Storm Forecast
Valid: Mon 02 Aug 2010 06:00 to Tue 03 Aug 2010 06:00 UTC
Issued: Sun 01 Aug 2010 20:35
Forecaster: DAHL

A level 2 was issued over S Germany ... the Czech Republic ... W Poland mainly for large hail ... damaging winds ... and excessive convective rain.

A level 1 was issued over S France and NE Spain, Switzerland, E and S Germany, and SE Sweden mainly for large hail, damaging winds, and heavy rain.

A level 1 was issued for western Finland mainly for damaging wind gusts and tornadoes.

SYNOPSIS

An upper long-wave trough is present over western Europe and will make slow eastward progress during the period. GFS indicates that the DCVA region associated with this trough will break up into two separate areas late on Monday, one moving across the NE Mediterranean, the other one lifting out across western Germany into the Baltic States. The latter feature will support cyclogenesis over the Czech Republic/Poland on Monday evening. Otherwise, Atlantic high pressure builds into the western parts of Europe in the wake of a cold front curving from SW Europe into Scandinavia. Quiescent synoptic-scale SFC conditions prevail in the air mass east of this boundary. This boundary will be the main focus for potentially severe convective storms on Monday.

DISCUSSION

... NE Spain ... S France ... S and E Germany ... Czech Republic ... W Poland ... E Sweden ... W Finland ... Baltic Sea ...

It seems that weak, possibly elevated showers and thunderstorms may persist along the front throughout the period as numerous small-scale upper-level short-wave troughs are present at the main trough's eastern periphery. These thunderstorms may briefly attain severe strength, the main threats being marginally severe winds/hail. An isolated tornado or two could occur over western Finland late in the day as LLS increases beyond 10 m/s.

The first more significant evolution should occur over southern France in the early afternoon hours in response to the approaching vorticity maximum. Low-level lift should be provided along the large-scale thermal boundary and along outflow boundaries laid out by Sunday's convection. Deep shear should be in the 15 to 20 m/s range, with MLCAPEs on the order of 1000 to 1500 J/kg. This should be sufficient for well-organized muticellular storms and imbedded supercells, spreading northeastwards into Switzerland and southern Germany. The main threat will be large hail, severe outflow winds and excessive convective rainfall.

Although the mesoscale details remain unclear ... it seems that another round of convective development will occur over southeastern Germany in response to the falling SFC pressure. Whether this activity will develop as new entity of manifest itself as re-intensification of the storms moving in from the southwest is unclear at this time. WRF12 seems to favor the former scenario.
CAPE will likely be on the weak end, possible lower than 1000 J/kg due to Alpine lee effects and a correspondingly dry boundary layer. However, the development of initially isolated storms, some of them supercells, that will gradually grow into an MCS is anticipated. Though large hail and damaging winds will the the main threats initially, heavy rain and flooding will become an increasing issue as the system expands and develops a growing quasi-stratiform precipitation region. The MCS should last through much of the night and affect western Poland early Monday morning.

Over NE Spain, the thermodynamic and kinematic fields remain strong, with high conditional probabilities of supercells capable of large hail and damaging winds ... the condition being convective initiation. GFS and WRF at least show weak signals in the QPF fields, and given rather robust large-scale forcing for ascent, this region will be included in the risk area.

... Benelux ... NW Germany ...

In the wake of the front over NW Germany and Benelux, a few non-mesocyclonic funnels and maybe a brief or two tornado may occur, especially in the coastal areas. The overall threat posed by this activity is too low, however, to issue a LVL1 threat.

... western Russia ...

Widely scattered thunderstorms should develop over the western portions of Russia in weakly capped and mostly weakly unstable air. Quite deep adiabatic subcloud layers suggest that isolated severe outflow winds may occur, but the coverage should be too low to warrant a LVL1 risk.