Storm Forecast
Valid: Tue 12 Jun 2012 06:00 to Wed 13 Jun 2012 06:00 UTC
Issued: Mon 11 Jun 2012 23:17
Forecaster: TUSCHY

A level 2 was issued for parts of Slovenia, Croatia, SE Austria, Hungary, parts of Slovakia, SE Poland, W/SW Ukraine and W-Romania mainly for severe to damaging wind gusts, large hail (an isolated very large hail event possible), a few tornadoes and locally excessive rainfall amounts.

A level 2 was issued for an area east/northeast of St. Petersburg mainly for a few tornadoes, large hail and severe wind gusts.

A level 1 surrounds both level 2 areas with large hail, severe wind gusts, an isolated tornado event and locally excessive rainfall likely.

A level 1 was issued for parts of N-Italy, E-Switzerland, W/NW Austria and SE Germany mainly for heavy to excessive rainfall and an isolated large hail event.

A level 1 was issued for parts of N-Germany and NE Netherlands mainly for heavy to locally excessive rainfall amounts.

SYNOPSIS

Given the magnitude of the broad upper trough over most of Europe, the calculated blocking index reveals quite low numbers, mainly caused by an overall smooth height gradient over most of the E-Atlantic and Europe and weak geopotential height minima in the trough itself. Nevertheless, ongoing blocking pattern assists in another round of active weather (convective-wise) for parts of C/E-Europe. Of main interest will be short-waves, which round the main trough and race to the E/NE. One is currently centered over S-France, fostered by a sharp moisture gradient along its tip (WV channel) and a structuring warm conveyor belt ahead (11th June, 21 Z). This feature exits NE-Italy until noon and lifts out to the NE thereafter (with a decrease in strength). Another impulse exits Belarus during the morning hours with a rapid progression to the north. This evolution assists in a broadening of the zone of CAPE/shear overlap and hence for an extensive area with organized DMC.

At the surface, a very complex surface pattern has evolved with latest map analysis placing a well defined wavy boundary over Switzerland and Austria, arching more towards SE Czech Republic further east, where one of numerous weak meso-beta scale lows is analyzed (11th June, 21Z). This quasi-stationary boundary oscillates north and south, modulated by enhanced segments of DMC and adjacent southward surging outflows. Arrival of a strong mid-layer impulse finally transforms that boundary in a gradually consolidating and NE ward moving warm front over E-Austria and Hungary around noon and a southbound moving cold front from C-Italy to the east (although near parallel alignment to the background flow keeps propagation speed to the south on the lower-end side). To pinpoint any developing surface depression is hard with the most likely development from E-Austria to the NE. However, an extensive W-E aligned surface pressure channel extends from W-Italy all the way to SE-Poland, where variable model outputs hint on numerous weak LL waves/depression in the prevailing SW-erly flow. Given that high and coarse resolved models all show the same surface pressure channel, confidence increases somewhat, that another baroclinic zone over Switzerland, Austria and S-Germany evolves during the forecast, as easterly flow to the north of the pressure channel advects moisture to the west.

DISCUSSION

Latest map analysis (21Z) places the 15°C isodrosotherm from far NE Italy to SE Austria and Hungary with the 20 °C isodrosotherm confined to the Aegean Sea up to the Ukraine. Latest soundings indicate that a deep moisture pool is still present with faint lowering of LL mixing ratios due to diabatic heating forecast. A sharp decline of magnitude and depth of BL moisture is noted over north C-Italy, Switzerland, W-Austria and most parts of Germany with surface dewpoints in the lower tens (however still offering a deep BL moisture coulmn in sounding data, at least for adequate CAPE build-up during the forecast).

EML, although still present, continues to fan out (especially over SE Europe), with somewhat better magnitude over Switzerland/ Germany and rapidly increasing values over NW Italy. This dispersal equilibrates aforementioned BL distribution and hence, C/E Europe will see a broad swath with 500-1000 J/kg MLCAPE and even higher values over Bulgaria, Romania and W-Russia (exceeding 2000 J/kg MCLCAPE).

Shear increases during the forecast, as a strong short wave lifts out of the trough's base with 30 m/s isoshear (6 km bulk shear) overspreading most of the warm sector (e.g. Croatia, Hungary and W-Romania). 0-3 km shear responds a bit later but reaches 15-20 m/s during the evening hours onwards. Right now, we declined impressive shear magnitude of the LL wind field along the depression's path due to convective feedback problems with GFS. However, LL shear increases during the afternoon hours onwards, reaching magnitudes, supportive for storm organization also at lower parts of the updrafts. Shear rapidly decreases on both sides of the jet (towards Germany and Bulgaria).

... N-Italy, Austria, Slovenia, Croatia, Hungary, Slovakia, W-Romania and parts of the Ukraine ...

Once again, overnight's convection plays an important role in exact DMC evolution. Latest satellite/radar data indicates a well anticipated (WRF) cluster of storms over S-Switzerland and numerous upslope bands of showers/thunderstorms over far N-Italy/S-Austria (as of 21Z). This convection continues to build northeastwards with a decrease in strength expected during the start of the forecast (still dampening the rate of diabatic heating over parts of the Alps and Germany due to anvil cirrus contamination). Further south and east, no significant overnight cluster is currently forecast (despite numerous smaller-scale thunderstorm clusters), so areas along and south of analyzed boundary should start with good insolation (here: level 2 area). Current thinking is that the main thunderstorm activity continues or evolves along the consolidating warm front over SE Austria, Slovenia and Croatia around noon with a rapid NE-ward propagation thereafter. Thunderstorm coverage increases during the afternoon hours, as dynamics and thermodynamics both increase and we expect numerous, large storm clusters to evolve out of this activity.

