issued: Fri 13 Aug 2010 06:07
Forecaster: TUSCHY

A level 1 was issued for N-Italy, Slovenia and Croatia mainly for excessive rainfall, large to very large hail and strong to isolated severe wind gusts

A level 1 was issued for Austria and areas to the north/northeast mainly for a localized heavy rainfall risk, next to large hail and strong wind gusts

A level 1 was issued for SE-Spain mainly for large to very large hail



SYNOPSIS and DISCUSSION

Broad and quasi-stationary trough over the English Channel affects major parts of Europe and assists in widespread convection from the western/central Mediterranean all the way to Finland/Sweden. No major short wave is embedded in this SW-erly to southerly flow, so wind field features uniform speeds. This keeps shear values on the moderate side with 10 - 15 m/s DLS forecast. Also, BL air mass reveals only modest LL averaged mixing ratios, regionally enhanced by moisture pooling or numerous convergent stream line patterns, so MLCAPE will peak already between 500 - 1000 J/kg for most parts of the highlighted area. However, NE-Italy, Slovenia and Croatia will be an exception, where better moisture pushes CAPE to locally well above 1000 J/kg. Hence, this spot will see the highest threat for organized convection, producing large to isolated very large hail , strong to severe wind gusts and locally excessive rainfall amounts. In fact, the rainfall risk becomes more widespread over NW/N-Italy, whereas an isolated significant hail risk exists over parts of Slovenia and Croatia. A level 1 should cover that risk, although Croatia may be along a level 1/2 border, if indeed sufficient instability can build up. Areas in and north of Austria may see pulsating/multicell storms, regionally consolidating into long-lived clusters with heavy rainfall, isolated large hail and strong to isolated severe wind gusts. Right now, no corridor with enhanced severe is seen, so a level 1 mainly for the rain risk will cover all that for now. Models agree surprisingly well in a swath of heavy rainfall from S-Germany to the north during the evening and night hours, hence the level 1 was expanded well to the west. It's not yet clear how well GFS handles the wind field over E-Germany during the evening / the night hours due to the long-lived model QPF "bomb". Depending on the deepening rate of the depression over the Czech Republic, backing wind field would increase SRH1 and SRH3 values over E/NE-Germany. In fact, GFS 00Z places 20m/s LL shear with SRH1 around 400 m^2/s^2 at the NE-quadrant of the depression, which would indicate a substantial tornado risk, if surface based convection evolves.

A severe risk exists over SE-Spain and areas just offshore. Onshore moisture flow keeps BL quite moist, as cold air atop overspreads the area from the west. Hence, roughly 800 J/kg MLCAPE are forecast beneath strong mid-/high-level jet. Main risk will be large hail, an isolated significant event well possible and severe wind gusts. During the night hours, a cluster of storms moves offshore, probably affecting the Balearic Islands around/after midnight. Severe risk diminishes, as CAPE decreases offshore.

The area south of the Balearic Islands will be monitored for increasing DMC chances beneath very strong shear during the night hours. If confidence increases in respect of initiation, a substantial severe risk will exist with large hail and damaging wind gusts.

Ridging persists over E-Europe with mostly stable conditions.