Storm Forecast
Valid: Sat 21 Jul 2012 06:00 to Sun 22 Jul 2012 06:00 UTC
Issued: Fri 20 Jul 2012 23:15
Forecaster: TUSCHY

A level 2 was issued for parts of the Ukraine mainly for a swath of severe to damaging wind gusts, large to very large hail and an isolated tornado event.

A level 1 surrounds the level 2 and expands to Italy mainly for large hail (an isolated very large hail event likely), strong to severe wind gusts and heavy rain.

SYNOPSIS

An amplifying upper trough over C-Europe dictates the weather over most of Europe. Brisk SW-erly flow still overruns a surface boundary over E-Europe, which results in a broad area with enhanced and organized DMC probabilities. The Mediterranean remains once again capped by a hot air mass, before the severe risk increases significantly during the upcoming days. Cool and unsettled conditions persist over N-Europe with numerous daytime driven showers and thunderstorms expected.

DISCUSSION

... N-Italy, SW-Austria, Slovenia, Croatia, Serbia, Bosnia and Herzegovina, Romania and the Ukraine ...

Very complex forecast scenario evolves in the area of interest due to the interaction of various mesoscale alpha and beta forcing mechanism.

A wavy and quasi-stationary baroclinic zone is hard to analyze in surface maps with somewhat stronger gradients in the 1-2 km thetae fields. A first,weak depression at mid-levels moves from Slovakia to the Ukraine with backing winds upstream, so the quasi-stationary front transforms into a cold front over the N-Balkan States with no serious net motion elsewhere. During the day, a weak lee cyclogenesis over the N-Italy also keeps the western part of the boundary in place with marginal motion to the south.

Abundant periods with moderate forcing are forecast as numerous disturbances emerge from the base of the main trough over the Alps, which ride to the NE mainly to the north of the synoptic boundary. As a result, a strong diabatic heating gradient evolves along the boundary with cloudy skies to its north and clear skies to its south. Hence a gradually strengthening thermally direct frontal circulation may help to produce a few storms, although the coverage will probably be more isolated with this forcing mechanism (e.g. N-Serbia and W-Romania during the daytime hours). Also, the strong UVV maximum of the upper trough itself starts to affect N-Italy/the N-Adriatic Sea during the night with storms spreading to the east. Further east, over the Ukraine, a combination of weak forcing of the cyclone, which moves from Slovakia to the Ukraine and a transient coupled mid/upper jet configuration assist in another hot spot for more widespread DMC over parts of the Ukraine.

To summarize that, we expect early CI south of the Alps which builds to the S and E, also affecting Slovenia and Croatia. Only sporadic CI is forecast to the east over N-Serbia and W-Romania during the daytime hours. During the late afternoon hours onwards, rapid and widespread CI is forecast over W-Ukraine, building eastwards. Also during the night, another round of scattered storms evolves over N-Italy, N-Adriatic Sea, Croatia, Bosnia and Herzegovina, parts of Serbia and W-Romania as the main upper trough approaches.

500-1000 J/kg MLCAPE, 20-25 m/s shear in the lowest 3 km and noticeably augmented SRH values along the boundary all assist in well organized multicells and supercells. Mature cells will be capable of producing large hail (a few very large hail events are likely), strong to severe wind gusts and an isolated tornado event . I'm a bit concerned about a swath of severe to damaging wind gusts over far W-Ukraine and into C-Ukraine with a 20 m/s jet max at 3 km overrunning the boundary, so went ahead an upgraded that area to a level 2 as long lived bow echoes seem possible.

The risk of thunderstorms continues along the boundary all night long, although the severe risk gradually decreases with diminishing CAPE and mainly elevated storm activity (especially where southbound cold front undercuts eastward moving mid/upper forcing over the N-Balkan States). However, forecast soundings indicate that not much BL modification is needed to get a few near surface based storms going over W-Romania, so will also include an isolated severe wind gust risk next to large hail and heavy rain.

Heavy rain also affects areas just north of the boundary, where storm clusters (mainly elevated in nature) may produce numerous rounds of heavy rain over the same areas. However, with a gradually southward motion of the main synoptic front, no focus is seen for excessive rainfall and hence the level 1 was kept inside the lightning areas. An exception is far NE-Italy, SW-Austria and far W-Slovenia with prolonged period of strong SW-erly flow towards the Alps and orographic/background lift all day long. Would not be surprised to see a few 100 l/qm/24h reports, but confidence in the exact spot remains too low for a heavy rainfall level 2 right now. Nuged the level 1 more towards the north over Austria to reflect the risk.

... Parts of Germany ...

The passage of the upper trough axis during peak heating with surface dewpoints in the lower tens assist in moderate SBCAPE build-up in the order of 500-800 J/kg. Speed shear up to 15 m/s is strong enough for a few better organized multicells with marginal to isolated large hail and strong wind gusts. Heavy rain accompanies storms with ill defined storm motion vectors forecast. Despite a gradual decrease in thunderstorm intensity after sunset, storms may keep going well into the night over SE-Germany/W-Czech Republic in case a weakly organized storm cluster evolves.

A marginal level 1 would have been possible over N-Bavaria, but as only a few storms will temporarily acquire an organized stage, we keep this forecast with lightning areas only.

... Rest of the lightning areas ...

Weak shear and CAPE of around 500 J/kg assist in pulsating storms with marginal hail and strong wind gusts. The thunderstorm activity decays after sunset.