Storm Forecast
Valid: Fri 09 Jun 2017 06:00 to Sat 10 Jun 2017 06:00 UTC
Issued: Thu 08 Jun 2017 21:12
Forecaster: TUSCHY

A level 2 was issued for E-Denmark and SW-Sweden mainly for severe wind gusts and to a lesser extent for large hail and an isolated tornado threat.

A level 1 surrounds the level 2 mainly for isolated large hail, an isolated tornado risk and severe wind gusts.

A level 1 was issued for SE Norway mainly for excessive rain.

A level 1 was issued for E-France, E-Belgium/Netherlands and Luxembourg, most of Germany, far W-Poland and the far NW-Czech Republic mainly for isolated large hail and isolated severe wind gusts. A localized tornado threat may evolve.

A level 1 was issued for W-Turkey mainly for large hail, excessive rain and isolated severe wind gusts.

SYNOPSIS

A progressive and negative tilted diffluent upper trough exits the English Channel east during the start of the forecast and crosses NW-Germany/Denmark thereafter. This feature pushes a cold front east, which affects Germany during the day and the Alps, the W-Czech Republic and W-Poland during the night. The structure of that front is complex and features a split-front appearance with a distinct prefrontal wind shift/convergence zone ... all influencing the final outcome of the severe risk.

Another (synoptic-wise) interesting scenario unfolds over SE-Europe with the interaction of two mid-/upper-level disturbances. A SE-ward drifting upper low interacts with a NE-ward lifting disturbance over W-Turkey. Given the proximity of both features, the Fujiwhara-effect causes the northern one to constantly drift to the SE while evolving into the dominant vortex over far NW-Turkey, whereas the other one speeds up, while weakening and moving more to the N. Resulting 500 hPa geopotential geometry offers a complex mid-level wind field with numerous local wind and shear maxima.

Substantial positive geopotential thickness advection occurs over the Iberian Peninusla and the Bay of Biscay ahead of another intense vortex further offshore (to the west). Very warm mid-levels (500 hPa temperatur above -10 °C) accompany that advective regime.

DISCUSSION

... W-Turkey ...

A broad LL depression along the coast of S-Turkey shifts east and out of our forecast area. Behind that vortex, a fetch with NW-erlies becomes established from Romania to W-Turkey. This results in the advection of a slightly less moist low-tropospheric air mass to W-Turkey. In addition, 850 hPa temperatures of 10 °C or less with a slight warm-up of the mid-levels result in weakening mid-level lapse rates. Nevertheless, diurnal heating combined with a weak cap, enhanced LL forcing by the complex orography and residual outflow boundaries from the previous night support thunderstorm development until noon.

DLS of 10-15 m/s and 400-800 J/kg MLCAPE result in a few strong to isolated severe clusters with large hail and heavy rain. An inversion around 650 hPa in forecast soundings limits depth and thickness of CAPE profiles and the general severe risk. Regionally inverted-V like forecast profiles add an isolated severe wind gust (downburst) threat to the overall risk.

Beyond 12 UTC, the approaching mid-level vortex interacts with a increasingly dry and less unstable air mass, so despite enhanced lift and upper-level shear, not much re-development over NW-Turkey is forecast. However as this vortex grazes the extreme SW-Black Sea, some weakly sheared offshore CAPE may support a few overnight storms with an isolated waterspout risk.

... E-France, Luxembourg, E-Belgium, Germany, Alps into W-Czech Republic and W-Poland ...

Synoptic-wise a complex forecast is in store, as smaller-scale features like the structure of the cold front/prefrontal convergence zone and questionable clearing with BL airmass recovery between both features all cast uncertainties in how the event unfolds. A broad area with a general severe risk was split into the following smaller regions:

- Localized high wind threat over NE-Germany and far W-Poland:

At 12 UTC, latest model data places the upper trough over Benelux and into far W-Germany. Along its leading edge, an elongated N-S aligned vorticity lobe is placed ahead of the actual surface cold front and is responsible for a band of more stratiform rain to affect NW/W-Germany before noon. During the afternoon/evening, this forcing crosses E-Germany, where diurnal heating and a combination of marginally steepened mid-level lapse rates atop modest BL moisture yield 400-800 J/kg MLCAPE. DLS will be in the 15-20 m/s range and hence strong enough for organized updrafts.

