Storm Forecast
Valid: Mon 28 May 2018 06:00 to Tue 29 May 2018 06:00 UTC
Issued: Sun 27 May 2018 23:24
Forecaster: PISTOTNIK

A level 1 is issued for Spain mainly for excessive convective precipitation.

A level 1 and level 2 are issued for a large region between NE Spain (Catalonia), Germany, W Romania and Italy for excessive convective precipitation and large hail.

A level 1 and level 2 are issued for W Turkey mainly for excessive convective precipitation and to a lesser degree large hail.

A level 1 and level 2 are issued for N and E Turkey mainly for a large hail and to a lesser degree for excessive convective precipitation, severe convective wind gusts and tornadoes.

A level 1 is issued for NE Algeria and NW Tunisia mainly for large hail and to a lesser degree for severe convective wind gusts.

SYNOPSIS

Weak pressure and temperature gradients remain in places across most of Europe. The formerly Scandinavian anticyclone shifts a bit east to the Baltics and Belarus, allowing a large but shallow upper-level low over Iberia to further extend its influence towards France and central Europe. Another upper-level low is placed over Turkey, and finally a more compact but deeper one over the North Sea.
The polar jet is deflected to the far north. The subtropic jet runs across the Maghreb States.

DISCUSSION

... continental Europe (including Spain) ...

Another round of scattered to widespread, diurnally driven thunderstorms will form in the warm and moist air across much of Europe. CAPE will be confined to some hundred J/kg where convection forms early, but may reach 1000-2000 J/kg in areas of steeper lapse rates and/or delayed convective initiation, as indicated by several of Sunday's 12z sounding in central Europe. Vertical wind shear is weak with values around or below 10 m/s across the lowest 6 km. In terms of synoptic lift, the Spanish upper-level low sends several subtle vorticity maxima across France and the greater Alpine region, whereas synoptic lift is absent everywhere else.
Storms can bring moderately large hail in intiating stages and flash floods plus isolated cold-pool-driven severe wind gusts later on, when large clusters form. The hail risk is maximized to the N and E of larger mountains (Pyrenees, Alps, Apennines), where thermal upslope circulations beneath weak SW-erly mid-level flow may enhance vertical wind shear just enough to favor multicellular organization. Otherwise, flash floods are the dominant risk. Despite a lack of distinct convergence zones, two level 2 areas were introduced for those areas where models largely agree on widespread convective initiation under robust CAPE.
Weaker lapse rates limit CAPE magnitude over Spain, reducing the hail risk. However, partly embedded storms in vicinity of the upper-level low center could still cause a few heavy rain events, hence the level 1 was extended there.

The eastern boundary of the thunderstorm-prone area is formed by drier and slightly cooler air that infiltrates into E-central Europe, but this zone is quite diffuse. It should be noted that GFS and WRF show much more CAPE in this zone (E Germany, Poland, Slovakia, Romania) than the rest of the models. The thunder and risk level areas were drawn rather conservatively here, following the majority of the model pool and considering the presence of weak synoptic subsidence, which should reduce storm coverage even in case the higher CAPE values should yet materialize.

... Turkey, Armenia, Georgia ...

This convective environment is characterized by deeply mixed, dry air with limited CAPE inland (see Sun 12z's Ankara and Erzurum soundings) and a "loaded gun" situation along the north coast (see Sun 12z's Samsun sounding), where an elevated mixed layer on top of 19-22°C dewpoints creates 1000-3000 J/kg CAPE under enhanced vertical wind shear (~15 m/s across the lowest 3 km) in the sea breeze and upslope flow regime. The stout cap makes it unclear how many storms will form in the most "interesting" belt, but weak synoptic forcing ahead of the Turkish upper-level low favors convective initiation. Storms that form will likely organize into multi- and supercells with a possibility of all kinds of severe weather. The main risk is large to very large hail.
On the back side of the upper-level low, vertical wind shear is much weaker over the W half of Turkey, but Black Sea moisture is advected farther inland. Scattered to widespread afternoon storms are expected to form and to grow upscale into several large clusters. Flash floods are the main risk and large hail is the secondary risk.

... NE Algeria, N Tunisia ...

Beneath the subtropical jet, an impressive kinematic environment (0-3 km shear of 15-25 m/s, 0-3 km storm-relative helicity up to 600 m^2/s^2 per GFS) overlaps with a narrow belt of CAPE in the sea breeze and upslope flow regime north of the Tell Atlas mountains. Convective initiation is an even bigger concern than in N Turkey, as the capping over the Mediterranean Sea will be very strong (see Sun 12z's Decimomannu sounding). Air parcels originating from the sea breezes will need to be exposed to a lot of daytime heating to break this cap. Despite the lift support of a distinct mid-level vorticity maximum, forecast models are reluctant to show precipitation signals. However, any storm that forms and persist can become highly organized and bring large to very large hail and severe downbursts. A level 1 was drawn along with a low probability thunder area to reflect this conditional scenario.
Some elevated, weakly electrified convection without a severe weather risk may also move across the W Mediterranean Sea ahead of the mentioned vorticity lobe.