Storm Forecast
Valid: Thu 29 Jun 2017 06:00 to Fri 30 Jun 2017 06:00 UTC
Issued: Thu 29 Jun 2017 05:42
Forecaster: GATZEN

A level 2 was issued for Romania, southern Ukraine mainly for large hail, severe wind gusts, excessive rain and to a lesser extent tornadoes.

A level 2 was issued for northern Ukraine, Belarus, and eastern Poland mainly for severe wind gusts, excessive rain, large hail and tornadoes.

A level 1 was issued for Poland, eastern Germany, Lithuania mainly for excessive precipitation and to a lesser extent tornadoes.

A level 1 was issued for northern Italy and Bulgaria mainly for large hail.

A level 1 was issued for the west Mediterranean mainly for large hail.

SYNOPSIS

An unseasonably intense trough digs across western Europe. It is surrounded by strong mid-level flow with a jet axis from Iberia to the central Mediterranean. Downstream short-wave trough moves across eastern Europe, while a shallow ridge crosses the Mediterranean today. At low levels, cyclogenesis over eastern Europe will result in a strong low pressure system over northern Poland at the end of the period. Axis of warm low-level air in its warm sector is pushed to the southern Balkans, central Ukraine, and Belarus already.

DISCUSSION

Balkans to Ukraine and Belarus

Soundings from the Balkans to Poland indicate elevated mixed layers within a tongue of warm air masses ahead of a cold front moving eastwards into the Balkans. Rich moisture at low levels overlap with the steep lapse rates, and models indicate CAPE in the order of 1500 J/kg, increasing towards Romania and Bulgaria. A cold front and mid-level trough axis will spread into this region during the day.

Thunderstorms are ongoing along the cold front over Bosnia and Herzegovina, Croatia, Hungary into Slovakia and will spread eastward in the morning hours. New initiation is expected to the east over the mountains of the Carpathians and southern Dinaric mountains that move eastward into the Ukraine, Romania, and Bulgaria. Strong vertical wind shear exceeding 25 m/s in the lowest 6 km will support well-organized storms including supercells, capable of producing all kinds of severe weather. Extremely severe weather is not ruled out, including very large hail, strong tornadoes, and wind gusts in excess of 33 m/s. Given stronger capping in the southern portions, isolated storms are expected. Especially over northern Romania and the Ukraine and Belarus, clustering of storms is likely. Severe threat will therefore shift to high winds and excessive precipitation in the afternoon and evening with the potential of a long-lived high wind event. In the wake of the first storms, further storms may follow from Poland in the afternoon and evening. Increasing low-level vertical wind shear to more than 10 m/s in the lowest km indicates a risk of (strong) tornadoes. The time window for such events is small, as severe weather potential will rapidly decrease from the west later on as cold air advection sets in.

Poland, Lithuania, eastern Germany

Warm air advection affects the regions further north over Poland and the Baltic States and surroundings. Moist low-level air masses are present and lapse rates increase to the east where CAPE can exceed 1500 J/kg over eastern Poland. Due to strong forcing today, widespread storms are forecast that will merge to large clusters. Vertical wind shear will be weak in the western portions, where excessive rain is the main threat, but better storm organization is likely further east with increasing vertical wind shear, especially at the lower levels. Bow echoes and supercells are not ruled out, capable of producing high winds and tornadoes. In the evening, cold air advection is expected from the west and storms will weaken.

Northern Italy, southern Austria, south-eastern France

Some remaining moisture to the south of the Alps and weak lee cyclogenesis can allow for storms over the southern Alps today. Long straight-line hodographs pose a threat of well-developed storms capable of producing large or very large hail. Severe wind gusts are not ruled out. Locally, storms may also produce excessive rain. Storms can continue during the night ahead of an approaching new trough.

Pyrenees to west Mediterranean

Ahead of the approaching trough axis, a strong mid-level flow affects this region. Although low-level moisture is very limited, some CAPE is expected and models indicate also precipitation. Any isolated storm that forms can be capable of producing large hail. Overall threat is limited, though.

Southern and central France

Within the base of the trough, showers and thunderstorms are forecast today. Low-level vertical wind shear is about 7-10 m/s, together with low LCL heights. Some shallow mesocyclones are not ruled out capable of producing tornadoes.