The current idea is that a pocket of somewhat steeper lapse rates in the 500-650 hPa layer offer adequate CAPE for elevated thunderstorms beyond 12 Z over NE Germany. Linear forcing along that vorticity lobe and along the prefrontal wind shift point to a rapid line-up of storms with rotating cells possible given enhanced directional shear (especially resolved in convective allowing models). This activity then shifts east into an increasingly dry and well mixed air mass with LCLs approaching 2 km. Forecast soundings ahead of that line show deep mixing up to 2.5km and DCAPE of up to 800 J/kg. Line of storms therefore poses a distinct wind gust threat with concentrated swaths of severe possible. The highest risk extends from N-CNTRL to NE-Germany into NW-Poland but may also include E-CNTRL Germany.


- Isolated tornado threat over W-/W-CNTRL Germany:

Another focus for organized thunderstorms exists behind the leading wind shift along the eastward moving surface cold front. Clearing ahead of the cold front looks increasingly likely over SW-/W-CNTRL Germany in a region with BL moisture recovery. Forecast soundings show a veer-back-veer profile as CAA at 700 hPa spreads east atop an ongoing LL WAA regime. Attendant mid-level cooling boosts lapse rates and MLCAPE to 400-800 J/kg with peaks aoa 1kJ/kg. LCLs remain very low with roughly 600-800 m.

Kineamtics remain supportive for organized DMC with 15-20 m/s DLS and enhanced negative helicity (given the aforemention differential temperature advection regime). Although LL wind field yields weak shear magnitude, strong veering ahead of the cold front and potential deviation of storm motion may locally enhance LL shear and SRH-1.

Forcing arrives with the surface front itself but also beneath the left exit of a strong mid-level jet over far NE-France and W-Germany. In addition the eastward pushing sharp upper trough also adds to the general deep/strong forcing which overspreads W-Germany during the forecast.

Exact timing and location of CI remains a bit nebulous, but it could occur as far west as E-France and E-Belgium with isolated severe. During the day as convection moves east, a increasingly more favorable CAPE/shear space results in organized multicells/isolated supercells over W-CNTRL and SW-Germany, which spread east. Point forecast soundings over Hesse f.ex. show favorable conditions for rotating updrafts with more than 200 J/kg CAPE in the 0-3 km layer next to strong LL veering. Aforementioned weakeness of the strength of the LL wind field may be partially offset by deviating storms and/or locally stronger shear.

Progressive nature of convection should limit the excessive rainfall risk with isolated large hail and strong to severe wind gusts becoming the dominant threat. A tornado risk exists but the final magnitude remains still uncertain as nowcast/mesoscale dictate that risk.

The level 1 was expanded to E-Beligum/Netherlands, where 300-600 J/kg MLCAPE and 15 m/s DLS combined with the eastbound moving cold front may result in a temporal severe risk before/around noon and over NW Germany thereafter. Isolated large hail, strong wind gusts and an isolated tornado threat are forecast as well.

- Alpine level 1:

Interaction of the vorticity lobe with the W-Alps also points to a developing late-afternoon/evening cluster along the CNTRL Alps, which could result in isolated severe (large hail with heavy rain and isolated severe wind gusts). The lightning and threat areas were expanded east accordingly. During the night, the overall severe risk diminishes with weakening CAPE.

... Denmark, SE-Norway and S-Sweden ...

A warm front surges north during the day with a cold front approaching the area of interest from the W. A cluster of storms probably develops over Denmark early in the forecast, moving off to the north towards SE-Norway while outrunning favorable CAPE/shear conditions. Excessive rain may bring flash-flood problems to SE-Norway - therefore the level 1.

In the broad warm sector, numerous thunderstorms develop during the day. 400-800 J/kg MLCAPE with 15 m/s DLS and substantially enhanced SRH-3 result in well organized multicells/isolated supercells with large hail and severe wind gusts. The severe wind gust risk may peak over SW-Sweden with the strongest 850 hPa flow. Clustering storms may cause localized flash flood problems. Denmark was added to the level 1/2 despite uncertainties, if the morning cluster will start a bit further north or atop Denmark. Displacement to the north would keep the air mass more unstable over Denmark with initiating storms ahead of the cold front experiencing enough shear for all kind of severe.

Models inidcate a tornado risk with enhanced LL speed and directional shear next to LCLs below 1000 m....especially in proximity to the warm front over S-Sweden and along a wind shift over E-Denmark. It is still unclear if an evolving LL depression north of Denmark (which causes most parts of the augmented LL shear) is not a NWP result of a QPF maximum (convective feedback problem f.ex. in GFS). Placement to the upper trough however would support such a senario and hence there is enough confidence in the development of that LL depression to mention the tornado risk. A small level 2 was issued, where confidence in more widespread severe is highest.

The thunderstorm activity decreases after sunset with loss of diurnal heating/CAPE.