Storm Forecast
Valid: Thu 25 May 2017 06:00 to Fri 26 May 2017 06:00 UTC
Issued: Thu 25 May 2017 06:33
Forecaster: GATZEN

A level 1 was issued for western Russia mainly for tornadoes, severe wind gusts and to a lesser extent large hail.

A level 1 was issued for western Iberia mainly for large or very large hail and severe wind gusts.

SYNOPSIS

A ridge is located across western Europe. It is flanked by a mid-level short-wave trough over Iberia and downstream low geopotential across central and eastern Europe. Embedded short-wave troughs move eastward over north-western Russia and southward across the Balkans, with a northerly jet streak pointing from eastern Germany towards the central Mediterranean. Ahead of the trough axis, a weakly baroclinic frontal zone is associated with warm air advection from Bulgaria into Romania, Moldova and Ukraine as well as over western Russia.

At lower levels, dry polar air masses have spread into eastern Europe. Slightly better moisture along the frontal zone from the Balkans towards western Russia overlaps with weak lapse rates as indicated by latest soundings that will not improve too much as broad cloud coverage is indicated by latest visible satellite images. Better moisture extends from the Mediterranean to western Europe where is partly overlaps with an elevated mixed layer extending from northern Africa to Iberia and the Bay of Biscay.

DISCUSSION

Western Russia

An intense short-wave trough travels east, associated with strong DCVA at the cyclonically sheared flank of a jet streak curving around its southern portions. At lower levels, a cold front moves east that provides strong low-level lift. Downstream, a moist air mass with dewpoints around 12°C overlaps with elevated steep lapse rates as indicated by latest Moscow sounding. Weak CAPE is present and a strongly forced convective line has developed that is currently half way between the Moscow area and the border to Belarus. Although low-level CIN is rather high in the 00 UTC sounding, diurnal heating can result in a zone of SBCAPE in the Moscow region and further south. The northern portions of this zone will see significant low-level vertical wind shear from the Moscow region towards the east due to increasing south-westerly flow at 850 hPa.

Current thinking is that storms will go on along the cold front with additional storms forming further east during the noon and afternoon hours. While deep layer vertical wind shear is about 15 m/s with limited veering and associated weak SRH, strong low-level vertical wind shear exceeding 10 m/s in the lowest km may result in rotating storms and the possibility of tornadoes, and even a strong tornado cannot be ruled out. Furthermore, severe wind gusts can be expected along the convective line and pre-frontal storms may produce large hail, especially when mesocyclones form. Excessive rain at the northern portions of the pre-frontal warm air advection regime will be mostly non-convective given a dry and stable low-level air mass. Storms will weaken after sunset due to low-level stabilization but may continue along the cold front well into the night.

Iberia

Storms have formed yesterday across western Iberia in a well-mixed air mass. Today, chances are even higher given a weak mid-level trough spreading north across the region. Rather large CAPE will overlap with about 10-15 m/s 0-3 km vertical wind shear that develops in response to the daytime thermal low over Iberia. First storms are already ongoing over Portugal. New initiation is expected in the afternoon especially over northwestern Iberia. A few isolated thunderstorms can also evolve further south-east. Storms can become quite intense given CAPE around 1000 J/kg and the possibility of multicells. Large CAPE is the main threat, however, severe wind gusts are not ruled out due to initially large DCAPE and later also due to cold-pool driven storm clusters.

Northern Africa

Storms are expected to form over the slopes of the Atlas. Weak vertical wind shear, weak CAPE, and rather high CIN over most places will limit severe potential. Locally, large hail and severe wind gusts cannot be ruled out, though.

Italy to Balkans and Greece

A cold front moves across Italy in the morning hours, and thunderstorms have formed that move southward in the morning hours. Severe potential with these storms is limited given the cool boundary layer and weak CAPE. In the wake of the cold front, cold air advection will be present over Italy today and new storms are rather unlikely. Further east, prefrontal air mass is rather moist with dewpoints about 15°C, but cloudiness may limit diurnal heating today. However, weak CAPE can be possible especially from northern Greece to Bulgaria and Romania, where storms are expected. Weak vertical wind shear will limit storm organization, but locally augmented vertical wind shear can support some better developed multicells or even mesocyclones, capable of producing large hail and excessive precipitation. A level 1 was not issued because o a missing focus of augmented severe potential.

Turkey

Strong diurnal heating of a rather cool and dry air mass is expected today. Thunderstorms are likely to form along sea breeze convergence zones and over mountains. Weak CAPE and vertical wind shear are present, and storms are not expected to become severe except for a slight chance of dry microbursts due to strong DCAPE in the well-mixed boundary layer.