Storm Forecast
Valid: Fri 03 Aug 2018 06:00 to Sat 04 Aug 2018 06:00 UTC
Issued: Fri 03 Aug 2018 00:17
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for NE Turkey, Georgia, Armenia and the Russian Caucasus region mainly for flash floods towards the W and mainly for large hail and severe convective wind gusts towards the E.

A level 1 is issued for parts of Finland and NW Russia mainly for flash floods and to a lesser degree for severe convective wind gusts and large hail.

A level 1 is issued for parts of Latvia, Lithuania, Belarus and Poland mainly for flash floods and to a lesser degree for severe convective wind gusts and large hail.

A level 1 is issued for the Alps, Hungary, W Romania, the Balkans, Greece, Italy and Corsica mainly for flash floods and to a lesser degree for severe convective wind gusts and large hail.

Level 1 areas are issued for the E Black Sea, the NW Ionian Sea and the NW Tyrrhenian Sea for waterspouts.

SYNOPSIS

Zonal flow sterengthens over the British Isles and Scandinavia. Slightly cooler maritime air is advected eastward and brings the latest heat wave to a gradual ending.
Hot conditions under weak pressure gradients remain in place everywhere further south. At 500 hPa, the subtropical mid-level ridge flattens a bit and increases its positive tilt from Iberia towards Belarus, opposed by an axis of slightly lower geopotential with several shallow minima from the Black Sea to Tunisia.

DISCUSSION

... NE Turkey, E Black Sea, Georgia, Armenia, Russian Caucasus region ...

Persistent lift overspreads the region in the warm air advection regime ahead of one of the cut-off lows. It partly overlaps with some hundred J/kg of CAPE, possibly more than 1000 J/kg in coastal areas where abundant low-level moisture with 2m dewpoints in excess of 20C is advected onshore, and enhanced 0-3 km shear around 15 m/s. At least scattered, often clustered and/or embedded storms will be active almost throughout the forecast period, and flash floods become a major issue, especially at the SW slopes of the Caucasus mountain range. Further inland into E Georgia, Armenia and NE Turkey as well as the Russian Caucasus forelands, limited low-level moisture but steeper lapse rates suggest that the heavy rain risk is lower but large hail and severe wind gusts become more likely with discrete daytime storms. Activity will continue into the night while it slowly fades.
Other, less organized showers or thunderstorms will repeatedly form near the center of a weak surface cyclone over the E Black Sea. A few waterspouts are possible.

... Alpine region to Hungary and W Romania, Balkans, Greece, Italy, Corsica ...

Another round of daytime-driven storms is expected under moderate CAPE on the order of 1000 J/kg, mostly weak vertical wind shear and weak synoptic forcing. Activity will be mostly tied to orographic features, apart from Hungary, Slovenia and Croatia, where a subtle vorticity maximum facilitates initiation also over flat terrain, e.g. along outflow boundaries or insolation gradients. A few flash floods are the main risk. In addition, large hail and severe downbursts might accompany the strongest pulse storms.
The most robust, albeit conditional, severe weather risk is present over the Po Valley in N Italy, where radiosondes from Cuneo, Milano and Rivolto have indicated CAPE on the order of 1000 to 3000 J/kg under a capping inversion for several days. In addition, thermal uslope circulation at the southern Alpine rim slightly enhance the veritcal wind shear to the NE-erly flow aloft. It appears still unlikely that this abundant CAPE reservoir will be touched on Friday, hence no upgrade from a low probability lightning area and a level 1 are performed. However, if indeed storm clusters descend from the southern Alps towards the evening and the cap breaks over their forelands, widespread severe weather may ensue.
Late in the night to Saturday, forecast models also show scattered convection along nocturnal land breeze fronts over sea waters, most notably near the SE coast of Italy and near the E coasts of Corsica and Sardegna. These areas are also covered with a low-end level 1 for the possibility of some waterspouts.

... Finland into NW Russia, Finland into central Germany ...

A cyclone travels slowly eastward across N Finland. Its warm front stretches into NW Russia, its cold front crosses Finland and the Baltic States before it becomes rather stationary across central Poland and central Germany. Low-level moisture is accumulated along both fronts, leading to an inversely Y-shaped area of substantial CAPE framing the cyclone's broad warm sector. Forecast models even show CAPE on the order of 1000 to 2000 J/kg, which seems plausible as rich low-level moisture from the extremely warm Baltic Sea is picked up by the flow. On Thursday, CAPE values of this magnitude were already sampled by the 12 UTC Leba and Kaliningrad soundings, and 2m dewpoints often exceeded 20C in coastal areas.
Vertical wind shear is weak. Storms will stay disorganized but may grow into large clusters. Some flash floods are the main risk. Marginally large hail and locally severe downbursts pose additional risks, especially in case of more discrete storms. Level 1 areas are issued were CAPE is predicted to exceed 1000 J/kg.
The majority of storms will form in the afternoon and can survive into the first half of the night, though a part of the activity may be unrelated to the diurnal cycle, especially closer to the cyclone's center.

... British Isles to S Scandinavia ...

A pronounced short-wave trough travels eastward in the postfrontal environment. It translates from Ireland at Fri 06 UTC to S Sweden at Sat 06 UTC. However, its contribution to lift is largely cancelled by cold air advection. Stratification of the maritime air will be neutral to slightly unstable, and a few hundred J/kg of CAPE might form especially after some daytime heating over land. Vertical wind shear is enhanced to 10-15 m/s across the lowest 3 km beneath the mid-level jet.
Storms that form may organize into multicells, and marginally large hail or severe wind gusts are not ruled out. Due to various limiting factors (lack of synoptic lift as described, weak lapse rates, low cloud top heights), no level 1 is issued. The relatively highest risk of isolated to scattered thundery showers translates from Scotland and N England around noon to S Norway and Denmark in the evening and S Sweden overnight.