Storm Forecast
Valid: Sun 14 Jul 2019 06:00 to Mon 15 Jul 2019 06:00 UTC
Issued: Sat 13 Jul 2019 16:48
Forecaster: TUSCHY

A level 2 was issued across extreme E-CNTRL Spain and the Balearic Islands for large to very large hail, severe to extremely severe wind gusts, heavy rain and an isolated but potential significant tornado risk.

A level 2 was issued across parts of the Golf of Genova, NE Corsica, the Tyrrhenian Sea, far W-Italy and far S-Italy including Sicily for large to very large hail, severe to extremely severe wind gusts, heavy rain and an isolated but potential significant tornado risk.

A level 2 was issued across parts of the Aegean Sea, E-/NE Greece and W-Turkey for large to very large hail, severe to extremely severe wind gusts, heavy rain and an isolated tornado risk.

The surrounding level 1 areas have similar hazards with lower probabilities.

A level 1 was issued for parts of S-Norway/Sweden mainly for isolated large hail and heavy rain. A low-end tornado risk exists.

A level 1 was issued for parts of E-CNTRL Europe mainly for isolated large hail and heavy rain. A low-end tornado risk exists.

A level 1 was issued for parts of the S-Black Sea and far N-Turkey mainly for isolated large hail and heavy rain.

SYNOPSIS

Ridging centered along 10 W faces an extensive area with negative standardized geopotential height anomalies over the Mediterranean and E/NE Europe with one peak over Bulgaria into Turkey and another one evolving around/south of the Maritime Alps.
Main players in today's outlook are numerous more or less pronounced short-waves which circle the ridge and trough axes and affect broad areas over Europe. Only the major waves will be mentioned:

a) A mobile upper trough starts out over Benelux/NE France and drops south towards the Maritime Alps during the night. It is accompanied by a broad area with surface pressure fall over the CNTRL Mediterranean during the night.

b) A closed upper low over N-Spain opens up into a progressive and zonally aligned wave, which moves east. It gets captured by trough a) somewhere over Sardegna and Corsica and continues its track east towards the Tyrrhenian Sea during the night. Weakening commences during the night as it moves into the mid-level short-wave ridge ahead of the dominant trough a).

Elsewhere further longwave and short-wave troughs certainly play a role in today's outlook but are placed in a less favorable environment for organized DMC activity.

Surface fronts are hard to find and to follow with a surge of cooler/drier air over the Adriatic and Ionian Sea to the south probably the most important feature during the forecast period.

DISCUSSION

... S-Norway, parts of Sweden, S-/SE-/E-Germany, parts of SW/S-Poland, the Czech-Republic, Austria and the extensive corridor from the Baltic States to Bulgaria ...

Placed beneath low geopotential heights with cold mid-levels atop a warm/moist low-tropospheric layer creates widespread weakly capped to uncapped 400-800 J/kg SBCAPE. Peak values aoa 1000 J/kg are possible along mesoscale convergence zones / in confluent low-tropospheric flow regimes where pockets of mean LL mixing ratios aoa 10 g/kg can be realized. These areas are not prone to diurnal/diabatical mixing.
Weak shear/steering flow with DLS of 10 m/s or less is in place, so early, slow-moving and rapidly clustering convection is forecast. Initiating storms bring isolated large hail but the main risk will be localized flash flooding - especially along convergence zone. Amount of LLCAPE and expected messy LL flow regime probably result in a few short-lived tornado events, too. This activity weakens beyond sunset but a few lingering storms are possible until 06Z.

... NE-/E-Spain and the Balearic Islands ...

Beneath the base of the eastbound travelling mid-level wave, weak DLS and 400-800 J/kg SBCAPE offer an environment for scattered weakly organized but slow moving thunderstorms with heavy rain and small hail. This activity weakens beyond sunset.

Of concern is a corridor from extreme E-CNTRL Spain to the Balearic Islands where rich BL moisture (Td readings in the upper tens/lower twenties) overlap with moderate mid-level lapse rates, offering 1-2 kJ/kg MLCAPE.
A 35 m/s mid-level speed max crosses the area from W to E until 18 UTC and spreads out an impressive kinematic environment. Point source forecast soundings from the Balearic Islands at 12Z show 35 m/s 0-6 km shear, 20 m/s 0-3 km shear and SRH-3 in excess of 150-200 m^2/s^2. Directional shear is maximized along a zonally alinged surface pressure channel over/just N of the Balearic Islands.
Constantly improving synoptic- and mesoscale lift favors CI with long tracked supercells possible. Large to very large hail and severe to isolated extremely severe wind gusts will be the dominant threat. However with strong LL veering profiles and better LL shear along the LL confluence zone/surface pressure channel, a tornado threat (with a potential significant event) is possible.

Further E (offshore of the Balearic Islands) NWP guidance agrees surprisingly well in showing a lull of QPF although ingredients remain in place (with slight decoupling of best shear/CAPE). In addition cap remains a bit stronger. However any organized DMC activity from the W may survive for some time and hence we kept a level 1 for this area with a low prob. lightning area to reflect increasing uncertainties in storm coverage.


... N-Italy, the Gulf of Genoa to the Tyrrhenian Sea, Corsica and W-CNTRL Italy...