During the more discret stage of forecast storms (SE Austria, Croatia, Slovenia and Hungary), well organized multicells and a few long lived supercells are forecast with large hail and severe wind gusts the primary risk. Given long/straight hodographs, both splitting supercells and bowing line segments are likely with swaths of damaging wind gusts and an isolated extreme hail event. The rainfall risk remains hampered by rapid storm motion, but local flash flooding can't be ruled out (e.g. during cell merging). The final tornado risk depends on how strong the LL depression evolves and therefore assists in a backing LL wind field with augmented SRH-1. Current idea is that Croatia and parts of C/S Hungary see highest LL shear values. However, any boundary can locally provide strong helicity, so an isolated tornado risk can be ruled out nowhere. The overall tornado risk probably increases during the evening and first night hours, as strengthening LL shear and lowering LCLs go hand to hand. Betimes, storm-produced outflows probably merge into a cold pool driven MCS event (most likely over NE Hungary, E-Slovakia into W-Ukraine and W-Romania). Severe wind gusts, isolated large hail/an isolated tornado event and heavy rain accompany that feature. Given anticipated strength of shear and incoming forcing, we decided to upgrade parts of the level 1 area to a level 2.

Further west (here: Switzerland, parts of W/N-Austria and S-Germany), limited diabatic heating by decaying clusters from the night probably keep MLCAPE on the lower end side (200-500 J/kg), but stout impulse impacting with lingering moisture may yield numerous rounds of partially organized storm clusters. Right now, the main risk seems to be excessive rainfall, as some upslope component over the N-Alps within diffluent mid layer flow and high PWAT overlap. We can't rule out an isolated severe wind gust or marginal hail event in case of a temporarily organized cluster, but issued level 1 mainly covers the enhanced rain risk.

Signals over NW-Italy are double tracked with a moistening and gradually destabilizing BL layer behind overnight's/morning convection and steepening mid-level lapse rates. However, departing vort max assists in strong NVA/subsidence, which may indeed suppress DMC completely, as some models forecast. Right now, only isolated activity is anticipated and a low-end level 1 was extended far to the west with large hail/heavy rainfall the main hazard.

Further south (here: Bulgaria and parts of C/E Romania), initiation is again bound to orography and/or to any existing outflow boundary/enhanced moisture pool. As forcing increases during the night, isolated storms are likely and hence the level 1 covers most of Romania and parts of Bulgaria. With DLS of 20 m/s and MLCAPE in excess of 1 kJ/kg, any storm will become well organized/supercellular, posing a large hail and severe wind gust risk (an isolated extreme event is well possible). In the past, similar set-ups sometimes resulted in self-induced higher storm coverage and right now, we won't exclude an overnight MCS event over all of Romania, which may force an eastward expansion of the level 2 in later updates.

... N-Ukraine into W-Russia ...

The air mass from the past 2 days over SE Europe still covers the area of interest and is featured by high MLCAPE values between 1000-2000 J/kg. Most models agree well in the northward motion of a strengthening depression from SE-Belarus towards St Petersburg. A sharpening cold front leisurely shifts to the SE and assists in scattered to widespread thunderstorm initiation over W-Russia. Overlap of 15 m/s DLS and aforementioned CAPE assist in well organized multicells and isolated supercells with large hail and strong to severe wind gusts. Of main concern will be the eastern fringe of the evolving 995 hPa depression around/east of St.Petersburg, where surface wind field backs (both due to ageostrophic deflection of the LL wind field but also due to the sharpening warm front itself) and a few tornadoes are well possible (given up to 15 m/s LL shear and 200 J/kg SRH-1). We upgraded to a level 2 due to the enhanced probabilities for a few tornadic supercells. The level 2 may have be drawn more to the SE (high storm coverage of severe storms), but expected rapid clustering prevented a large level-2 for now. Later in the night, numerous ongoing clusters pose an heavy rainfall and isolated large hail/severe wind gust risk. We expanded the level 1 far to the N, to account for a few elevated overnight storms along the intensifying and northward moving warm front (into N-Komi).

... NE France, Benelux and parts of Germany ...

Adequate BL moisture beneath 500 hPa temperature of at or below (aob) -20 °C result in widespread SBCAPE of 500 to 1000 J/kg. Despite weak shear (aob 10 m/s), this amount of instability, forecast diffluent flow aloft and up to 150 J/kg LL CAPE may assist in a few strong to isolated severe thunderstorms with an isolated large hail and strong wind gust event. Given active convection and aforementioned LL CAPE, a 'mesoscale accident' can't be ruled out and hence an isolated tornado event will be mentioned. Given a persistent signal of a weak surface low over E-Germany in WRF/GFS/EZ, concerns exists that excessive rainfall amounts may become possible due to slow moving storms, which travel along the W-E aligned theta-e axis (from NE Germany to the Netherlands). Also, ageostrophic deflection of the wind mainly within the 1 to 3 km layer may result in a temporarily more organized multicell event. Reflecting that risk in the probability scheme, we issued a confined level 1 area. Thunderstorms gradually decay after sunset.

... Rest of the lightning areas ...

Mainly sub-severe storms are forecast beneath the upper trough. Activity will be daytime driven with stronger cells containing marginal hail. The activity diminishes after sunset.