Storm Forecast
Valid: Fri 17 Jun 2016 06:00 to Sat 18 Jun 2016 06:00 UTC
Issued: Thu 16 Jun 2016 20:12
Forecaster: TUSCHY

A level 3 was issued for E-Hungary, W-Romania to SW-Ukraine mainly for damaging hail and severe wind gusts.

A level 2 surrounds that level 3 for a lesser risk of extreme events. Still large/very large hail, severe wind gusts, excessive rain and an isolated tornado event are possible.

A level 3 was issued for W-Belarus, NE Poland and Lithuania mainly for an augmented tornado risk (a significant event is possible), damaging wind gusts, large or very large hail and excessive rain.

A level 2 surrounds that level 3 for a lesser risk of extreme events. Still large/very large hail, severe wind gusts, excessive rain and an isolated tornado event are possible.

A level 1 surrounds all other level areas. All kind of severe is possible, but no extreme events are forecast.

A level 1 was issued for S-Germany mainly for isolated large hail and an isolated tornado event.

A level 1 was issued for N/W-Germany, Benelux, parts of France and S-UK mainly for excessive rain and an isolated tornado event.

A level 1 was issued for the Balearic Islands and NE Spain mainly for large hail and excessive rain. An isolated tornado event is possible.

A level 1 was issued for far W-Russia mainly for large hail and excessive rainfall amounts.

... SYNOPSIS ...

Positive tilted upper trough remains anchored over W-/CNTRL- and N-Europe. A significant impulse evolves along its eastern fringe over the Czech Republic/far E-Germany and Poland. This vortex deepens rapidly along a pronounced baroclinic zone while moving to the N/NE. On the one hand, northeastbound surging tongue of high IPV values indicates the substantial dynamics, which accompany that process, on the other hand they highlight a pronounced dry slot, which works its way from the Czech Republich over Poland to the NE.

Accompanied LL-vortex features a strong deepening rate for this time of year with surface pressure falling to 995 - 990 hPa. Phase diagrams feature transformation towards a shallow warm-core vortex, which reflects an healthy looking wrap-around occlusion. This occlusion advects a warm/moist and rather unstable air mass to the depression's center. A blend of different models places the depression over N/NW Poland at 12 UTC and over the CNTRL Baltic Sea during peak strength around 00 UTC. There, models diverge regarding further timing and track during its approach towards Finland. Reasoning probably is growing uncertainty how fast this vortex becomes vertically stacked. Until then, models in general show a unique picture with EZMW a tad slower compared to the German model suite.

Regarding fronts, the aforementioned depression pushes a sharpening warm front from Belarus to Estonia with a pronounced cold front surging over NE Poland to the E/NE. Betimes, the warm sector shrinks and the occlusion becomes the main boundary, which affects Finland.
Trailing cold front opens out into a wavy boundary, which runs from the W-Ukraine all the way to S-CNTRL Italy. An older front (already occluded for most parts) extends from N-Finland to the White Sea to W-Russia and features no substantial eastbound motion during the forecast. Regions like CNTRL/W-Europe continue to be affected by weak pressure gradients, where numerous mesoscale boundaries once again act as foci for CI.

A dangerous outbreak of severe thunderstorms is forecast over parts of E-CNTRL Europe which includes large and damaging hail, severe wind gusts, flash-flood producing rain and tornadoes!

... Parts of Poland, parts of Belaurs, Lithuania, Latvia and Estonia all the way to S-Finland ...

NNE-ward moving and deepening cyclone over the Baltic Sea results in a responding low-/mid-tropospheric wind field over the warm sector, which covers the area of interest. Backed LL flow causes enlarged hodographs with SRH-1/3km of more than 250/200 m^2/s^2 respectively. Maximized backing occurs along and just to the south of northbound moving warm front. The warm sector features 1-2 kJ/kg MLCAPE with decreasing values towards Latvia and Estonia. Cap weakens during the day as diabatic heating sets in, although impressive deep-layer forcing most likely erodes existing cap quite early. LCLs drop to 1 km or less, especially along the warm front, where moisture pooling offers abundant BL moisture (BL mixing ratios in excess of 12 g/kg). This moisture also offsets only modest mid-level lapse rates. Those weak lapse rates could lessen the overall hail risk a bit.

