Storm Forecast
Valid: Fri 06 Jun 2014 06:00 to Sat 07 Jun 2014 06:00 UTC
Issued: Fri 06 Jun 2014 06:15
Forecaster: PISTOTNIK

A level 1 was issued for W France and S England for severe wind gusts and to a lesser extent for excessive precipitation, large hail and tornadoes.

A level 1 was issed for E Spain, S and Central France mainly for large hail and severe wind gusts.

A level 1 was issued for W-most Russia, parts of Belarus and the Ukraine mainly for large hail and excessive precipitation.

A level 1 was issued for E Romania, E Bulgaria and N Turkey mainly for excessive precipitation and to a lesser extent for waterspouts.

SYNOPSIS

Between a pronounced cyclone over the Atlantic Ocean near 45°N and a weak ridge over Central Europe, a deep Southwesterly flow with strong warm air advection spreads over Spain, France, England, BeNeLux and Germany. This configuration will open up a prolonged severe thunderstorm episode in these countries.

Further East, a compact upper-level low slowly moves from Denmark into Southern Sweden. Rather low geopotential connects it with a second upper-level low over Turkey. An anticyclone sits over Western Russia.

DISCUSSION

E Spain, France, England

Under strong insolation and a capping inversion created by the warm air advection, rapid diurnal heating and moistening of low-levels can be expected. Forecast models agree on the buildup of CAPE on the order of 1000 to 2000 J/kg in France till the evening with realistically looking surface dewpoints of 16-18°C. The instability maximum is expected in the Northern foreland of the Pyrenees. Deep-layer shear strengthens to 20-25 m/s, and at least slightly enhanced shear and helicity will also be present at lower levels. The conditions look prime that any forming storm can turn into a well-developed supercell. Main uncertainties will be if convective initiation can be achieved with such a strong cap, and if forming updrafts can grow persistent enough to withstand entrainment of very dry mid-level air.

Diurnal timing of large-scale lift is adverse to widespread storm development: after a first short-wave departs eastward until 09 UTC, peak daytime heating hours will rather see large-scale subsidence over most of Spain and France. Only in the evening hours, stronger vorticity maxima will start to overspread the Bay of Biscay and Western France. Though their exact timing and placement is still uncertain, their contribution to lift will likely erode and finally break the cap. The most plausible scenario is that elevated storms will form over the Bay of Biscay in the evening and move northward into Western France (Pays de la Loire, Bretagne) overnight. Severe wind gusts will be the main risk. Other kinds of severe weather (excessive precipitation, large hail and tornadoes) are also possible, but do more critically depend on the question if storms will be able to root down to the surface. If they succeed, they can fully benefit from moist low-level air and further improving wind profiles overnight.
Late in the forecast period, elevated storms will likely extend into Southern England with a slowly decreasing severe weather risk.

Further East and South, isolated storms may form over orographic features of France and Eastern Spain between noon and sunset, but will struggle to still penetrate the strong cap as soon as the detach from their points of initiation. If they succeed, supercells with a risk of large hail and severe downbursts are forecast, an even an isolated extreme event is possible.
Relatively highest probability for initiation seem to exist over Southwestern France in the wake of the Pyrenees, where Northeasterly upslope flow meets with Northwesterly onshore flow from the Bay of Biscay. This region will also see a second chance for initiation overnight when an outflow boundary will move onshore in the wake of the aforementioned MCS. However, with slim to none precipitation signals in all models, confidence in convective initiation is not high enough for a level 2 despite the impressive thermodynamic and kinematic setup.

Alps

Isolated afternoon storms will form over the Alps, which are placed under the Eastern fringe of the broad warm air advection regime. Along the Northern Alpine rim of Bavaria and Austria, CAPE on the order of 500-1000 J/kg and a further improvement of favourably veering profiles by NE-erly upvalley flow would support organized convection, but the cap will likely be too strong to allow initiation.
Apart from that, both CAPE and shear seem to be too weak to enhance the severe weather risk sufficiently for a level 1. An isolated large hail event cannot be excluded, though. More storms will only form and possibly persist into the first half of the night over Northwestern Italy, which gets overspread by a vorticity maximum. The rest of the activity will mostly diminish around sunset.

Finland, Baltics, Russia, Belarus, Ukraine

Hot and dry air from Russia will slowly advect westward and overspread a belt of rich low-level moisture. With temporarily suppressed mixing, CAPE can be boosted to values in excess of 1000 J/kg in this belt, especially over Belarus and surroundings. Deep-layer shear is mostly weak and will exceed 10 m/s only near the Western rim of the unstable air mass. Scattered to widespread storms will form in response to daytime heating. Stronger pulse storms can bring localized large hail, and the isolated spin-up of a spout-type tornado is possible. Later on, storms will likely merge into several large clusters which will either move northward or follow erratic, outflow-driven paths while they persist well into the night. Excessive precipitation can be responsible for a few flash flood events.
Within the hot and dry air mass over Russia, isolated high-based storms may form in the afternoon. Strong evaporative cooling may promote a severe downburst.

E Romania, E Bulgaria, Turkey

Widespread thunderstorm activity can also be expected further South, but thermodynamic profiles are moister and allow only some hundred J/kg of CAPE under weak vertical wind shear and onshore flow from the Black Sea. Excessive precipitation will be the main concern. One or two waterspouts are possible in the morning along the Black Sea coast.

E Poland, Lithuania, S Scandinavia

Isolated to scattered afternoon storms will form near the core of the upper-level low and along a decaying occlusion which wraps around it. Limited CAPE and weak vertical wind shear should preclude any severe weather.