Forecast Update
Valid: Fri 06 Aug 2010 10:00 to Sat 07 Aug 2010 06:00 UTC
Issued: Fri 06 Aug 2010 10:44
Forecaster: TUSCHY

A level 1 was issued for most parts of CNTRL Europe mainly for excessive rainfall amounts and large hail. To a lesser extent for strong to severe wind gusts and an isolated tornado threat.

SYNOPSIS

Large-scale upper trough over central Europe shifts slowly east/northeastwards during the forecast with falling pressure at the surface. This causes a broad area to become unstable with widespread showers/thunderstorms. The rest of the forecast area won't see organized convection during the rest of the forecast period.

DISCUSSION

... Poland, the Czech Republic, Slovakia, parts of Austria and the Balkan States...

Modest upper jet rounds the base of an upper trough, forcing the trough axis in an increasingly more negative tilt. Beneath that feature, large-scale pressure fall fosters weak/diffuse surface vortex with a gradual northeastward/northward shift betimes. This depression layaways along a wavy north-south aligned baroclinic zone, with still no real hint in exact track forecast for this surface feature. This will be relevant in highlighting highest potential for enhanced convective rainfall magnitude along gradually sharpening deformation zone.

Airmass to the east of that boundary gets characterized by surface dewpoints in excess of 20°C (09 UTC readings). Thickness of this very moist BL air mass increases towards the baroclinic zone and locally exceeds 1 km in vertical depth (e.g. Poprad-Ganovce). History of a slowly decaying trough over SE-Europe precluded a more westward expansion of a stout EML, so only modest mid-level lapse rates expected, increasing again beneath the approaching upper trough from the central Mediterranean. This results in a wide warm sector with 1-1.5 J/kg, probably doubling when using MUCAPE magnitudes.

Wind field at all levels are weak to modest with DLS ranging from 10-15 m/s beneath anticyclonically curved streamline pattern over Poland to 15 - 20 m/s over Hungary beneath the appraoching left exit jet. Diretional shear component is enhanced either due QPF model bombs or due to a strong tilt of the LL depression and associated upper low.

Right now, there exist two areas with enhanced severe potential. The first one will be found along the western fringe of the gradually consolidating surface vortex along a line NE-Adriatic coast to E-Austria to N-Poland. Where strengthening surface depression finally builds in from the SE, frontogenesis will be enhanced with widespread rain forecasts (also due to embedded thunderstorms). High rainfall amounts may concentrate along upslope region of Slovenia (along the S-Alps) but also to a lesser extent over the N-Czech Republic/SW-Poland, where locally excessive rainfall amounts are forecast with embedded storms. We went with an high-end level 1, as widespread 90l/6h and 120l/12h don't seem realistic. The other area is already in full swing with scattered deep moist convection ongoing in the central parts of the level area. Combination of persistent influx of very mosit air from the south in slowly northward propagating storms causes a regionally enhanced excessive rainfall risk. As LL inflow increases during the night, backward building storms become even more likely with training forecast. Next to that, large hail during the initiation will be possible before rapid storm clustering occurs...strong wind gusts and an isolated tornado all can't be ruled out, the latter risk due to various outflow boundaries in a low LCL and modest LL-CAPE environment. Thunderstorm clusters keep going all night long, while moving to the north/northwest, decreasing from south to north.

Again, we went with a level 1, as widespread nature of high rainfall amounts will probably not be the case. Nevertheless, conditions are supportive for locally excessive and flash flood producing rainfall amounts in this large-scale level area, especially from central-Poland to Hungary and over the upslope faced terrain of Slovenia and to a lesser extent over the N-Czech Republic/W-Poland!

The severe risk to the northeast and southeast of that level becomes more isolated in nature.