Storm Forecast
Valid: Fri 09 Aug 2013 06:00 to Sat 10 Aug 2013 06:00 UTC
Issued: Thu 08 Aug 2013 23:05
Forecaster: PISTOTNIK

A level 1 was issued for SE Finland and NW Russia mainly for excessive rain and tornadoes.

A level 1 was issued for parts of Poland, the Czech Republic and Austria mainly for excessive rain and to a lesser extent for severe wind gusts.

A level 1 was issued for the Southern Alpine region, parts of the Adriatic Sea and Italy for excessive rain, large hail and severe wind gusts.

SYNOPSIS

A cut-off low moves from Corsica to the Southern Adriatic Sea during the forecast period, while the de-amplifying parent trough shifts from France into Central Europe. A fairly strong upper-level flow rounds these features, with >20 m/s at 500 hPa from the British Isles to Sicily and from the Baltic States into Russia.
Near the surface, a low-pressure system moves from Finland into Northern Russia. Its long cold front extends all the way southwestward across Belarus, Poland, the Czech Republic and the Alpine region into Italy. Ahead of this front, a still very hot air mass with an African origin is advected to higher latitudes over Eastern Europe, supported by (and in turn supporting) a broad upper-level ridge centered over the Ukraine.
The atmosphere starts to recover from the transient break of the mid-summer heat wave over Western Europe, where a humid Atlantic warm air advection regime re-establishes over the British Isles and a subtropical high spreads into the Iberian Peninsula.

DISCUSSION

... SE Finland, NW Russia ...

Latest observations (Thu 21 UTC) confirm rich low-level moisture in the warm sector of the Finnish low with widespread dew points in the 18 to 20°C range. Even despite rather poor lapse rates, this should result in low to moderate CAPE on Friday. Subtle vorticity maxima move over the region and will facilitate convective initiation ahead of and along the cold front. At least scattered storms are expected to form in an environment of ~20 m/s deep-layer shear. Low cloud bases and low-level shear in excess of 10 m/s point to an enhanced risk of a tornado or two with this activity. In case a storm can organize into a supercell, severe wind gusts and large hail are not ruled out, though the weak lapse rates and the low cloud bases are somewhat detrimental. In the course of the day, convection can grow into large clusters with a threat of excessive rain while it continues to move eastward.
The latest observations seem to support the ECMWF solution, which show a slower motion of the cyclone into Russia, compared with GFS. This would also place SE Finland into the same favourable environment, which is why the level 1 was extended there mainly for excessive precipitation. The bulk of the convective activity will likely be embedded in the stratiform rain shield of the low, but any convective updraft which manages to stay more isolated could spin up a (weak) tornado, too.

... the Baltics, Belarus, Poland, Czech Republic, Slovakia, Austria ...

Despite an impressive-looking CAPE-and-shear overlap in the model forecasts, convection struggled hard to form along the slowly eastward moving cold front in the last two days. Only the radiosondes revealed that this CAPE was in fact lower and confined to a belt behind the surface cold front, where cooler but moister air with Northwesterly surface winds undercut the hot and dry Southwesterly flow. The resulting overlap of CAPE around 1000 J/kg and enhanced deep-layer shear of 15-20 m/s was in most places too strongly capped to enable convective initiation. In contrast, the very hot pre-frontal air was deeply mixed, which resulted in stunning profiles with surface temperatures up to 41°C on Thursday (e.g., new all-time temperature records in Austria), but surface dew points too low (~10°C) to produce any substantial CAPE.
According to the model forecasts, this separation of postfrontal moisture and shear from the prefrontal steep lapse rates will become even more pronounced on Friday: In the absence of any frontogenetic forcing, the cold front will likely continue to move eastward as a dry and shallow ana front till the afternoon hours. Confidence in convective initiation is low, and even if storms form, they will have difficulties to become or stay surface-based.
More and more storms will only start to form in the afternoon along the main Alpine crest in NE Italy and Austria, which blocks the inactive front for a longer time. Together with increasing vorticity advection, this may result in several big precipitation clusters with embedded convection which then travel northeastward into the Czech Republic and Poland in the evening and night hours. Though most of the convection will likely be elevated, isolated severe wind gusts are possible, and the motion parallel to the almost stationary front means that heavy precipitation over several hours poses a threat of localized flooding.

... Southern Alpine region, Western Balkans, Italy ...

A highly complex situation evolves in the wake of the Alps. The low-level cold front will be blocked by the Alpine chain, but accelerated around its corners until its two branches are forecast to converge with gusto over the Adriatic Sea late in the forecast period. The Southern half of Italy and (later) the Adriatic Sea see enhanced deep-layer shear (15-20 m/s) and QG lift ahead of the upper-level low, whereas forcing is merely neutral and shear rapidly decreases further North near the saddle point of the 500 hPa field.
On Thursday, observations showed very hot and dry conditions inland and some "knee-deep" moisture along the shores, meaning that convective initiation stands and falls with low-level winds that converge long enough to significantly deepen the maritime moisture or advect it inland. Current thinking is that scattered thunderstorms will go on or form early in the forecast period over the Ligurian Sea, Corsica and the Tyrrhenian Sea, then the main activity will shift to the Italian mainland and the Southern Alpine region over noon. Large hail, severe wind gusts and excessive precipitation are possible. Parallel to that, first afternoon storms are also possible in the hot, dry and weakly sheared air mass over orographic features of the Western Balkans, which will bring little precipitation but may cause isolated (dry) downbursts.
A second round of convection is forecast in the evening and night hours, when the cooler but significantly moister flow around the Eastern Alps will enter the scene from the Northeast. Together with increasing upper-level forcing, scattered to widespread storms may form over the Western Balkans and the Adriatic Sea. With Northeasterly surface winds (bora) but still Southwesterly flow aloft, convection may remain stationary for a longer while or slowly backbuild towards the Italian East coast. Excessive rain is possible, and the combination of fairly strong instability release over the warm water and deep-layer shear around 15 m/s may still be sufficient to produce isolated large hail or severe wind gusts.

... North Sea and coastal areas ...

Showers and isolated, low-topped thunderstorms are expected in the well-mixed, weakly sheared maritime air beneath the base of the long-wave trough. An isolated waterspout near the shorelines is not ruled out.