Two foci exist. The first one is the warm sector convection over Lithuania, NE Poland and W-Belarus. Cap probably suppresses initiation until noon. Between 12-18 UTC however, arriving deep-layer lift and ongoing diabatic heating should result in scattered supercells with all kind of hazards, including large hail (isolated very large hail possible with mature supercells despite modest mid-level lapse rates), severe wind gusts and excessive rain. The tornado risk is enhanced in the warm sector and a significant tornado event is possible for storms near the warm front.

The second focus is the NEward surging cold front with dry slot approaching from the SW. Main uncertainty is ongoing DMC from the previous night over SW/CNTRL Poland. Despite some temporal weakening during the forenoon hours (still capable of producing severe events!), this convection re-intensifies rapidly when crossing E/NE Poland. Intense 30 m/s speed max at 700 hPa approaches this convection from the SW and a potential scenario will be a developing severe MCS, which races to the N/NE. Widespread severe/damaging wind gusts will be the main hazards, although expected shear will probably support a few tornado events as well. Large hail is also possible and combined with severe wind gusts, damaging wind-driven hail could be a life-threatening risk!

Due to the high coverage of severe thunderstorms, a confined level 3 was issued.

Over N-Estonia and S-Finland, convection becomes increasingly elevated with the main hazard being heavy rain. However in case of local near surface based convection, an ongoing isolated tornado risk still exists.

... W-Ukraine to E-Croatia ...

Very warm 850/700 hPa temperatures overspread most of SE-Europe and cause robust capping. This cap gradually erodes along its western fringe due to marginal cooling and faint forcing by passing short-waves. Main focus for CI will be a SE-ward trailing frontal boundary, which interacts with the rough orography like the Carpathians and the Dinaric mountains. Isolated to scattered thunderstorms erupt in an environment with 1-2 kJ/kg MLCAPE and MUCAPE peaks of more than 3000 J/kg! Very fat mid-level CAPE profiles overlap with 30 m/s 0-6 km shear. 25 m/s 0-3 km shear support well organized and long-lived supercells with a damaging hail threat (hail diameter in excess of 5 cm is likely). A deep and well mixed subcloud layer also assists in very intense downdrafts, which probably cause a few hurricane-force wind gust events - combined with the hail, life-threatening storms with high-end impact are possible. Another confined level 3 area was issued despite ongoing uncertainties of final thunderstorm coverage.

During the night, supercells grow upscale into one or two severe MCSs (mainly driven by intense cold pools) and hence the level 2 was expanded far to the E/SE. Severe/damaging wind gusts, large or very large hail and an excessive rainfall risk exist. With a cooling/moistening BL, the tornado risk also increases.

... Extreme NE Germany, NW Poland, covering most of the Baltic Sea and E-Sweden ...

Impressive wrap-around occlusion advects a very moist/unstable air mass to the depression's center and therefore beneath the left exit region of an astonishing (for June) 60 m/s upper-level jet. Deepening vortex supports influx of that air mass to the CNTRL/S-Baltic Sea before weakening during the overnight hours in response of a weakening trend of that vortex. Forecast soundings inside that occlusion show 700-1000 J/kg MUCAPE and PWATs of 30-40 mm. Unidirectional shear within this pronounced deformation band probably results in embedded enhanced/banded convection, which causes excessive rainfall amounts. 12 hourly rainfall amounts beneath that deformation zone will probably be in the range of 40-80 mm with locally higher amounts. Therefore an high-end level 1 and a level 2 were issued for the S-CNTRL Baltic Sea (level 2 where maximized thunderstorm activity exists). Due to ongoing track uncertainties of that depression, also SE/E-Sweden was included into the enhanced rainfall risk.

... Balearic Islands to E-Spain ...

