Storm Forecast
Valid: Fri 08 Aug 2008 06:00 to Sat 09 Aug 2008 06:00 UTC
Issued: Thu 07 Aug 2008 14:53
Forecaster: TUSCHY

SYNOPSIS

The active weather pattern over most parts of Europe continues as a large upper trough shifts eastwards. Strong WAA on its downstream side means hot and stable conditions for SE / E-Europe and the same for the Mediterranean as a desert airmass affects this region. A cool and maritime airmass spreads southward over central Europe behind an eastward moving cold front, and unstable conditions continue in this CAA regime.

DISCUSSION

... SE Austria, Slovenia, Hungary, parts of Croatia and Slovakia ...

The predominant upper trough starts to lift out to the northeast and a complex process begins as parts of the trough start to fill / weaken while re-intensification is on its way in its SW-quadrant as a short-wave moves in from the NW. As a consequence, the trough commences to amplify again, while racing eastwards. This evolution supports a compact and quite strong UVV field to cross our areas from the SW during the evening hours. Despite that clear signal, various features will be available for a prolonged period of initiation during the day as short-waves move out of the base during the day and an active cold front pushes SE-ward. Main attention for widespread initiation will be the cold front itself during the midday / afternoon hours and the approach of the upper trough axis during the evening hours from the SW.

Final degree of instability is somewhat questionable. Pool of rich BL moisture is present (e.g. 20 °C isodrosotherm already present over N-Italy), but downward mixing of drier mid-level air and mixing at the surface should keep dewpoints just below 20°C over our area of interest. Lapse rates steepen as tongue of constantly fading EML overspreads this BL airmass and MLCAPE values of widespread 500 to 1000 J/kg are likely to develop with locally up to 1.5 kJ/kg.

DLS is so-so with readings between 15 and 20 m/s due to a lull in the wind field between a departing streak and another, strong one, reaching N-Italy during the night hours. Dependant on the final strength of the instability release, this is more than adequate for organized storms. In addition, veering throughout the atmosphere is augmented as SRH-3 values climb well above 200 m^2/s^2.

At the moment the most widespread and enhanced severe thunderstorm risk is anticipated to evolve over SE Austria, W-Slovakia, W/CNTRL Hungary and Slovenia after 12Z. During the midday hours, a leisurely SSE-ward moving cold front is placed over central Austria and serves as the focus for initiation. At the beginning, storms should tend to move NE-wards, producing large to isolated very large hail and strong to severe wind gusts, but rapidly raising surface pressure over N-Austria should help to push the surface front more rapidly to the S / SE during the afternoon hours. This could enhance the severe wind gust risk, if a well structured MCS with attendant cold pool can evolve ( despite unfavorable orientation of the shear vektor ). This front will continue to be the focus for intense thunderstorm development also over Slovenia and W-Hungary with the primary threat being large hail and severe wind gusts. An isolated tornado can occur due to good LL CAPE release but more straight-line formed hodographs should limit this threat. Latest model runs were consistent in developing a large cluster of storms over Slovenia, moving eastwards during the evening and night hours so there will be also a flash flood risk.

We want to wait for new model/surface data and for the degree of diabatic heating, before upgrading those areas.

... NE of Slovakia ...

Dependant on the final BL quality, a tongue of modest instability covers SE Poland and W-Ukraine just ahead of the approaching cold front. A few short waves strip those regions and combined with the surface front, enough forcing should be available for scattered thunderstorm initiation. Degree of instability and modest shear point to multicells/isolated supercells with large hail, strong wind gusts and an isolated tornado due to increasing LL shear during the evening hours. After sunset, instability vanishes and so does the severe risk.

... N-Adriatic Sea ...

Confidence is high that very unstable profilers will develop offshore and just onshore as dewpoints in the lower twenties will spread onshore in the level-2 area. Atop this very hot and moist BL, the EML is present, resulting in capped 2 - 3 kJ/kg MLCAPE in a very narrow corridor. 20-25 m/s DLS and SRH-3 values of more than 200 m^2/s^2 overlap and if thunderstorms form, explosive development with a significant hail threat will occur. As upper trough axis moves through during the evening and early night hours and as surface front is present during this time-frame, thunderstorm initiation is most likely. Strong cap should limit offshore extend of the threat area but we have to monitor that region for modifications.

... Parts of Latvia, Lithuania, Estonia and the Gulf of Finland...

A strongly sheared warm sector will be the focus for organized thunderstorms and a potential tornado threat. In the past few model runs of GFS the instability release was quite marginal and the same for the EL temperatures, which were too warm for deep convection. For now the area with the highest thunderstorm chances will be the Gulf of Finland and surrounding areas. We decided not to include any level area for now as uncertainty regarding initiation and coverage is very high and an update may be issued later-on, if new model/surface data hint on a special region.

... SW-Finland...

The warm front of the intense depression moves onshore during the morning hours and continues to spread northward during the day. Hence S/SW-Finland enters the strongly sheared warm sector and as daytime heating starts, a broad area of low-end to modest instability release looks reasonable. We think that a small fetch onshore could see enough instability build-up for a few stronger showers/thunderstorms. Intense LL shear in the range of 15m/s and impressive SRH-1 values are present. Despite limited instability access and quite warm EL temperatures, shallow supercells could develop with a enhanced tornado / severe wind gust risk.


... The rest of the highlighted area ...

As the main upper trough crosses central Europe from the west during the day, daytime heating and cooling mid-levels should help to steepen lapse rates. Widespread thunderstorm development is expected in a weakly sheared environment. Locally up to 800 J/kg MLCAPE are forecast and a few severe thunderstorms with isolated large hail could evolve but this is too marginal for a broad level-1. Low LCLs and locally strong updraft strength could also result in a funnel/tornado report, but a lot depends on mesoscale features like boundaries which can't be forecast that far out.