Storm Forecast
Valid: Fri 27 Aug 2010 06:00 to Sat 28 Aug 2010 06:00 UTC
Issued: Thu 26 Aug 2010 20:15
Forecaster: TUSCHY

A level 2 was issued for N-Italy, parts of Slovenia and S-Austria mainly for large to very large hail, severe wind gusts and a few tornadoes.

A level 2 was issued for parts of Poland mainly for tornadoes, severe wind gusts and an isolated large hail risk.

A level 1 surrounds both level 2 areas with similar risk, but with a lower storm (expected report) coverage.

A level 1 was issued for parts of E-France and S-Germany mainly for strong to severe wind gusts, a few tornado events and marginal to isolated large hail.

SYNOPSIS

A potent upper trough over Ireland shifts towards the east and enters the North Sea during the latter part of the forecast. Around this base of this trough, numerous upper waves, embedded in a strong W-/SW-erly flow, cross W/central Europe, which fosters unsettled conditions over a broad area. Hot and stable conditions prevail for the Mediterranean.

As a side-note, environmental conditions northwest of the Azores become increasingly supportive for a potential subtropical development. A closed upper low NW of the Azores meanders to the east/northeast/north and models like GFS/EZMWF indicate a gradually developing warm core structure during the upcoming 48 h. However, subtropical development during the forecast period (24 h) seems unlikely due to the merging of a quasi-barotropic depression, with a baroclinically coined one from the north, so rapid strengthening of those morphing features is forecast with a pressure drop, justifying the term "explosive development". Despite the interesting synoptic background of this feature, it will also increase the blocking over the E-Atlantic, which is supported by the latest geopotential height gradient index forecast for the N-Atlantic.

DISCUSSION

... N-Italy, parts of Slovenia and most parts of Austria ...

A potent upper trough is about to cross the Alpine region around noon onwards with a sharp cyclonic kink seen in upper-level streamline pattern analysis. Despite some flattening of the amplitude during its NE-ward progression, still abundant PVA forecast to cross the area from the SW with a concentrated UVV maximum. BL airmass features Italian-style values for late summer, as dewpoints oscillate around 20°C. Yesterday's strong cap weakens with ongoing CAA, so initiation over N-Italy probably occurs around noon and onwards. Mid-level lapse rates remain modest, so MLCAPE will probably already peak at 1000 J/kg. Shear will be strong ( 30 m/s 0-6 km bulk shear, nearly the same magnitude in the lowest 3 km with strong directional shear signals). Storms, which move off the mountainous area tap into a very moist BL and supercells are well possible. Large hail and severe wind gusts will be the main risk with tornado chances increasing for discrete, long-lived supercells. Another concern is the augmented chances for bowing segments with a damaging wind gust risk, especially if storms line up due to the strong forcing/intense LL shear.

Thunderstorms move rapidly to the east/northeast with favorable conditions for organized, long-lived supercells over parts of Slovenia and S-Austria, moving along the gradually SE-ward dragging cold front. The passage of that cold front will also mark the end of the severe episode with model pool indicating the later afternoon/early evening hours, where CAPE values decrease. Till then, large to very large hail, severe wind gusts and an isolated tornado will be possible, if storms enter that area in a more discrete fashion. Another possibility is rapid clustering over NE-Italy, which would increase the severe wind gust risk (organized MCS with bowing segments) and decrease the tornado risk. Models still diverge, but mesoscale data currently support more discrete storm structures. Hence the level 2 was expanded well to the east. Locally heavy rainfall spreads eastwards and affects E-Austria, W-Hungary and parts of Slovakia during the evening/night hours although progressive nature of this event ought to limit the excessive rainfall risk.

... Poland ...

A gradually deepening depression enters Poland during the morning hours with a rapid east/northeastward movement thereafter. The warm sector overspreads most parts of S/central Poland during peaktime heating, so modest CAPE (locally up to 800 J/kg SBCAPE) could be realized in a concentrated swath. 20-25 m/s 0-6 km and 0-3 km bulk shear support organized updrafts, potentially long-lived, if a few discrete storms manage to evolve. BL airmass is warm and moist (low LCLs) and strongly sheared with augmentend speed/directional shear, supportive for LL updraft rotation. All this indicates a risk for tornadic supercells, also accompanied by large hail and severe wind gusts. If discrete storms indeed evolve, an isolated strong tornado event can't be ruled out. Overall severe risk rapidly diminishes after sunset, as BL stabilizes and CAPE wanes.

... Parts of France and S-Germany ...

Prolonged period of enhanced convection forecast with 2 strong waves affecting the area of responsibility. The first one crosses S-Germany around noon (please refer to the paragraph about Italy, Slovenia and Austria) and assists in scattered to widespread shower/thunderstorm initiation from E-France to S-Germany. Shear increases towards Germany with abundant shear forecast for organized deep moist convection. Strong to severe wind gusts and an isolated large hail risk go along with stronger storms. LCL around 600 m, roughly 150 J/kg LL CAPE, 150 m^2/s^2 SRH-1 and 10-15 m/s LL shear overlap mainly over S-Germany, so a few tornadic cells will be well possible.

The second round of storms occurs during the evening and night hours over E-France and S-Germany, as another sharp upper trough races eastwards with a strong UVV maximum. CAPE will be on a rapid decline, but a forced line of storms is partially indicated by mesoscale model data. The main risk will be marginal hail and a few severe wind gust reports.