Storm Forecast
Valid: Wed 24 Aug 2011 06:00 to Thu 25 Aug 2011 06:00 UTC
Issued: Wed 24 Aug 2011 00:16
Forecaster: TOMAS PUCIK

A level 1 was issued for parts of Eastern France, Germany, Switzerland, Northern Austria, Western Czech Republic and Western Poland mainly for large hail, severe wind gusts and to the lesser extent for the excessive precipitation.

A level 1 was issued for Southwestern Ukraine and parts of Northeastern Romania mainly for large hail and severe wind gusts.

SYNOPSIS

A large cyclonic vortex at all levels of troposphere over Atlantic seems to be a dominant player in the synoptic scenario with southwesterly flow between the forward flank of the low and a ridge stretching from the Central Mediterranean into Scandinavia. The ridge covers a plume of high Theta-E airmass with temperatures exceeding 20°C at 850 hPa. A frontal boundary encloses this plume of very warm airmass, with the most active zone - a cold front over France and Germany - lying under the moderate southwesterly flow with several vorticity maxima travelling over it towards north/northeast.

Very warm airmass is characterised by an environment of moderate to high latent instability, as suggested by Tuesday 12 UTC soundings from the region thanks to the steep mid level lapse rates and high dew points at lower levels of troposphere. Initiation will likely by linked to the surface frontal boundaries, passage of PV max (especially concerning France and Germany), orographically driven circulations and also outflow boundaries laid out by previous storms. It is very difficult to anticipate storm coverage and exact spots of initiation at the moment, especially for regions further away from the frontal boundaries.

DISCUSSION

... Southern France to Germany, Western Poland and Western Czech Republic ...

A belt of moderate to high latent instability is forecast by both GFS and ECMWF simulations, with MLCAPE values likely exceeding 2000 J/kg especially over the eastern extents of Level 1. This seems reasonable also with respect to the Tuesday 12 UTC soundings from the region. Speaking of wind shear bulk values of 15-20 m/s in the 0-6 km layer are anticipated. Most of the shear will probably be confined to the lower levels, especially in the western parts of Level 1. Moreover, close to the surface frontal boundary, slightly enhanced SREH values will be observed. Southeastern part of Level 1 should have a weaker wind shear, albeit bulk values might approach 15 m/s at 0-6 km layer. Overall conditions justify the potential for strong multicellular and also briefly supercellular convection. A potential for a growth of clusters into MCS will also be present, especially during the evening hours, when PV max spreads over Germany.

Question of initiation is more difficult to answer, as the foci for initiation will likely be influenced by the overnight convection and its outflow boundaries. Nevertheless, it seems that especially towards evening hours, initiation will be likely over France and Germany thanks to the PV max crossing over the frontal boundary. Storms will then likely quickly spread northeastwards. Even before that, local development is well possible, especially over areas with rich terrain.

A broad Level 1 is issued due to the uncertainities with initiation for the region, where a threat of mostly large hail and also severe wind gusts will be present. Large hail threat will be most prominent over the areas with the highest CAPE, i.e. eastern half. An excessive precipitation might be observed as well and the best conditions seem to be over Northwestern Germany, where MCS passage seems to be most likely. During the day, the forecast might be updated as mesoscale details of the situation become more apparent.

... Southwestern Ukraine to Northern Romania ...

Moderate instability (over 1000 J/kg of MLCAPE), combined with weak to moderate wind shear (10-15 m/s of DLS), enhanced SREH (close to the frontal boundary) and high lapse rates is conducive for a few strong multicellular storms posing the threat of isolated large hail and downbursts. The downburst threat should be present due to the dry and deep boundary layer and this notion is supported by enhanced Delta Theta-E values over the region. Threat should be most prominent over Romania, where the initiation is most likely and at the same time, also enhanced SREH.