Storm Forecast
Valid: Sun 09 Jul 2017 06:00 to Mon 10 Jul 2017 06:00 UTC
Issued: Sat 08 Jul 2017 21:45
Forecaster: TUSCHY

A level 2 was issued for NE-Italy and SE-Austria mainly for very large hail, severe wind gusts and excessive rain. An isolated tornado event is possible.

A level 2 was issued for parts of France, Switzerland and Germany mainly for excessive rainfall, large to very large hail and severe wind gusts.

A level 1 surrounds the level 2 for similar hazards but with less coverage.

A level 1 was issued for parts of Latvia, Lithuania, Belarus and NW-Ukraine mainly for isolated large hail, severe wind gusts and an isolated tornado threat.

A level 1 was issued for parts of W-Russia mainly for an isolated tornado event and an isolated large hail and severe wind gust threat.

A level 1 was issued for N-Spain mainly for isolated large hail and severe wind gusts.

SYNOPSIS

The zonal flow regime from UK to the Ukraine weakens a bit and becomes more wavy in response to an approaching strong and mobile trough over the far E-Atlantic/Bay of Biscay with attendant downstream development response (ridging over CNTRL-Europe and a deepening trough over E-Europe). Phasing is forecast between the aforementioned Atlantic trough and a long-lived upper-level cold-core low over SW-Europe, which opens and lifts to the NE. Both features merge into a broad trough over W-Europe during the end of the forecast.

This mid-/upper level pattern is accompanied by a broad warm sector over CNTRL-Europe, encompassed by a wavy front which runs from the English Channel to N-Germany to S-Poland and to the far W-Black Sea at 12 UTC. The air mass south of this boundary features high moisture content and is partly covered by a NE-ward fanning EML plume (emanating from Algeria/Tunisia to the NE). Lower BL moisture and weaker mid-level lapse rates result in less CAPE build-up over SE-/E-Europe.

DISCUSSION

... France to the Alps and S-Germany ...

As the mid-/upper-level impulse lifts NE, a constant weakening trend is anticipated downstream of the approaching main trough over the Bay of Biscay. This weakening wave however interacts with a warm and moist air mass, so adequate lift within a weakly capped environment and with enough orographic support exists for widespread CI.

Kinematic set-up bisects the broad area of interest in two separate regions:

a) The NE-ward lifting impulse is accompanied by a 20 m/s mid-level flow, pushing 0-6 km bulk shear into the 10-20 m/s range over N-Italy and S-/E-Austria. Rich BL moisture is verified by latest surface data with surface dewpoints in the lower twenties at a few places over N-CNTRL Italy. Despite some WAA cirrus clouds, not much shielding for diabatic heating can be expected, so insolation and attendant heating will erode any existing cap by noon ... in time with the approaching impulse. Steep mid-level lapse rates atop yield strong MLCAPE signals in the 1.5-2.5 kJ/kg range with even higher values in local models. Combined with the shear, a favorable environment for long-lived multicells/ isolated supercells will be the result.

The main hazard will be very large hail in excess of 5 cm and severe wind gusts ... the latter risk partially by diurnal BL mixing but also later-on with cold-pool driven activity in case of upscale growth into small clusters. Although not striking, environmental conditions could support an isolated tornado event especially for cells maturing in the very moist air mass over N-CNTRL Italy and later-on with lowering (daytime driven) LCLs over S-Austria (including the more complex orography, which regionally enhances LL shear). Storms spread E/NE into SE-/E-Austria, Slovenia and probably also into W-Croatia with the aforementiond risks. After sunset, both nocturnal BL stabilization and decreasing CAPE along the E-edge of the CAPE plume should result in a gradual weakening trend over Slovakia and Hungary. We expanded the level 1 far east however to account for internal storm dynamics, which could drive storms further east to what numerical models propose.

b) DLS drops substantially towards France and S-Germany with DLS in the 5-10 m/s range as a result of a diffluent and also divergent upper flow pattern. Forecast soundings show very moist and deep profiles with adequate lapse-rate for MLCAPE in the 0.8-1.5 kJ/kg range for France and 1-2 kJ/kg range over S-Germany. Effective PWs gradually increase to more than 35 mm, which is also advertised by local models and forecast storm motion varies between 15-25kt over France and 5-10kt over S-Germany. In addition, high-level flow remains enhanced with 1-8 km shear of 30 kt, which could support some ventillation of updrafts (evacuating precipitation) and a prolonged separation of up-/downdrafts.

The main question will be the coverage of storms as rapidly eroding cap and increasing forcing should insert scattered to widespread CI. However, any initiating storm will be intense with large/isolated very large hail (> 5cm in diameter) possible as DLS seems to be still adequate for organized storms when combined with those CAPE values (next to some model guidance offering 15 m/s DLS over far S-Germany, which would be enough for isolated supercells). Before forward propagation starts by developing cold pools, quasi-stationary storms pose a threat for flash-flood producing rainfall amounts (locally in the extreme range). Betimes, upscale growth into messy but still strong to severe clusters is forecast with an ongoing risk of large hail and excessive rain. Strong to isolated severe wind gusts remain confined to local wet downburst events or cold-pool meso-high/-low pressure perturbations.

The W-Alps reside between both regimes but with a similar robust severe risk. Mixed mode of organized multicells/isolated supercells cause all kind of hazards with a few extreme (e.g. rainfall) events possible.

Another area with a substantially severe risk arises over W-France, where the incoming main upper trough with impressive deep forcing interacts with the CAPE plume during the afternoon/evening, resulting in a big MCS and potential MCC event. This cluster poses all kind of hazards inlcuding large to very large hail, excessive and locally extreme rainfall and severe wind gusts. This cluster will push NE and we expanded the level areas accordingly to the NE to account for potential model problems regarding internal thunderstorm dynamics. In addition, DLS increases to 20 m/s over W-France, which should support organized convection all night long.

During the night, ongoing clustering and lowering CAPE should limit the severe risk over CNTRL Europe, while storms gradually spread north with an ongoing isolated large hail and heavy/locally excessive rainfall risk.

... Lithuania, Latvia, Belarus to the Ukraine and far W-Russia ...

Along/east of a quasi-stationary occlusion, rich BL moisture offsets meager mid-level lapse rates with 400-800 J/kg MLCAPE forecast. DLS increases from 10 m/s to 20 m/s while values increase from east to west with strongest flow confined to the front itself. This supports a few organized multicells and isolated supercells with large hail and strong to isolated severe wind gusts. Enhanced LL flow along the front itself may add an isolated tornado risk with LCL heights in general of less than 1000 m due to moisture pooling.This risk diminishes after sunset.

A similar risk exists further to the NE over far W-Russia along another occlusion, although DLS is weaker with mostly 10 m/s deep-layer shear in place. Much stronger LL shear (including augmented SRH) results in a higher tornado risk but with a more conditional large hail and severe wind gust risk. This risk also vanishes beyond sunset as CAPE decreases.

... N-Spain ...

Scattered well organized multicells with large hail and severe wind gust are forecast to develop over N-Spain during the day ahead of the approaching upper trough. NW-erly onshore flow advects highest BL moisture (marine air mass) to far N-Spain, where strong orographic convection is anticipated (400-800 J/kg MLCAPE and 20 m/s DLS). This activity spreads east during the night with an ongoing isolated large hail risk (especially when the activity approaches NE-Spain, where onshore flow advects rich moisture to the NW, keeping MLCAPE in the 400-800 J/kg range until 06 UTC).