Storm Forecast
Valid: Tue 06 Aug 2013 06:00 to Wed 07 Aug 2013 06:00 UTC
Issued: Mon 05 Aug 2013 22:13
Forecaster: TUSCHY

A level 2 was issued for the Pyrenees and SW/S France mainly for widespread severe to damaging wind gusts, very large hail (hail diameter in excess of 5 cm) and an isolated tornado risk. Flash flooding could occur with the potential for a more serious flash flood event.

A level 2 was issued for E France, Switzerland, NW-Austria, NW Italy and parts of Germany mainly for large to very large hail (hail diameter in excess of 5 cm), severe to damaging downbursts and swaths of severe to damaging wind gusts, heavy to excessive rainfall amounts and an isolated tornado event.

A level 1 surrounds the level 2 with a similar risk but lower confidence in extreme events.

A level 1 was issued for Sweden /SE Norway mainly for a few large hail and strong to severe wind gust events. Heavy rain and an isolated tornado event is another concern.

SYNOPSIS

An ongoing highly amplified steering flow regime continues over Europe with a pronounced upper-level trough building south towards the Bay of Biscay. Downstream ridging, which extends from Italy toward Belarus does not look healthy at all with rather low mid-level geopotential heights and notable height falls along its western fringe. Another upper trough over far W-Russia exits the forecast area to the east and gradually loses ground on the E-European weather.

In the lower troposphere, an extensive frontal boundary extends from S-France to C-Germany to Sweden. Considerable displacement of that boundary in E-W direction is forecast either due to regionally enhanced thunderstorm activity (MCS events) or due to some CAA from NE Germany into Sweden. Also, numerous short-waves seem to result in some BL wind response (e.g. temporal backing) and also support a rather complex frontal boundary structure.

DISCUSSION

... N-Spain, France, Switzerland, NW Italy parts of Austria and Germany ...

An outbreak of severe thunderstorms is forecast!

First question will be how thunderstorm activity from the previous night could mess with today's set-up. Latest satellite data (5th Aug. at 22 Z) indicate some thunderstorm activity over Benelux in response to a NE-ward moving short wave and numerous thunderstorm clusters over E-France, Switzerland and W-Austria. The current idea is that convection from Benelux will exit to the north with no serious activity thereafter until 06Z. This activity however could leave outflow boundaries behind over N-Germany, which could serve as foci for CI. More tricky is the scenario with the other clusters over France and the W-Alps which see some support by the departing short-wave and attendant mid-level speed max. Hence, we think numerous clusters will continue over Germany but also over south/central France at 06Z although showing some temporal weakening.

In the mid/upper troposphere, we expect a rather brisk SW-erly flow regime from SE-France into NE-Germany. Numerous embedded short-waves eject out of the base from the gradually approaching trough over the Bay of Biscay. Also, ongoing lowering mid-level heights affect the area of interest.

Before noon, we expect aforementioned mess of weakening or re-structuring thunderstorm clusters of the previous night to already insert an isolated severe risk (hail/wind) due to 15-20 m/s DLS and MLCAPE of 1000 J/kg. Beyond noon however, regionally enhanced thunderstorm development (likely bound to the passing short-waves) is forecast from C-France all the way to NE-Germany. Current indications are that both NE Germany and C/E-France could see the best forcing for scattered CI. In-between, e.g. over S/C-Germany, CI might be more isolated in nature until the early afternoon and we might see a lull in activity. Thereafter however, a gradual upscale growth into thunderstorm clusters is also forecast. Complex interaction of those clusters is expected with subsidence next to any mature thunderstorm cluster probably affecting its immediate environment.

The kinematic environment is very supportive for organized DMC with 20 to 25 m/s 6 km bulk shear over C/E France, decreasing to 15 to 20 m/s further to the east/northeast. Brisk SW-erly flow at 700 hPa also supports 3 km shear on the order of 15 to 20 m/s along the frontal boundary itself. LL shear increases somewhat over C/E France during the evening hours, as 850 hPa winds start to increase and back in response to the deepening trough just to the west. LL shear remains low otherwise. Ongoing WAA supports strongly veered profiles over the complete forecast area.

The overlap of rich BL moisture along and east of the boundary with a northward expanding EML plume result in widespread 1000 to 2000 J/kg MLCAPE with regionally higher values.

