Storm Forecast
Valid: Fri 04 Jul 2014 06:00 to Sat 05 Jul 2014 06:00 UTC
Issued: Thu 03 Jul 2014 23:58
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for E France, Switzerland, W Austria and SW Germany mainly for large hail and severe wind gusts.

A level 1 and level 2 were issued for S France, the Ligurian Sea and N Italy mainly for large hail, excessive precipitation and severe wind gusts, and to a lesser extent for tornadoes.

SYNOPSIS

A meandering zonal flow is active over Europe. A negatively tilted short-wave troughs moves from Northern Spain into the Alpine region. An intense surge of warm air advection pushes northward between this trough and a transient ridge over East-Central Europe. It includes elevated mixed layers which spread from the Sahara over the Central Mediterranean and from the Alps over much of Germany. Cooler maritime air advances eastward over the British Isles, France and Iberia, though a distinct frontal boundary is missing.
Southeastern Europe is dominated by anticyclonic influence with mostly friendly conditions and seasonable temperatures,
while a decaying low over Finland brings cool and unsettled weather into Northeastern Europe.

DISCUSSION

... S and E France, Switzerland, SW Germany ...

Low-level moisture will continuously increase on Friday under the influence of strong insolation and suppressed mixing beneath the warm air advection. Moderate CAPE with peak values around 1000 J/kg or higher is predicted to build till the evening, initially capped by a rather strong inversion. Deep-layer shear will be between 15 and 20 m/s, in general increasing towards the Southeast.
Intense QG lift overspreads Southern and Eastern France during daytime and the Alpine region in the evening and overnight. It is provided both by the short-wave trough and by the left exit region of a pronounced mid-level jet (35 m/s at 500 hPa) that swerves in from the Western Mediterranean.
The day will likely start with rather cloudy conditions, scattered rain and partly embedded thunderstorms over Southern France. Chances for surface-based convection increase in the course of the day, as a Southerly low-level jet advects Mediterranean moisture northward through the Rhone valley. Multi- and supercells are expected, some of which may be backbuilding and remain almost stationary as long as they encounter sustained Southerly inflow. Large hail and flash flooding are the primary risks along with isolated severe wind gusts. One or two tornadoes are not ruled out, either, due to rather low cloud bases and enhanced low-level wind shear and veering.

Almost unimpeded insolation will prevail further North and East till the early afternoon, followed by increasing high cloudiness thereafter. The sharpening thermal contrast and northeastward moving outflow boundaries will likely trigger scattered storms from noon onwards over Eastern France and Switzerland with an increasing hail and wind risk.
Highest uncertainty exists with respect to the degree of low-level moisture recovery and the behaviour of the near-surface wind field on the Northern side of the Alps. Unseasonably strong Southerly flow across the Alpine chain may well push hot and dry Foehn winds some distance into the Northern foreland. The resulting dryline boundary with the moister air to the North will need particular monitoring ahead of the approaching French convection. If it aligns favourably with the track of one or several of these storms, they can grow into a long-lived bow echo, which will be capable of producing a larger swath of damaging winds while it moves northeastward into Germany in the late afternoon and evening. Highest chances for such a development exist along an axis from Lyon to Karlsruhe.

... S Germany, W Austria ...

Forecast uncertainties culminate along the border between S Germany (Bavaria) and Austria, where e.g. WRF predicts Southerly Foehn winds with dewpoints of only 10°C and hardly any CAPE by 18 UTC, whereas ECWMF shows persistent upslope flow with dewpoints up to 20°C and a resulting narrow CAPE maximum up to 3000 J/kg. Again, the dryline between the South Foehn and the moist Northeasterly upslope flow will provide the main focus for convective initiation. The longer it stays attached to the Alpine rim, the more likely it gets that one or two storms will form along this boundary.
With strongly sheared and veering wind profiles and almost no competition, any isolated storm can quickly organize into a supercell and can bring (very) large hail and severe downbursts. Due to the uncertainties about initiation and coverage of storms, it was decided to leave large parts of Bavaria only under a level 1 and a low probability thunder area. However, more widespread severe weather events will materialize if the ECMWF scenario comes true.

Storms will weaken as they encounter a strengthening cap and decreasing instability along their path to the Northeast. Nonetheless, some elevated "convective debris" can move into Eastern Germany and the Western Czech Republic overnight.

... Ligurian Sea, N and Central Italy ...

Despite very strong shear (up to 35 m/s between 0 and 6 km) and helicity (up to 350 m^2/s^2 between 0 and 3 km) and moderate CAPE signals in the forecast models, various doubts remain about the convective potential in the Italian region. Most prominently, a strong capping inversion will stay in place beneath the warm air advection aloft.
Current thinking is that the ongoing warm air advection and orographic lift will bring rather thick cloudiness, some rain and elevated thunderstorms through much of the day in Northern Italy. With the arrival of the strong vorticity maximum in the evening, convective activity is expected to increase and one or two large clusters will likely move eastward overnight. Heavy precipitation appears to be the main risk, and especially tail-end storms to the South can also be accompanied by large hail and severe wind gusts. Any storm that manages to root down into the boundary layer could also produce a tornado. However, the expectation of rather messy and predominantly elevated convection are the limiting factors which preclude a level 2.

Further South, a very strong cap and a lack of lift support will likely preclude any convection despite an equally impressive CAPE-and-shear overlap across much of the Central Mediterranean region.

... other thunderstorm areas (W France into BeNeLux, Black Sea coasts, and parts of Finland and NW Russia) ...

Scattered thundery showers will form in response to diurnal heating of rather cool air masses. Limited CAPE and weak shear make severe weather unlikely.
Along the Black Sea coasts, initiation will mostly be tied to sea breeze fronsts. Locally higher rainfall accumulations may result in Eastern Romania, where the low-level moisture is maximized and the orientation of this sea breeze front is parallel to the Northerly background flow. However, this threat does not seem high enough for a level 1.