During the day the focus for organized DMC activity evolves over NW-Italy, where easterly BL flow advects a moist air mass (10 g/kg LL mean mixing ratios and TPWs aoa 30 mm) towards the Maritime Alps. Not much mid-level cooling and only weak Alpine EML advection keeps MLCAPE in the 800-1200 J/kg range. Mid-level flow remains enhanced ahead of the approaching trough from the N with DLS aoa 20 m/s. Main caveat is ongoing discrepancies about degree of mid-level temperature profile and variable LL moisture forecast in various models. Most likely scenario will be scattered CI along the orography with isolated CI over the lowlands. Organized multicells / isolated supercells bring large hail and severe wind gusts.

A similar area with better NWP agreement in adequate CAPE/shear overlap exists over NE Italy, where similar hazards are forecast (hail and severe wind gusts).

During the night, falling surface pressure over the Golf of Genoa with enlarging mid-level hodographs over NW-Italy (indicative of deepening WAA) show a general tendency of storms to cluster with a lowering severe risk (although a local flash flood risk continues).

A more serious severe risk evolves during the night over the Golf of Genoa, Corsica and all the way to the Tyrrhenian Sea. Low-tropospheric flow regime responds to the falling pressure over the Golf of Genoa and an elongated N-S aligned low-tropospheric confluence zone is forecast from Corsica to the SE with an E-W aligned convergence zone over the Golf of Genoa. Impressive moisture advection and pooling beneath meager mid-level lapse rates support 1-2 kJ/kg over the Golf and 2-3 kJ/kg SE of Corsica. DLS increases from N to S from 20 m/s to 30 m/s over the Tyrrhenian Sea.
Severe/organized convection (multicell/supercells mixture) is forecast during the night mainly along those convergence zones and long-tracked supercells are possible. Large to very large hail and severe wind gusts will be the main hazard although a tornado risk also exists especially next to the aforementioned convergence zones. This activity also affects far W-CNTRL Italy during the night.

Depending on how far south the structuring LL vortex over the Golf of Genoa shifts, a nocturnal excessive rainfall threat may evolve over far N-Corsica within a strengthening and confluent LL flow regime including a moist and very unstable air mass. This risk increases beyond 06Z.

... CNTRL-Greece, Aegean Sea and W-Turkey ...

A passing upper-level trough is accompanied by a 30 m/s mid-level speed max, which translates E/SE rapidly during the day. It overspreads a warm and moist air mass with 800-1200 J/kg MLCAPE. Point source forecast soundings at 09 Z over CNTRL-Greece show 0-6 km shear of 30-35 m/s, 0-3 km shear of 15 m/s and SRH-3 of 100 m^2/s^2 with slightly weaker DLS but stronger SRH and CAPE over the Aegean Sea and far W-Turkey (although some NWP disagreement exists over the Aegean Sea about the CAPE/shear overlap).
A questionmark exists with CAPE magnitude over CNTRL-Greece due to lots of cloudiness from nocturnal convection from the previous night including ongoing and onshore moving thunderstorms from the Ionian Sea. In case of temporal diabatic heating, a mix of multicells/supercells with large hail, severe wind gusts and an isolated tornado risk is anticipated. Uncertainties preclude a level 2 upgrade for now.

Confidence in early supercell development increases over the Aegean Sea, E/NE-Greece and W-Turkey with all kind of hazards possible including severe wind gusts, large to very large hail, flash flooding and a conditional tornado risk. The severe risk diminishes beyond sunset with a lingering risk along the SW-coast of Turkey, where CAPE/shear overlaps continues.

... S-Black Sea ...

A growing nocturnal thunderstorm cluster with severe wind gusts, large hail and heavy rain streaks far N-Turkey although the main severe risk should stay offshore.


... Sicily and Calabria ...

Until 21Z this region remains capped beneath a NE-ward fanning EML with 850 hPa temperatures in excess of 20 C. Mobile and flattening ridge axis crosses the area of interest during the evening hours and pushes quickly E/SE thereafter.

Beyond 18Z, the approaching wave from the W and a gradually cooling column of low-/mid-tropospheric air coincide with the collision of the SW-ward fanning and weakening cold surge from S-Italy and strengthening westerly moisture advection with the approaching wave. Hence, synoptic-scale and mesoscale forcing increase constantly during the night over Sicily and surroundings.
A dry air mass over N-Tunisia and the Strait of Sicily with surface dewpoint readings in the single digits and lower tens moistens up during the night, so MLCAPE readings in the 1-2 kJ/kg range are likely to evolve. With the passing mid-level wave a strong 40 m/s mid-level speed max crosses Sicily during the night and pushes DLS in the 30-40 m/s range. In fact convectively uncontaminated point-source forecast soundings over Sicily around 03Z show 0-6 km shear of 40 m/s, 0-3 km shear of 30 m/s and SRH-3 in excess of 400 m^2/s^2 which is in the strong to extreme range of wind shear.

Increasing forcing from the W probably sparks numerous thunderstorms just W of Sicily during the night, which organize rapidly into long-tracked supercells. Large to very large hail and severe to extremely severe wind gusts are forecast. The tornado risk depends on the BL moisture quality onshore with most models keeping convection slightly elevated and with somewhat weak 0-1 km shear magnitude forecast. However in case of passing supercells in such a thermodynamic and kinematic environment, tornadoes can't be ruled out with a significant event still possible.