Storm Forecast

Storm Forecast
Valid: Mon 12 Jun 2017 06:00 to Tue 13 Jun 2017 06:00 UTC
Issued: Sun 11 Jun 2017 23:51
Forecaster: PISTOTNIK/GATZEN

A level 1 was issued for E Germany, Poland, parts of Lithuania and Belarus mainly for severe wind gusts, large hail and tornadoes.

A level 1 and level 2 were issued for SE Germany, the Czech Republic, Slovakia, Austria, and parts of Hungary and Slovenia mainly for large hail, severe wind gusts and to a lesser extent for tornadoes.

A level 1 was issued for N Italy, Switzerland and the French Massif Central for large hail and excessive precipitation.

A level 1 was issued for SW Turkey for large hail and excessive precipitation.

SYNOPSIS

Between a deep cyclone over S Scandinavia and a subtropical ridge over the W Mediterranean, an unseasonably strong mid-level jet streak runs from the British Isles to Belarus. It overspreads a plume of warm and moist subtropic air that has temporarily entered central Europe, but will be pushed eastward by the cold front of the Scandinavian cyclone. This process provides an interesting setup for a possible severe weather outbreak!
Another focus for convection is the vicinity of a cut-off low over the Aegean Sea.

DISCUSSION

... Poland, Belarus, Czech Republic, SE Germany, Austria and around ...

The model pool is confident that 2m dewpoints around 15C will create a belt with a few hundred J/kg of CAPE ahead of the cold front all the way north to the Polish coastline. Further south, the more aggressive scenarios (e.g., WRF and GFS) with 2m dewpoints around 19C would yield CAPE up to 1500 J/kg in SE Germany, the Czech Republic and Austria. Lifting mechanisms are manifold, though none of them is utterly prominent: At upper levels, a distinct short-wave trough will cross Poland and later Belarus, and a second, weaker and more fragile trough will dig SE-ward into Austria. Near the surface, the passage of a prefrontal convergence line will bring a wind shift from SE to W (along with the highest dewpoints), while convergence signals at the cold front itself are modest.
To sum it up, the more conservative scenarios indicate a robust overlap of limited CAPE, manifold but patchy lift mechanisms, and strong vertical wind shear (around 20 m/s across the lowest 3 km). The more aggressive scenarios increase both CAPE and forcing sufficiently to expect a possible outbreak of strong and long-lived supercells.
Long and straight hodographs indicate a high probability for any forming storms to split into pairs of right- and left-moving supercells. Towards evening, when cold air advection makes low-level winds veer towards the NW, even left-movers (with anticyclonic rotation) may be favored, though the convective activity should be on the downswing by then. Despite the limitation of rather weak lapse rates, any of these organized storms may bring large hail and severe downbursts. In addition, 0-1 km shear around 10 m/s and a possible favorable interaction of secondary storms with pre-existing outflow boundaries may suffice to create a few tornadoes.
However, remaining uncertainties make this forecast particularly challenging. Convective initiation may be dispersed across a wide area, but focal points are difficult to find. It appears likely that initially elevated convection will emerge from mid-level clouds under the influence of the trough's lift, which may then take some time to break the capping inversion (left by overnight's warm air advection) and to root down to the surface. If this happens already in the morning, first organized and severe storms may appear as far west as E and SE Germany. The focus will quickly shift east with any further hour that passes before surface-based storms form. This probability is maximized from central Poland to central Austria, where the various lift mechanisms may come best together during the peak heating hours. However, it cannot even be ruled out that convection may largely fail to initiate or to root down to the surface (see e.g. the Polish UM model, which hardly shows any precipitation signals anywhere south of Prague).
Convective initiation is a fairly safe bet in the N part of the discussed area, centered over Poland. Due to the unclear situation how early or late (and hence how far west or east) it will occur, though, we stick to a large level 1. The wind risk will be maximized there, in particular if storms should morph into bowing line segments. Concerning the areas further south with the strongly varying model outputs, latest observational data show dewpoints as high as predicted (15-18C) in the warm air tongue (placed across E France and Germany at 22 UTC on Sunday). In addition, also the southern short-wave trough (headed for the area of maximized CAPE around Austria on Monday) is already well visible on water vapor satellite imagery. Confidence in the more aggressive forecast scenarios with at least scattered, long-lived supercells therefore increases, and Austria and surroundings were upgraded to a level 2 due to a possibility of numerous severe and even isolated extreme weather events (i.e., hail >5 cm or wind gusts >32 m/s).
Storms will move east in the evening and night and will gradually turn elevated and weaken while they move across Belarus and into the Ukraine, Slovakia and Hungary. Severe weather risk, mainly in form of wind gusts, remains enhanced for a while but decreases betimes.

... N Italy across S France into N Spain ...

The tail of the cold front becomes stationary and inactive over Switzerland and central France. Hot air is present to its south, and low-level moisture will accumulate beneath a capping inversion due to evapotranspiration and converging low-level winds. Forecast models show patches of CAPE up to 1500 J/kg where this moistening is most effective. Vertical wind shear is increased over Switzerland and Italy, which are still grazed by the mentioned mid-level jet, and more locally along the Spanish coastlines where sea breeze fronts will establish.
Pulse storms are expected in the afternoon, which may turn multi- or supercellular especially over the Italian Alps in case updrafts can penetrate the capping inversion long enough. Large hail is the main risk. In addition, one or two flash floods are not ruled out due to anchoring of storms over orographic features. A level 1 was issued for the exposed regions from the Italian Alps to the French Massif Central. Further west, the stronger cap and a lack of synoptic lift support makes convective initiation too unlikely to issue a level 1 or a high probability thunder area.

... SW Turkey ...

Steep lapse rates allow CAPE up to 1000 J/Kg over the Turkish mainland in response to daytime heating, and synoptic lift ahead of the Aegean cut-off low as well as local lift by onshore and upslope winds allow scattered afternoon storms. Vertical wind shear is low in general but may be locally enhanced by orographic influences on the low-level wind field. Single cells and multicells are expected, and a few large hail and excessive rain events are possible.

... S Norway and S Sweden ...

Daytime heating of the polar air mass will allow scattered to widespread thundery showers. With weak vertical wind shear and low cloud bases, localized flash floods are not ruled out. Inflow/outflow interactions may also create a short-lived, non-mesocyclonic tornado. These risks appears just too marginal for a level 1.

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