Storm Forecast
Valid: Fri 08 Jul 2016 06:00 to Sat 09 Jul 2016 06:00 UTC
Issued: Thu 07 Jul 2016 22:37
Forecaster: PISTOTNIK

A level 1 is issued for parts of Finland and adjacent Russia mainly for excessive convective precipitation and to a lesser extent for (non-supercellular) tornadoes.

A level 1 is issued for N Germany and W Poland mainly for severe convective wind gusts and to a lesser extent for tornadoes.

A level 1 and level 2 are issued for S Austria, N Italy and Slovenia for large hail, severe convective wind gusts and excessive convective precipitation.

A level 1 is issued for S France and N Spain mainly for excessive convective precipitation and large hail.

A level 1 is issued for E Spain and N Algeria for large hail, severe convective wind gusts and excessive convective precipitation.

SYNOPSIS

A "high index" pattern is in place. Low-pressure systems are placed near Iceland and over Finland/Karelia, while a subtropical high covers the Mediterranean region. In-between, a strong WNW-erly mid-level flow and an associated diffuse frontal zone stretch from the British Isles across central Europe towards Romania and the Black Sea.

DISCUSSION

... Scandinavia across Baltic States and Belarus into NW Russia ...

The cool maritime air to the north of the frontal zone is characterized by good mixing and weak vertical wind shear. Despite poor lapse rates, low-level moisture is sufficient to create a few hundred J/kg of CAPE in response to daytime heating over land, and independently from the time of day over the Baltic Sea.
More CAPE, probably around 500 J/kg, may materialize over Karelia, where low-level moisture is augmented in the convergent flow near the cyclone's center. A high storm coverage is expected and a level 1 is issued for a risk of localized flooding. A second area with expected CAPE between 500 and 1000 J/kg is the bordering region of S Norway and S Sweden, where regionally steeper lapse rates originating from the Norwegian mountains may promote the formation of marginally large hail.
Otherwise, scattered thundery showers will affect wide regions. Localized heavy rainfall is possible and one or two non-supercellular tornadoes may spin up, but the lack of clear foci precludes a level 1.

... BeNeLux, Denmark, Germany, Poland, Czech Republic ...

A strong upper-level trough overspreads the frontal zone and pushes it temporarily towards the SE in form of a cold front. Daytime heating ahead of it will allow CAPE around 500 J/kg, which is overspread by 0-3 km shear on the order of 15 m/s. Convection will likely initiate across Denmark and the northern half of Germany around noon and organize into multicells while moving east. Fusion into a several hundred kilometer long squall line is possible later on. Isolated severe wind gusts may occur with such a system, and a tornado is not ruled out in case more discrete updrafts can be maintained. Convection will move into Poland and the Czech Republic in the evening while it gradually weakens.
A second, smaller round of postfrontal thundery storms may affect E England in the morning and move onshore in the Netherlands and NW Germany towards evening. Under similar CAPE and shear conditions, isolated severe wind gusts are not ruled out with this system, either.

... S Alpine region ...

The potentially most dangerous, albeit uncertain, setup evolves in the wake of the Alps. With strong WNW-erly mid-level flow (>15 m/s at 700 hPa), an elevated mixed layer will spread from the mountains over the southern forelands. Moisture accumulation beneath the capping inversion enables the build-up of CAPE, and upvalley/upslope circulations create excellent wind profiles for organized storms (0-3 km shear around 20 m/s, 0-3 km storm-relative helicity between 100 and 300 m^2/s^2).
The same configuration has been in place for several days. Numerous severe supercells formed on Tuesday, while initiation happened more sporadically on Wednesday and completely failed on Thursday due to large-scale subsidence and cold air advection across the main Alpine crest.
With a return of warm air advection, CAPE will increase again (500-1000 J/kg over S Austria and Slovenia, possibly up to 2000 J/kg in N Italy) and an upswing of thunderstorm activity is expected on Friday. Storms will fire up to the immediate south of the main Alpine crest along the nose of the moist upslope flow, and will move SE-ward until they succumb to the strong cap sooner or later. Each storm that forms may easily turn multi- or supercellular and poses a distinct risk of large (or locally even very large) hail and severe downbursts. Flash floods are also possible in the case of backbuilding convection. Chances for convective initiation are best in S Austria (Carinthia) and NE Italy (Friuli), where a level 2 is issued. Otherwise, initiation is questionable enough to stay with a level 1.

... W Mediterranean region including S France, E Spain and N Algeria ...

CAPE up to 1000 J/kg or locally even more develops in response to daytime heating of onshore/upslope flow and allows another round of scattered afternoon storms, supported by the remnants of a decaying upper-level low. In most of Spain and S France, weak vertical wind shear will keep the degree of convective organization low, and localized flash floods and marginally large hail are the main hazard.
Vertical wind shear rises towards the SE, owing similarly to increasing SW-erly flow aloft and to opposing sea breezes near the surface. GFS suggests 0-3 km shear between 10 and 15 m/s for the Spanish east coast and up to 25 m/s for coastal areas in Algeria. It is unclear whether these sea breezes will be heated enough to be consumed by convective updrafts. If they do, the risk for large to very large hail and severe downbursts increases, but uncertainties are too high for a level 2.