Storm Forecast
Valid: Tue 24 Jun 2014 06:00 to Wed 25 Jun 2014 06:00 UTC
Issued: Mon 23 Jun 2014 23:17
Forecaster: GATZEN

A level 2 was issued for north-eastern Italy,Slovenia, Croatia, Bosnia, Serbia, Bulgaria, and Romania mainly for large hail and severe wind gusts. Excessive precipitation and tornadoes are additional threats.

A level 2 was issued ofr northern Algeria mainly for very large hail and severe wind gusts.

A level 1 was issued for the central Mediterranean mainly for large hail and severe wind gusts. Tornadoes are also possible.

A level 1 was issued for the eastern Iberian Peninsula mainly for large hail.

SYNOPSIS

The north-European trough will remain through-out the period. Vort-maxima will travel around its periphery over north-western Russia, from the Baltic States into Belarus, from southern Scandinavia to northern Germany, and over northern Norway. In the cool air mass steep lapse rates develop in response to diurnal heating. Given around 6 g/kg 30 hPa mixing ratio, at least weak CAPE is observed. Deeper instability evolves near the vorticity centres. Weak vertical wind shear and instability will limit the potential of severe storms over most places. However, latest reports indicate that non-supercell tornadoes have evolved. On Tuesday, some more events may occur. Highest potential is expected over the central Baltic States region, where rather high moisture, low-level convergence and steep lapse rates will overlap below a vorticity maximum.

Main convective activity is expected to go on over southern Europe. A region of rich moisture is present from the west and central Mediterranean into the Alps, Balkans, and Black Sea region. A plume of steep lapse rates from the African continent overlaps with this moisture over most of the west and central Mediterranean. Strong QG forcing is expected due to a jet streak that moves north-eastward on Tuesday.

DISCUSSION

Western and central Mediterranean

From the African continent, an elevated mixed layer spreads over the Mediterranean Sea. The overlap with rich boundary-layer moisture will result in strongly capped instability over most places. The cap will be weaker in the northern portions where also stronger QG lift is expected at the cyclonically sheared flank of the approaching mid-level jet. Initiation is therefore most likely in a band from the Pyrenees across southern France into the southern Alps.

Vertical wind shear is mostly between 10 and 15 m/s in the lowest 6 km over the north Mediterranean. This may allow for some organized storms together with rather weakly-organized clusters. Locally large hail and excessive precipitation are forecast. In the eastern portions, vertical wind shear will be stronger, and severe potential will be higher with hail the main threat. Given increasing QG forcing, storm clusters are expected to go on during the night, posing a threat of excessive precipitation especially across the southern Alps.

During the evening and night hours, the cold front of the cut-off low approaches from the west, supporting initiation further south as well. A couple of storms are expected to spread eastward. Strong vertical wind shear in excess of 20 to 30 m/s will support well-organized convection, including supercells and bow echoes. Large hail, severe winds, and tornadoes are possible until the morning hours.

Iberian Peninsula

The current cut-off low will merge with the European long-wave trough and starts to lift north-east. Its centre will again affect the Iberian Peninsula. Rather cool air will evolve steep low-level lapse rates during the day, whereas low-level moisture is rather high, so that widespread CAPE is expected. Given low-level convergence and upslope flow due to the sea breeze, widespread thunderstorms are forecast again. Vertical wind shear is weak over most of the region, and single cells or clusters are forecast, capable of producing isolated large hail, severe winds, and excessive precipitation.

Better storm organization is forecast over north-eastern Spain, where 15 m/s 0-3 km vertical wind shear can evolve in the afternoon due to latest GFS model run. Favourably veering profiles can support supercells with large hail the main threat. Storms over Iberia will weaken after sunset. During the night hours, storms may go on only over the north-east, where QG lift will be present.

Northern Mediterranean into central Balkans and western Black Sea region

A well-developed frontal boundary will evolve over the central Balkans on Tuesday. To the north, a cold front will move across Austria and Hungary, whereas warm air advection sets in over the southern Balkans ahead of the approaching mid-level jet. With rich boundary-layer moisture and steep lapse rates still in place, the potential of severe storms generally increases, especially because vertical wind shear will be strong over a broad area.

Current thinking is that convective storms will go on along the cold front, spreading from Hungary into Romania. In the wake of the cold front, new storms are expected to initiate in the late morning hours front northern Italy across the central Balkans to Bulgaria.

High winds due to a jet streaks along the cold front and within the warm air advection regime will result in strong vertical wind shear in excess of 15 m/s in the lowest 3 km. Large hodographs are forecast especially in the warm air advection regime. Current thinking is that rather well organized MCSs will move into Romania that may evolve into bow echoes. Damaging wind gusts seem to be the main threat with these storms, together with large hail and excessive precipitation. Tornadoes are not ruled out.

Further south and south-west, storms will be more isolated, and main mode will be well developed multicells and especially supercells, capable of producing very large hail, severe wind gusts, and tornadoes. Convective activity will weaken during the night hours, but some storms may go on along the frontal boundary. Bow echoes and supercells are not ruled out, capable of producing high winds, large hail, and tornadoes.

Belarus into western Russia

Ahead of an approaching short-wave trough, 15-20 m/s deep layer vertical wind shear are expected to overlap with 6 g/kg low-level mixing ratio and steep low-level lapse rates due to daytime heating. Storms that form will be organized, and multicells and supercells are forecast, capable of producing large hail and severe wind gusts. The overall threat is limited due to weak CAPE and weak low-level convergence, so that a level 1 is not warranted.