Storm Forecast
Valid: Sat 23 Aug 2014 06:00 to Sun 24 Aug 2014 06:00 UTC
Issued: Fri 22 Aug 2014 22:25
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for parts of Italy, the Balkans, Hungary, Romania and Bulgaria for large hail, severe wind gusts, excessive precipitation and to a lesser extent for tornadoes.

A level 1 was issued for some coastlines of S Norway, Sweden, Finland, Estonia, Germany and Denmark for waterspouts.

SYNOPSIS

A broad long-wave trough resides over Central Europe. Its base is surrounded by moderate to strong mid-level flow, which exceeds 20 m/s at 500 hPa from France to Poland and from the Western Mediterranean to the Balkans.
Near the surface, a mature cyclone over Southern Norway and a long, diffuse frontal boundary from the Baltic States across Hungary and Northern Italy to Northern Spain are the primary features. Far Eastern and far Southern Europe as well as Iberia are under anticyclonic influence.

DISCUSSION

... Central Italy into the Balkans, Hungary, Romania and Bulgaria ...

Rich low-level moisture is advected inland from the warm sea surfaces and collected around the diffuse frontal boundary. It partly overlaps with a plume of steep lapse rates that spreads from Northern Africa east- to northeastward over Italy and the Balkans. This combination results in CAPE between 500 and 1500 J/kg on the warm side of the frontal boundary, probably peaking above 2000 J/kg over the Adriatic Sea. Towards the South, capping will get very strong, though (refer to Fri 12 UTC's impressive Brindisi sounding).
Weak warm air advection and a short-wave trough, which will translate from Italy into Romania, provide large-scale lift and favorable conditions for convective initiation over a wide area to the South of the frontal boundary. With constantly increasing Westerly mid-level flow, deep-layer shear around or above 20 m/s will be realized.

At least scattered storms will already be active in the morning near the trough axis over Northern Italy, while still some insolation is in place further East and South. From noon onwards, intensifying storms are expected to spread into Slovenia and Croatia, and further convection will initiate over orographic features of the Balkans and other sites of enhanced low-level convergence in vicinity of the main frontal zone. Another focus of initiation will be provided by an outflow boundary in the wake of the morning storms, which will likely propagate southward over the Adriatic Sea and enhance the sea breeze convergence along the Italian East coast later in the day.
Storms have good chances to organize into multi- and supercells, and a large level 2 area seems to be warranted mainly for large hail and severe wind gusts. Later in the afternoon or evening, it is possible that the activity will culminate in a bowing line, which would preferably ride eastward along the frontal boundary and pose a risk of widespread severe wind gusts in Southern Hungary, inland Croatia, Northern Bosnia-Herzegovina and Northern Serbia. However, the position of the frontal boundary is difficult to forecast and subject to modulations by mesoscale wave developments. For example, Friday's 12 UTC model runs have substantially delayed and deepened a forming frontal wave in the Hungarian sector, which was only weakly indicated in earlier forecasts. Such a development could also retard the formation of thunderstorms and possibly bring them more northward into Hungary than anticipated.
Further East over Romania and Bulgaria, stronger warm air advection and the later arrival of the trough suggest that storms will tend to form later and might even be elevated first. However, as soon as they become surface-based, they can benefit from similar CAPE and shear values and can organize equally well, which is why the level 2 was extended all the way to the Black Sea coast.

In addition to the primary risk of large hail and severe wind gusts, a few tornadoes are possible as well. They may form in areas where temporarily suppressed vertical mixing can maintain enhanced low-level shear. This is most notably the case in the sea breeze regime along the Italian East coast, where even a strong tornado cannot be ruled out in an environment of abundant CAPE, low cloud bases and 0-1 km shear above 10 m/s!

By the evening, numerous storms - including several large clusters - are expected to travel over the Balkans, Romania and parts of Bulgaria as the trough axis proceeds eastward. The main risk will then shift to excessive precipitation which may cause a few flash floods. Storms will slowly weaken but will continue through much of the night.

... N Italy ...

The further development after the first wave of morning storms is somewhat uncertain. However, a weak Genoa cyclogenesis may re-establish a Southeasterly flow of warm and moist air. With the aid of a few hours of insolation, some hundred J/kg of CAPE could become available under deep-layer shear that will rise from 15 to 25 m/s in the course of the day. Isolated to scattered multi- or even supercells are possible throughout the forecast period. A level 1 was issued for large hail and severe wind gusts. In addition, the Southern Alps may see a period of heavy rainfall with a possibility of embedded thunderstorms.
In each morning hours (i.e., at the beginning and again towards the end of the forecast period), a few waterspouts may occur over the Northern Adriatic Sea, where the cyclonic background wind field is further concentrated by converging land breezes.

... W Mediterranean region ...

Forecast models show spotty precipitation signals in an environment of robust CAPE and strong deep-layer shear over the Western Mediterranean Sea and Northern Algeria. However, with a strong cap in place and a lack of lift, they are likely representing elevated convection originating from Altocumulus clouds with little or no lightning activity.
If at all, isolated surface-based thunderstorms may form in the afternoon in Eastern Spain in case the sea breeze can once more bring thick moisture onshore. Any such storm could organize very well, but the probability of initiation is too low for a level 1.

... N Europe ...

A well-mixed and weakly sheared polar air mass has invaded large parts of Northern Europe. Numerous vorticity maxima round the base of the long-wave trough and overspread the North and Baltic Sea plus their surroundings. Scattered to widespread thundery showers are expected throughout the forecast period over the sea and mainly in the noon and afternoon hours over land.
A few waterspouts are possible, especially in the second half of the forecast period when the coldest upper levels arrive, the background flow starts to relax and nocturnal land breezes provide convergence zones which are favorable for a spin-up. Level 1 areas were issued where this chance seems to be highest.