Storm Forecast
Valid: Fri 17 Jul 2009 06:00 to Sat 18 Jul 2009 06:00 UTC
Issued: Thu 16 Jul 2009 22:01
Forecaster: TUSCHY

A level 2 was issued for N-Italy, SE-France, most parts of Switzerland and W-Austria mainly for large hail, severe wind gusts, tornadoes and excessive rain.

A level 2 was issued for most parts of Germany mainly for large hail, severe wind gusts, excessive rain and to a lesser extent for tornadoes.

A level 1 was issued for areas surrounding the level-2 and central France mainly for large hail.

A level 1 was issued for parts of W-Russia mainly for large hail.

SYNOPSIS

A strong upper trough gradually gains a negative tilt during the forecast, with further amplification to the SE. Weak pressure gradients over E-Europe remain in place with hot and moist conditions, so diurnal driven thunderstorms are possible, increasing in intensity and coverage over W-Russia due to a broad surface depression. Cold and stable remains the atmosphere over Norway and Sweden with no deep convection expected.

The main feature at the surface is an extensive cold front over the level-1/-2 regions of W/-CNTRL Europe, which moves towards the south/southeast during the forecast. Widespread thunderstorm activity is forecast during the following 24h along that front.

DISCUSSION

An outbreak of severe thunderstorms is forecast over parts of Germany, Switzerland, SE-France and N-Italy with large hail, tornadoes and severe wind gusts

This set-up, although quite supportive for widespread severe thunderstorms, still features major uncertainties. The main uncertainties arise over SE-Germany and W-Germany, described in the respective paragraphs.

... SE-France, N-Italy, Switzerland and W-Austria ...

This area is the one with the highest degree of certainty that widespread severe will occur. As a sharp upper trough draws near from the west during the forecast and overspreads a hot and very moist airmass, that features surface dewpoints in the lower twenties (at 19 UTC, yesterday). In addition, lapse rates at mid-levels steepen due to an approaching EML, so MLCAPE of 1000 - 2000 J/kg is a reasonable forecast for the regions south of the Alps with up to 1000 J/kg over E-/S-Switzerland. South of the Alps, DLS increases to 25m/s with roughly 250 m^2/s^2 SRH-3, so strong mid-level updraft rotation is forecast with those thunderstorms. Supercells with large hail are forecast and hail in excess of 5cm is possible, next to organized downdrafts with severe wind gusts, given well mixed subcloud layer mainly over the southern/eastern parts of the level-2. During the afternoon hours, a depression at the surface is in progress and intensifies during the following hours, which enhances shear at low-levels. A sharp decrease of LCL heights from the south to the north is expected due to drier/warmer air to the south. For now, especially the southern parts of the level-2 have the highest tornado threat, which is also in line with the local WRF3, showing a sharp moisture gradient with impressive shear and EHI values of 2-3. An isolated strong tornado can't be ruled out. The activity spreads eastwards during the night with a continuos severe risk.

For Switzerland, a lot depends on how well models captured the activity from the night before. WRF and GFS bring the cold front into Switzerland during the morning hours, so a moist and unstable airmass still covers most parts of Switzerland. DLS increases to well above 20m/s during that time with steep lapse rates and some veering throughout the troposphere, so large hail and severe wind gusts are forecast. Although not resolved in the global models, LL wind field ought to feel a strong influence of the complex topography with regional higher LL shear. An isolated tornado is therefore possible. The main concern arises during the afternoon and night hours as the cold front stalls along the western Alps. A prolonged period of training/back-building is forecast over E-Switzerland and torrential rain with flash flooding is possible, probably serious over extreme E/SE-Switzerland. This hazard also continues mainly along the southern fringes of the Alps and especially in the level-2 area.

... The level-2 of Germany ...

... NW-Germany ...

Surprisingly, models with and without parameterized convection are all on a similar track, with numerous MCSs over Belgium and E-France/W-Germany. A more stable stratified BL and mainly MUCAPE release ought to keep the severe risk with this activity quite low with isolated large hail and severe wind gusts the primary risk. In addition, W-Germany is somewhat in a lull of mid-level forcing in-between a departing short wave and an approaching trough axis over central France.

