Storm Forecast
Valid: Sat 12 Oct 2013 06:00 to Sun 13 Oct 2013 06:00 UTC
Issued: Sat 12 Oct 2013 09:11
Forecaster: PISTOTNIK

A level 1 was issued for S Italy, Albania, Montenegro, Bosnia, Croatia, Slovenia and Hungary mainly for severe wind gusts, large hail and excessive rain.

A level 1 was issued for parts of Finland and NW Russia for severe wind gusts.

SYNOPSIS

A cut-off low over France has established as the dominant steering center. It has brought a body of unseasonably cold air over West-Central Europe and much of the Alps, while a tongue of warm and moist air wraps around the low from the Adriatic Sea into East-Central Europe and Poland. These two air masses are separated by a pronounced but slow-moving cold front. A series of powerful vorticity maxima rotates around the Southern, Eastern and Northern flank of the cut-off low, overrunning the postfrontal cold air with their better parts but also overlapping with the cold front, along which various mesoscale cyclogeneses start and decay in response. Model consistency is very low and makes this forecast particularly challenging.
This cut-off low is surrounded by a belt of higher geopotential, most notably a pronounced, negatively tilted ridge from the Black Sea all the way to Southern Scandinavia and the Northern Atlantic. Calm and mild conditions prevail in this belt.
Further downstream, a Northwesterly jet extends across Finland and Russia. A new strong cyclone travels along this jet. Its warm sector brings a short relief from a recent first winter greeting, only to be followed by another, even stronger outbreak of polar air late in the forecast period.

DISCUSSION

... Central Mediterranean to W Balkans ...

An elevated mixed layer from the Sahara becomes picked up by the Southwesterly flow and overspreads a moist maritime boundary layer with dew points between 17 and 21°C. This results in moderate to strong instability (on the order of 1000 to 2000 J/kg), but also a pronounced cap. Deep-layer shear around 15 m/s would be sufficient for an organization into multicells or isolated supercells, but in the absence of large-scale forcing confidence in convective initiation is limited. Certainly a good deal of persistent Southerly onshore flow is needed to trigger isolated to scattered storms.
Towards the Northwestern fringe of the unstable area, grazing vorticity lobes and the tail of the cold front provide more lift to erode and break the cap. Deep-layer shear increases to 25 m, while CAPE magnitude decreases. A severe-looking MCS has formed over the Tyrrhenian Sea in recent hours, but seems to decay now while it makes a landfall in South-Central Italy. Similarly well-organized storms can form till late in the evening, while their focus gradually shifts from Italy towards the West Balkans. Activity will slowly weaken overnight.
All in all, a level 1 was issued for much of the discussed areas, for a risk of large hail mainly towards South (steeper lapse rates) and a risk of severe wind gusts mainly towards North (stronger shear and forcing). Upscale growth of storms may also result in localized excessive precipitation. The tornado threat seems to be not particularly enhanced, given there is limited low-level shear and not too much low-level buoyancy, yet a tornado cannot be ruled out in case of supercells.

... Northern Adriatic Sea to East-Central Europe ...

The fairly pronounced but temporarily inactive cold front will soon be revived by the passage of healthy vorticity maxima. The prefrontal belt of warm and moist air is characterized by limited CAPE (a few hundred J/kg) and impressive shear (20-30 m/s between 0 and 6 km) and helicity.
With the wind shift of the cold front and the strong forcing aloft, it seems likely that some multi- or even supercells will form over the North Adriatic Sea in the afternoon hours. Upscale growth into one or two MCSes is possible, possibly with a pronounced sidewards propagation as they are undercut by the cold front, which would result in a northeastward rather than a northward motion. A level 1 was issued for a chance of severe wind gusts and (marginally) large hail, supplemented by a risk of excessive rain with onshore and upslope flow. Confidence is high enough that the storms can be sustained even well inland into Northern Bosnia, Croatia and Hungary in the evening hours, supported by a prefrontal tongue of dewpoints up to 16°C that is advected Northward with a low-level jet. Storms will gradually become elevated and decay overnight, but until then some late-season severe weather events are well possible. The seasonal lack of instability precludes a level 2.
Further North, forcing and shear remain equally strong, but the instability signals fade away, though the models all agree on at least weak patches of CAPE all the way into Western Poland and Eastern Germany. The strongest short-wave trough will arrive in the evening, and isolated storms are forecast to form within the narrow band of precipitatiom then. However, since the instability will be increasingly elevated after sunset (and in general near the Western fringe of the moisture tongue), severe weather potential seems to be low.

... Other parts of Central and West-Central Europe ...

Models largely agree that the dominant cyclogenesis along the cold front will happen somewhere between the Netherlands and England overnight, boosted by latent heat release from the North Sea. A few lightning strikes are possible towards the end of the forecast period in its core. Depending on its exact position and strength (which is still unclear), marginally severe and non-convective wind gusts are then possible in Southeastern England, the Normandy and Belgium.
The large body of cold air centered over France and the Western Alps will feature steep lapse rates but very limited moisture. A few showers are possible but will likely not become deep enough to produce thunder.

... Western Mediterranean ...

With moderate Westerlay flow, a frontal wave with an attendant short-wave trough will move into Northern Spain and Southern France later in the forecast period. It will bring lots of rain but no convection on land, until its cold front may pick up first appetizers of instability in the Western Mediterranean overnight. Otherwise, the increasing CAPE reservoir towards the South will likely remain untouched on Saturday due to the strong cap.

... Finland, NW-most Russia ...

The cold front of a strong cyclone moves southward Saturday night, overspread by a dry intrusion and a powerful short-wave trough. This strong forcing will likely create neutral to marginally unstable profiles in an environment of intense shear (20-30 m/s between 0 and 3 km). A shallow and fast-moving convective line is expected, and a level 1 was issued for severe wind gusts. Slight doubts about an involvement of convection preclude a level 2. Nonetheless it needs to be mentioned that even non-convective gusts can exceed 25 m/s.
Further post-frontal convection in a weakly-sheared environment may become deep enough to produce isolated thunder in the Russian oblasts of Murmansk and Archangelsk.