Storm Forecast
Valid: Wed 13 Jul 2011 06:00 to Thu 14 Jul 2011 06:00 UTC
Issued: Tue 12 Jul 2011 19:21
Forecaster: PUCIK

A level 2 was issued for Northern Italy mainly for large to very large hail, severe wind gusts and to the lesser degree for tornadoes and excessive precipitation.

A level 1 was issued for Switzerland, Southwestern Germany, Eastern Germany mainly for excessive precipitation and to the lesser degree for marginally large hail and severe wind gusts.

A level 2 was issued for Southeastern Germany, Northern Austria, Czech Republic mainly for large hail, severe wind gusts and to the lesser degree for tornadoes and excessive precipitation.

A level 1 was issued for Western Poland mainly for excessive precipitation and to the lesser degree for tornadoes and severe wind gusts.

SYNOPSIS

A large trough over Western Europe is expected to undergo some minor changes, including a transformation to a closed cyclonic vortex centered over the North Sea, moving slowly eastwards. A short-wave trough will extend into Iberia in the morning hours and then it is expected to rotate around the vortex in the prevailing flow towards north-east. Strong flow is forecast on the forward flank of the vortex, especially connected to the short-wave trough. To the east, high geopotentials prevail, albeit that a very insignificant cut-off low is observed over the Black Sea.

Closer to the surface, a complex, but strong waved frontal boundary will stretch from Germany to Switzerland, Northern Italy and Northern Iberia. At the occlusion spot, a surface low will deepen and progress from Central to Northwestern Germany. The boundary will progress eastwards during the day and a cyclogenesis is forecast along its cold front. Several closed centers might develop with the most prominent one being over the Czech Republic. Strong southerly flow is simulated in the evening hours over eastern pat of the country due to the steepening pressure gradient.


DISCUSSION

A complex scenario is evolving over Central Europe with a potential for local outbreaks of severe weather. Situation will likely depend strongly on the overnight development of convection. Current thinking is that as of06 UTC Wednesday, storms might be already ongoing in the belt from SE France to E Germany with new activity redeveloping over some areas during the day (SE Germany, N Italy). Mid to high level cloudiness from the anvils might hinder insolation in some places and outflow boundaries laid out by the overnight storms might also play a significant role in the initiation of new storms. Therefore, the whole scenario as written down for particular areas is very conditional and an Update might be required anytime during the day.

... Northern Italy ...

An overlap of moderate to strong instability (1000-2000 J/kg of MLCAPE likely) with unseasonably strong wind shear is forecast, thanks to the presence of mid and upper-level jet aloft. 20-35 m/s of DLS is simulated for the region with 15-20 m/s of bulk wind shear in the lowest 3 km. Moreover, enhanced SREH values are simulated as well thanks to the backed surface flow with values between 150 and 300 J/kg. Initiation of the storms should firstly commence in the northwestern part of Level 2, likely close to the mountain range. A complex of storms might evolve, quickly traveling northeastwards. Towards southeast, more isolated development is likely, especially in conjcution to the vort max passage at approximately 15 UTC. Storms will spread eastwards and another complex of storms might form during the late evening hours over the Alpine range. In the conditions mentioned above, well organised storms, including long-lived supercells are expected, posing threat of large to very large hail, severe wind gusts and even a tornado can not be ruled out. Excessive rainfall will also be a threat over northern parts of Italy where MCS might form.

... Switzerland, Southwestern Germany to Northeastern Germany and Western Poland ...

Storms will likely be ongoing already in the morning hours close to the western extent of the Level 1 with cloud debris likely hindering strong insolation allowing for a significant CAPE build-up. Along the wavy cold front, one or more MCS will likely form in the flow parallel to the boundary, travelling along it towards northeast. Such MCS should mostly pose a risk of excessive rainfall, especially over the Alpine range in the afternoon hours. Later in the evening, another MCS might travel from Southern Germany along Northwestern Czech Republic into Eastern Germany/Western Poland. Excessive precipitation will also be the main threat in these regions. Nevertheless, moderate to strong shear overlapping with instability might also result in a marginally severe hail or wind gusts. The wind gust threat will be most prominent over Western Poland during the late evening and night hours due to the enhanced low level shear.

... Eastern Bavaria, parts of the Czech Republic, Northern Austria ...

Both GFS, ECMWF and WRF models agree on the development of moderate instability in the region, with CAPE values likely exceeding 1000 J/kg. During the late afternoon and evening hours, as a mid-level jet overspreads the region, strong wind shear is simulated with DLS values exceeding 20 m/s. Shear in the lowest 0-3 km should stay around 15 m/s. Initiation should be first confined to the outflow boundaries laid out by an overnight convection or orography and afterwards will likely be tied to the frontal passage and a vort max influencing the area. The highest coverage of storms will likely be observed in the early to late evening hours. Over eastern parts of Level 2, ahead of the deepening surface low a low level jet is forecast to develop in the evening hours at 850 hPa with backing surface winds, enhancing low-level shear (reaching 10-15 m/s) and SREH.

With such conditions in mind, well organised convection is likely, including supercells, posing threats of large hail and severe wind gusts. Later into the evening, in conjuction with the passage of a vort max, MCS might form (possibly in the form of a bow-echo) with the threat of severe wind gusts and excessive precipitation, the latter one especially over northwestern part of Level 2. In the eastern part of Level 2 (Northern Austria, Moravia) and over Silesia/Western Poland, tornado threat will be enhanced thanks to the strong low level jet and low LCLs in the evening hours, especially if some isolated convection can be maintained in the region.