Thunderstorms again gain an higher degree of organisation already during the morning hours over NW-Germany, where better dynamic/shear ought to increase the risk of a more widespread severe wind gust risk despite limited instability release. Isolated large hail is possible. A lot depends on how far north the surface moisture can be advected ahead of the cold front with BOLAM and WRF being the more aggressive ones. The thunderstorm risk rapidly decreases behind the cold front, so this level-2 will probably expire during the late morning and afternoon hours from SW to NE.

... NE-/CNTRL-/E-Germany and W-Czech Republic ...

The prefrontal airmass heats up under the influence of strong WAA and at least a few hours of insolation. The cold front will enter the level-2 from the west during the late morning hours/around noon and slows down during the forecast, while moving eastwards. The risk for organized thunderstorms with large hail, severe wind gusts and isolated tornadoes increases during the afternoon hours, as airmass ahead continues to destabilize. During the early afternoon hours, the attention also shifts to a convergence zone ahead of the main front, where isolated and more discrete thunderstorms could evolve, primarily over E-Germany/W-Czech Republic. This could occur either due to exceeding the convective temperature or the convergence zone itself. Large hail and an isolated severe wind gust report are possible. Later-on during the subsequent afternoon and evening hours, the cold front finally also approaches NE/E-Germany. DLS constantly increases to 15-20m/s with SRH-3 values exceeding 300m^2/s^2, so the risk of very large hail is present next to severe wind gusts. An isolated tornado event can't be ruled out, however the probabilities were increased over extreme E/NE-Germany during the evening hours, as LL shear ramps up. This depends on the strength of a potential low-level depression over far E-Germany, so uncertainties still exist about the exact magnitude of the tornado risk.

During the night hours finally, the cold front looks like to stall just east of Germany with numerous MCSs moving northwards along its cold side. Some MUCAPE is still present, so an overall messy picture with large stratiform rain and small convective parts along their leading edge mainly pose a strong wind gust/marginal hail risk with locally excessive rain. Isolated flash flooding is possible.

... S/SE-Germany ...

The region with the highest run-to-tun discrepancies in the past few model runs was SE-Germany. Latest surface dewpoint reports indicate a moist airmass with dewpoints between 15-19°C, so approaching EML from the SW ought to yield strong MLCAPE release of up to 1000 J/kg. The foehn was the main concern, also affecting the well mixed pre-Alpine region due to a southerly component of the mid-level wind field, but this trend diminished with the depression over N-Italy. The 12Z model run finally convinced us to include also SE-Germany into a level-2. Isolated thunderstorms ought to initiate during the early afternoon hours along the Alps, which then move northwards into an increasingly unstable airmass, probably also supported by the passage of a weak convergence zone around noon from the west. DLS in the order of 15m/s and rapidly increasing directional shear in the lowest 3km point to large/very large hail events as thunderstorms stay more discrete (weak forcing, still moderate cap and flat CAPE profiles in the hail growth zone). During the afternoon hours, a developing thunderstorm cluster over E-Switzerland/W-Alps moves northeastwards along the gradually eastward moving cold front. 3km speed shear of 15-20m/s hint on an augmented severe wind gust risk with that activity as it continues to cross Bavaria and affects E/NE-Germany later-on. Excessive rain amounts with that cluster are forecast.

... Central France ...

As the cold core of the trough crosses France from the NW during the noon/afternoon hours, diurnal driven 200-600 J/kg SBCAPE in a well mixed postfrontal airmass are forecast. 15-20m/s at 850hPa and good downward transport of that high momentum air to the surface yield an isolated severe wind gust risk next to marginal hail. Thunderstorm activity decreases after sunset.

... Level-1 regions over W-Russia ...

DLS in the order of 15-locally 20m/s and 500-1000 J/kg MLCAPE overlap, so multicells could produce a few large hail events. This risk diminishes after sunset.

... UK and Scotland ...

Main focus for a few thunderstorms is the back-bent occlusion, which crosses UK during the afternoon/evening hours. However, despite a few thunderstorms, not much severe is forecast. Also, no level-1 for excessive rain was issued over S-Scotland/N-UK as the precipitation of the occlusion ought to be more of stratiform nature.