Storm Forecast
Valid: Tue 27 Aug 2013 06:00 to Wed 28 Aug 2013 06:00 UTC
Issued: Mon 26 Aug 2013 17:15
Forecaster: PISTOTNIK

A level 2 was issued for N Algeria and N Tunisia, and a level 1 for much of the W and Central Mediterranean, Italy and the W Balkans for large hail, severe wind gusts, excessive precipitation and tornadoes.

A level 1 was issued for the Alpine region mainly for excessive precipitation.

SYNOPSIS

An extensive upper-level low with several shallow centers stretches from Belarus and the Ukraine across Central Europe into France. The subtropical jet stream curves around its Southern flank, where strong vertical wind shear and cooling upper levels on top of the very warm sea set the stage for an active severe weather day over much of the Mediterranean region. The main foci for convective initiation are a long and diffuse frontal boundary across the Balkans and the Mediterranean and a series of passing vorticity maxima in its wake.
Further North, a large anticyclone slowly moves from Southern Scandinavia into Western Russia, connected by a bridge to a second high over the Atlantic Ocean. The polar jet and the main frontal zone are displaced to Northern Scandinavia, which gets crossed by a cold front and a progressive trough late in the forecast period.

DISCUSSION

... E Balkans into the Ukraine ...

A frontal wave is forecast to develop over Northern Romania and Moldova on Tuesday morning and to move into the Eastern Ukraine during daytime. WRF shows the strongest solution that would include a surge of rich Black Sea moisture into the warm sector, which features enhanced 0-6 km shear (15-20 m/s) and 0-3 km storm relative helicity (100-150 m^2/s^2). However, due to the diverging model forecasts it is doubtful if these shear values will overlap with more than just a few hundred J/kg of (possibly elevated) CAPE further inland, where lots of clouds from the warm front and from the previous day's convection will likely persist. Isolated to scattered, partly embedded thunderstorms are possible, but the severe weather threat does not seem high enough to warrant a level 1. Stronger instability release will probably be confined to coastal areas, where a slight risk of excessive rain might evolve. Marginally large hail and severe wind gusts are not ruled out in case of more discrete storms, either.
The wake of this frontal wave will leave a diffuse cold front across the Balkans. With decreasing lapse rates but still some residual low-level moisture, low-end CAPE can rebuild in an environment of 15-20 m/s deep-layer shear. Isolated to scattered and mostly non-severe thunderstorms are expected here as well, though not as many as on Monday any more.

... W and Central Mediterranean, Italy, W Balkans ...

The same diffuse boundary extends all the way across the Mediterranean region, where it comes to a temporary halt by a weak cyclogenesis over Central Italy while it continues to slowly move southeastward in the sea area between the Baleares and Algeria. Converging low-level winds and suppressed mixing by a strong cap help to push surface dew points into the 20 to 26°C range (confirmed by Monday's observations) on its warm side, overspread by several tongues of a Saharan elevated mixed layer. This combination will likely result in an impressive overlap of 2000-3000 J/kg CAPE with 20-30 m/s deep-layer shear, especially towards the Algerian and Tunisian coast (refer to Mon 12z's Dar El Beida and Tunis soundings).
Since synoptic-scale lift support fans out towards the South, the crucial question is in how far the lift along the boundary and/or upslope flow alone will suffice to break the cap, which makes this forecast particularly delicate. Without getting too much lost in details, isolated to scattered thunderstorms are forecast to travel eastward throughout the forecast period. Any storms that form can quickly turn into supercells or well-organized MCSes with a threat of large hail, severe wind gusts and excessive precipitation. Low cloud bases and various patches of enhanced low-level shear and helicity point to the possibility of one or two tornadoes, too.
Due to a lack of obvious hot spots for initiation, a broad level 1 area seems to be the best choice for the moment. The only exception is the North coast of Algeria and Tunisia, where Northeasterly onshore flow in response to the approaching boundary and to a thermal low over the Atlas mountains maximizes vertical wind shear and pumps abundant maritime moisture onshore. Despite rather reluctant precipitation signals in most of the forecast models, confidence in convective initiation is high enough to upgrade to a level 2 for a possible outbreak of supercells with all kinds of severe weather as soon as the cap breaks in the afternoon to evening hours, including a risk of very large hail. With upscale growth into one or two MCSes, serious flash flooding is also a possibility!

Behind the diffuse cold front, the still warm sea surface helps to maintain dew points in the 16 to 20°C range from Eastern Spain via much of the Western Mediterranean into Northern Italy, Slovenia and Croatia, which is sufficient for the buildup of almost uncapped CAPE on the order of 500 to 1000 J/kg beneath cool upper levels. A series of passing vorticity lobes further facilitates convective initiation. Scattered to widespread thunderstorms are expected throughout the forecast period over the sea and with an afternoon and evening maximum over land.
Vertical wind shear decreases and retreats to the jet levels towards the North, which makes it increasingly difficult to be consumed by the not particularly high-topped storms. However, with a remaining 0-6 km shear of 15 m/s, an organization into multicells with isolated events of large hail, severe wind gusts and excessive precipitation is still well possible. The most vigorous surge of convection will likely cross Northern Italy in the evening hours in response to the strongest short-wave trough aloft. Especially offshore convection over the Ligurian and Northern Adriatic Sea can go on all night long with a noteworthy risk of waterspouts, as the vertical wind shear further drops overnight.

... France, S Germany, Switzerland, Austria ...

A few hundred J/kg CAPE are forecast to form in response to diurnal heating, and scattered to widespread thundery showers are expected in the weakly sheared environment beneath the upper-level low. They will possibly grow into unorganized clusters and become ingested by large-scale rain areas later on. Localized flooding is possible. Low cloud bases, good low-level CAPE release and a cyclonic background wind field point to the possibility of a few funnel clouds and brief landspouts where vorticity can be concentrated along wind shift lines.