Storm Forecast
Valid: Thu 25 Jun 2015 06:00 to Fri 26 Jun 2015 06:00 UTC
Issued: Wed 24 Jun 2015 21:52
Forecaster: GATZEN

SYNOPSIS

A long-wave trough is still located across Europe, expanding from Scandinavia to the Black Sea and yielding a north-westerly flow over large portions of Europe. As a consequence, cool and rather dry air masses dominate the forecast region. Additionally, lapse rates are weak except for diurnally driven deeper boundary layers across higher terrain in southern Europe as well as steep lapse rates in a well-mixed polar air mass situated across southern Scandinavia.

Main focus of synoptic-scale forcing for ascent are two upper vort-maxima moving across southern Scandinavia and the central Balkans. Another trough moves across central and northern Finland.

Further east, an unstable air mass is expected to advect into parts of the Ukraine and Russia ahead of the European trough. Strong vertical wind shear will allow for a severe weather outbreak over this region.

DISCUSSION

SW Russia and NE Ukraine

Ahead of the European trough, an elevated mixed layer advects north-westward on Thursday. It will overlap with a moist boundary layer leading to large CAPE as indicated by latest Tuapse ascent. North of a low pressure centre situated across the Black Sea, string warm air advection will take place over the forecast area together with a low-level jet reaching 20 m/s in the lowest 3 km on Thursday evening.

Thunderstorms are forecast to form within the warm air advection regime during the day. Given the strong forcing, these storms are expected to cluster building an MCS in the afternoon and evening, spreading further NW into the northern Ukraine and south-western Russia. Severe wind gusts are forecast with this MCS. Furthermore, large hail and tornadoes are forecast with every supercell that forms given around 2000 J/kg MLCAPE overlapping with 15 m/s 0-3 km bulk shear.

During the night, MCS will move further west while gradually becoming elevated. Severe threat will weaken, but some severe gusts and tornadoes are not ruled out.

Central Finland

The eastern vort-max of the Scandinavian trough was located across southern Finland today and will propagate northward. A quite strong south-westerly flow will continue during the period, reaching 20 m/s at 700 hPa. Some warm air advection is possible in the wake of the trough leading to some QG forcing.

Affected air mass is quite unstable: Latest Helsinki sounding indicates a moist air mass, characterized by steep low-level lapse rates and CAPE especially below 650 hPa. This air mass is expected to advect further north on Thursday. Thunderstorms are expected given the weak CIN and some low-level heating.

Storms that form may organize given 15 m/s deep layer vertical wind shear. Low-level wind shear will be quite strong in northern central Finland, where a cooler boundary layer is located below the strong flow at 850 and 700 hPa. It is questionably if storms can evolve within this environment. However, rather moist low-levels, and overall weak CIN may allow for a few storms in a region with 10 m/s 0-1 km vertical wind shear. These storms may be capable of producing severe wind gusts and tornadoes. Overall threat is very limited, though, and a level 1 is not issued.

Northern Italy

Some showers and thunderstorms are expected to the south of the Alps, given upslope flow of rather moist low-level air mass overlapping with steep lapse rates that evolve in the north-westerly flow. A weak vort-max crosses the Alps in the afternoon and evening, providing some further lift. Storms that form will likely organize given deep layer vertical wind shear of 15 to 20 m/s. Main limiting factor is the uncertainty of low-level moisture and associated CAPE. GFS is likely quite optimistic. Storms that form may be capable of producing large hail. Given the limited threat due to the great uncertainty, a level 1 was not issued.