Storm Forecast
Valid: Fri 13 Jul 2018 06:00 to Sat 14 Jul 2018 06:00 UTC
Issued: Thu 12 Jul 2018 23:29
Forecaster: PISTOTNIK

A level 2 is issued for N Spain and S-ernmost France, and level 1 areas are issued from NE Italy to the W Balkans, and for the Russian and Georgian Caucasus forelands mainly for large hail and to a lesser extent for severe convective wind gusts and flash floods.

A level 1 is issued for parts of Poland, Belarus, Lithuania and the Russian exclave of Kaliningrad mainly for excessive convective precipitation and large hail. In addition, waterspouts are possibly at the coast.

A level 1 is issued for W Latvia and W Estonia for flash floods.

SYNOPSIS

A quiet summertime pattern with weak pressure and temperature gradients covers Europe. Upper-level lows are in place over N-central Sweden, NE Poland and off the Portuguese coast. Some hundred J/kg of CAPE over wide areas of the continent allow scattered afternoon and evening storms, but mostly under limited vertical wind shear.
The subtropic jet with somewhat stronger mid-level flow stretches with an anticyclonic curvature from Iberia to Italy, before it slowly fans out over the E Mediterranean region, Turkey and the Caucasus region. Hot conditions are confined to inland areas of Spain and Turkey.

DISCUSSION

... thunderstorm areas south of 45 deg N ...

CAPE mostly on the order of 500-1000 J/kg or locally more will build where elevated mixed layers are advected from mountains over adjacent lowlands, most notably over N Spain, S-ernmost France, N Italy, the W Balkans, the Romanian and Bulgarian Danube valley, and finally Georgia, Armenia, Azerbaijan and the Russian Caucasus forelands. It overlaps with 15-20 m/s deep-layer shear, possibly more where thermal upvalley and upslope circulations are opposed to the mid-level jet.
The overlap of CAPE and shear is robust enough to expect multi- and a few supercellular storms. Large hail is the main risk, possibly approaching 5 cm in size in case of supercells especially in the level 2 area. In addition, isolated severe downbursts or flash floods may accompany the strongest mature storms.
Limiting factor is the rather low storm coverage due to capping and a lack of synoptic lift. The only noteworthy short-wave trough translates from N Italy towards Slovenia, Croatia, Bosnia-Herzegovina and Serbia, and first storms may already form in the morning ahead of this trough. Otherwise, the time window is mostly confined to the afternoon and evening.
Coastal and offshore areas will be strongly capped and will stay devoid of storms. The only exception is the Black Sea, where isolated to scattered offshore convection may form in the course of the night. A few waterspouts are then possible in the early morning to Saturday, but they are not covered with a level 1 due to a lack of obvious hot spots.

... NE Poland, NW Ukraine, Belarus, Lithuania, Russian exclave of Kaliningrad ...

CAPE in these areas is overlapped by enhanced deep-layer shear between 10 and 15 m/s in a circular belt around the upper-level low. Lithuania, Kaliningrad and coastal areas in N Poland may experience several - partly embedded - rounds of storms around the clock, which may cause flash floods and maybe one or two waterspouts. Otherwise, scattered to widespread storms in the afternoon and evening will likely turn multicellular and may bring a few marginally large hail and heavy rain events. A donut-shaped level 1 is issued.

... other thunderstorm areas ...

Weak vertical wind shear will keep scattered daytime-driven storms disorganized. Localized heavy rain is the main risk. A level 1 is issued for the area of the expected highest storm coverage in W Latvia and W Estonia.