First chance for isolated severe storms already exists with ongoing convection from the previous night. A SW-NE aligned low-pressure channel weakens during the day, which results in a breakdown of the BL flow and in the evolution of numerous more or less pronounced convergence zones. Those will be a focus for isolated CI, as cap should be overcome at least on an isolated scale. Temporal drying of the low/mid-troposphere however limits overall coverage.
Main focus for CI resides on the final approach of the mid/upper trough during the night, where steepening mid-level lapse rates and cooling low-/mid tropospheric air atop warm SSTs result in weakly capped 1-1.5 kJ/kg SBCAPE.
DLS oscillates aoa 20 m/s during the day with enhanced LL shear along the convergence zones during the day and in general during the night. Hence, well organized multicells/isolated supercells with large hail, excessive rain and strong to isolated severe wind gusts are forecast. The tornado threat is enhanced along the convergence zones, but also exists with any supercell during the overnight hours.

... Pyrenées and NE Spain ...

Diurnal heating and a sharpening mid-level trough over the S-Bay of Biscay result in backed winds over NE Spain, which support the onshore advection of a modified maritime air mass. As BL spreads to the Pyrenées, steep mid-level lapse rates atop cause MLCAPE of 1 kJ/kg along the coast with decreasing values to the N/NW. Forecast soundings show thickening mid-level CAPE profiles with moderate shear of 15 m/s in the hail growth layer and hence the main risk will be large hail. In addition, an isolated severe downburst event is possible next to the Pyrenées with a deep/dry subcloud layer and an isolated tornado event is possible along the coast with a more humid BL air mass and stronger LL shear.

... N-Italy, far S-Austria and NW-Slovenia ...

As the main upper trough refuses to push east and keeps its positive tilt, deep SW-erly flow regime continues to affect N-Italy. Hence, BL moisture remains rather high with BL mixing ratios of 8-9 g/kg. Steepest mid-level lapse rates exist over NW Italy and along the S-Alps, where 500-1000 J/kg MLCAPE is forecast. Dependant on the exact shape of the trough and timing/track of short-waves, DLS oscillates aoa 15 m/s. Hence expect a few organized multicells/supercells with large hail and strong to isolated severe wind gusts. Betimes, thunderstorms cluster and the heavy rainfall risk increases. Southern most storms (which f.ex. approach the N Adriatic Sea) have to be monitored for and low-end tornado risk as depth of BL moisture increases with somewhat stronger backed BL flow. The risk gradually diminishes during the night.

... N-France, Belgium/parts of the Netherlands to N/NW-Germany and S-UK ...

Ageostrophic response to the deepening LL vortex to the east results in a pronounced confluent flow regime over the area of interest (mainly N-Germany). Complexity increases as a few models develop a second circulation over Benelux, which would result in another convergence zone from N-France to NW-Germany. Similar to the previous days, forecast soundings show very moist profiles with elongated/thin CAPE profiles and very low LCLs but also augmented LL lapse rates (mainly diabatically driven) with substantial LLCAPE build-up. Hence another round of slow moving and gradually clustering thunderstorms is forecast to evolve during the forenoon hours. Those clusters leisurely spread east and excessive rain is likely. Also, aforementioned favorable LL CAPE forecast and augmented 0-2 km convergence signals indicate enhanced probabilities for another round of a few funnel/isolated tornado reports.

... S-Germany ...

Just north of the Alps, a passing short-wave during the afternoon hours adds enough synoptic-scale lift for at least isolated CI. 15 m/s DLS with 200 m^2/s^2 SRH-3 support an isolated well organized multicell/supercell event. Large hail and strong/isolated severe wind gusts would be the main hazard, although dropping LCLs during the evening may offer a short time-frame for an isolated tornado event. This activity moves to N-Austria and weakens after sunset.

... W-Russia ...

A broad level 1 was issued for slow moving storms. 1-1.5 kJ/kg MLCAPE but weak DLS support pulsating thunderstorms with excessive rain and large hail. An high-end level 1 covers that risk. Activity diminishes after sunset.