Storm Forecast
Valid: Sat 13 Sep 2014 06:00 to Sun 14 Sep 2014 06:00 UTC
Issued: Fri 12 Sep 2014 23:14
Forecaster: PISTOTNIK

A level 1 was issued for the S Baltic Sea and surroundings for excessive precipitation and to a lesser extent for tornadoes.

Level 1 areas were issued for E Germany, W Poland, W Slovakia, E Czech Republic and E Austria for excessive precipitation.

A level 1 was issued for SE Italy, the N Ionian and S Adriatic Sea, Montenegro, E Bosnia-Hercegovina, Serbia and Hungary for excessive precipitation and to a lesser extent for large hail and severe wind gusts.

A level 1 was issued for Albania, FYROM and parts of Greece mainly for excessive precipitation and to a lesser extent for waterspouts.

A level 1 and level 2 were issued for E Spain mainly for large hail and severe wind gusts.

SYNOPSIS

Between the subtropical jet over the southern Mediterranean region and the polar jet over Iceland and northern Scandinavia, a broad "no man's land" covers most of Europe. It is characterized by several compact upper-level lows and otherwise weak geopotential gradients.
The strongest upper-level low is stationary near the Slovenian-Croatian border. Warm air advection and a series of pivoting vorticity maxima create lift on its southern and eastern flank from the Balkans towards Poland and continue to induce several surface cyclogeneses. Another small upper-level low - without any signal at low levels - slowly drifts from southern Sweden into eastern Germany. High-resolution models even agree on a third geopotential minimum prowling around over southwestern Poland.
The surface pressure field is more homogeneous with a large anticyclone over the North Sea and most of Scandinavia and broad low pressure over the Balkans, the central and eastern Mediterranean region. The mentioned small upper-tropospheric features in the absence of noteworthy surface signals make this forecast particularly tricky.

DISCUSSION

... S Baltic Sea and surrounding coastlines ...

A long fetch of northeasterly winds over the Baltic Sea creates a belt of augmented low-level moisture with surface dewpoints around 16°C per latest observations (Fri 21 UTC) and precipitable water above 30 mm. In vicinity of the upper-level low, limited CAPE is expected to build and "lake effect" showers and low-topped thunderstorms become likely on Saturday. They may align into bands parallel to the background flow, which can result in localized high rainfall accumulations when they move onshore in southern Sweden, on the Danish islands or in northeastern Germany.
On the eastern flank of the upper-level low, a confined area of increased 0-3 km wind shear around 15 m/s and veering wind profiles may overlap with this unstable area. More discrete convection might therefore even develop rotating updrafts with the possibility of an isolated tornado or two.
Activity will move more deeply into Germany overnight while it slowly decays.

... Poland, E Germany ...

A few hundred J/kg of CAPE are forecast to build in response to daytime heating. Due to a history of warm air advection, convective initiation will likely happen rather late. Scattered thunderstorms can form in the late afternoon and merge into several clusters in the evening. Despite uncertain details and litle agreement of the forecast models, a level 1 seems to be warranted around the Polish-German border, where precipitable water above 30 mm, slow storm motions and the expected high coverage indicate a risk of localized flash floods. Weak vertical wind shear and limited instability should preclude other severe weather.

... SE Italy into Montenegro, Bosnia-Hercegovina, Serbia, Croatia, Slovenia, Hungary, Austria. Slovakia, Czech Republic ...

A second, stronger surge of warm air advection overspreads these regions from the south. Friday's 12 UTC soundings (Budapest, Szeged, Belgrade) confirmed that CAPE around 500 J/kg can easily build after some hours of insolation, and scattered thunderstorms are expected to form from noon onwards on Saturday. Deep-layer shear is enhanced to values around 20 m/s as a small jet streak curls around the upper-level low from central Italy into Hungary. Storms may therefore organize into multicells or a few supercells, and marginally severe hail and wind events are not ruled out as long as they stay discrete. However, with large-scale lift in place, low cloud bases and weak capping, storms will likely merge into clusters soon and flash floods become the main risk in the evening and overnight (precipitable water between 30 and 40 mm and possible training storms).
Strong ascent in the left exit of the jet streak may keep some of these storms going while they move into western Slovakia, Moravia and Lower Austria especially in the second half of the forecast period. They will become elevated and embedded into stratiform clouds and can therefore not benefit from the vertical wind profiles any more, which would be excellent near the nose of the warm air advection. However, heavy rainfall with a risk of localized flooding may still be an issue.

Scattered storms will also still go on near the base of the upper-level low over Slovenia, Croatia and over the Adriatic Sea. In its wake, drier and cooler air spreads southward and suppresses thunderstorm activity over Italy and the Central Mediterranean region.

... Albania, FYROM, Greece, Turkey ...

Scattered storms are expected in an environment of low to moderate CAPE and weak vertical wind shear. Storm coverage will be highest in the afternoon over land. After sunset, the majority of remaining storms will propagate offshore along land breeze fronts.
A level 1 was issued for a risk of heavy precipitation, as well as isolated waterspouts overnight.

... E Spain ...

With moderate southwesterly mid-level flow, an elevated mixed layer from Morocco overspreads southern and eastern Spain and the adjacent Mediterranean Sea. Over the maritime boundary layer with dewpoints around 20°C, moderate to high CAPE will result, possibly up to 2000 J/kg (refer to Fri 12 UTC's Palma sounding). Deep-layer shear increases from 10 m/s to more than 20 m/s from northwest to southeast across the Iberian Peninsula.
Onshore and upslope flow is forecast to feed some moisture inland along the Spanish east coast, where the sea breeze front with dryline characteristics will act as the main trigger mechanism for thunderstorms. On synoptic scales, the passage of a weak short-wave trough indicates that some lift will be present until noon, followed by subsidence afterwards. Due to this unfavorable timing, storm coverage will likely not be too high, but any forming storm may turn into a well-organized multi- and supercell with a risk of large hail (possibly even exceeding 5 cm in size) and severe downbursts.
A level 2 was issued for a small area in eastern Spain where convective initiation appears likely enough under the robust CAPE-and-shear overlap. It is surrounded by a level 1 to the south, where the stronger capping makes convective initiation less likely, and to the north, where vertical wind shear is weaker. However, some models suggest that activity may go on through the evening and well into the night in Catalonia, which seems plausible in case outflow boundaries trigger secondary convection. Flash floods would become an additional risk in this case.
Storms will quickly decay when they move offshore, where a strong cap is in place.

... Portugal and NW Spain ...

Rich Atlantic moisture is advected inland and allows some hundred J/kg of CAPE in response to daytime heating. Scattered, mostly unorganized storms are expected, especially in the afternoon. Severe weather is unlikely.