This kind of shear/CAPE overlap supports well organized multicells and mature supercells mainly during the discrete initiation stage. Large to very large hail (hail diameter locally well in excess of 5 cm) is a big concern and this hazard could occur rather widespread with highest coverage probably from extreme E-France, N-Switzerland into S/C-Germany. A few severe downburst events are forecast as well. Slow storm motion and PWs in excess of 30 mm also favor excessive and potential flash flood producing rainfall amounts. An isolated tornado event can't be ruled out either with long-lived and mature supercell structures or along the boundary itself, where LCLs drop rather fast from E to W. This risk also exists over E-France, where backing LL winds increase the tornado risk during the afternoon/evening hours.
Betimes, those storm grow upscale into numerous large thunderstorm clusters. A few forward propagating cold pool driven MCS events are possible. Despite an ongoing large hail risk, swaths of severe wind gusts and heavy rainfall amounts will likely be the main hazard. It remains still unclear if those clusters will be able to remain connected to the frontal boundary, pushed to the NE by passing short-waves or if they tend to build more into the unstable air mass and hence showing a more easterly motion. The first option would indicate an enhanced severe to damaging wind gust risk with an elongated mid-level shear vector aligned parallel to the boundary, whereas the second possibility would indicate the chance for embedded supercell structures with enhanced helical inflow. Despite those uncertainties, the severe risk will be significantly enhanced either way. Those clusters continue all night long.

NW Italy was added into the level 2 with an enhanced risk for a large thunderstorm cluster during the afternoon/evening hours. Strong CAPE signals and shear support all kind of severe and an augmented risk for a few extreme events (hail/wind).

Further south, over N-Spain and SW-France, ingredients for a long-tracked and damaging MCS/bow echo event during the evening and overnight hours seem to merge. A strong vorticity maximum approaches N-Spain during the afternoon hours from the west, likely adding abundant forcig/lift to that set-up. Despite some uncertainties regarding timing, most models show a progressive and negative tilted wave rotating from the Bay of Biscay to the NE during the night. DLS gradually increases from 20 m/s to 30 m/s with 15 to 20 m/s shear in the lower 3 km of the troposphere. MLCAPE likely exceeds 2000 J/kg due to ongoing influx of a warm and very moist air mass from the W-Mediterranean. Expect explosive thunderstorm development over the Pyrenees during the afternoon hours and a rapid upscale growth into a severe thunderstorm cluster (potential MCC event) over SW France. Once again, very large hail in excess of 5 cm in diamater, severe downbursts and excessive rain will be the main hazard during the initiation stage. Betimes however, swaths of damaing wind gusts and large hail likely affect most of S-France. A level 3 may be added in case confidence in a derecho event increases.
Beside the wind damage risk, the concern of a major flash flood event exists in the regions of Midi-Pyrenees and Languedoc-Roussillon, where BL mean mixing ratios exceed 15 g/kg with persistent SE-erly flow from the W-Mediterranean .

Despite mixed model signals, the level 2 was expanded towards NE-Spain, as no real reason is seen to not expect isolated to scattered DMC ahead of the approaching trough. Shear and CAPE overlap favors supercells with very large hail, severe wind gusts, flash flood producing rainfall amounts and an isolated tornado risk. The same with the level 1, as very deep parcel layer depths and strong LL convergence overlap along the E Sistema Penibético. We think isolated CI is possible with shear/CAPE once again supporting rotating storms with all kind of severe. This risk however is isolated in nature.

... Sweden ...

Aforementioned wavy frontal boundary acquires more cold front characteristics over Sweden with a constant push to the east. Expect scattered DMC development during the day with 15-20 m/s DLS and 500 to 1000 J/kg MLCAPE. A few large hail and strong to severe wind gusts events are likely. LL shear seems to be somewhat enhanced along and ahead of the cold front and an isolated tornado event seems to be possible. Thunderstorms grow upscale into a large thunderstorm cluster during the afternoon and evening hours with an excessive rainfall and strong to isolated severe wind gust risk. Thunderstorms gradually weaken during the night.

... Far W-Russia into Ukraine ...

Weak shear but 500-1000 J/kg MLCAPE result in daytime driven scattered pulse storms. A few of those thunderstorms could be pretty strong with a few severe downburst and large hail events. The expected coverage of severe however preclude a level area for now. Upscale growth into a loosely organized thunderstorm cluster during the evening hours is forecast with rapid weakening beyond sunset.