Storm Forecast
Valid: Thu 04 May 2017 06:00 to Fri 05 May 2017 06:00 UTC
Issued: Thu 04 May 2017 00:05
Forecaster: GATZEN

A level 1 was issued for northern Greece and FYR Macedonia, W Bulgaria, Serbia, E Bosnia and Herzegovina, E Croatia, W and central Romania, W Ukraine, E Hungary, E Slovakia, SE Poland mainly for large hail and to a lesser extent severe wind gusts and excessive precipitation.

A level 1 was issued for south-eastern Germany, north-western Austria and the Czech Republic mainly for large hail.

A level 1 was issued for northern Italy mainly for large hail and to a lesser extent severe wind gusts and tornadoes.

A level 1 was issued for western Turkey mainly for large hail and to a lesser extent excessive precipitation.

SYNOPSIS

High-over low blocking with centres across Iceland and the Alps continues during the period. Vort-maxima rotate around the periphery of the Alpine trough, a pronounced short-wave trough moving west over Scotland, a jet streak moving across the north Mediterranean, and two vort-maxima that cross the Balkans into eastern Europe and later to the North Sea. Another intense trough is situated west of Iberia and starts to affect the forecast region. Ridging affects the south Mediterranean and extents into eastern Europe. While cold air masses advect into most of northern Europe, a tongue of warm air advects from Bulgaria and Romania into the western Ukraine, Belarus, and eastern Poland. Furthermore, strong warm air advection sets in across the west Mediterranean and Bay of Biscay ahead of the trough west of Iberia. Overlap of rich low-level moisture and steep lapse rates is expected in the warm air advection regimes as well as over the east Mediterranean.

DISCUSSION

Northern Greece and FYR Macedonia, W Bulgaria, Serbia, E Bosnia and Herzegovina, E Croatia, W and central Romania, W Ukraine, E Hungary, E Slovakia, SE Poland

Ahead of the Alpine trough, a quasi-stationary frontal boundary extends from south to north. Given steep lapse rates due to cool mid-levels as indicated by latest Sofia sounding and low-level mixing ratios around 8 g/kg (surface dewpoints around 13°C), CAPE in the order several 100 J/kg has developed today (LHUD sounding). Severe thunderstorms have formed on Wednesday with several large and very large hail reports from Hungary and Serbia. A MCS is ongoing from Hungary into south-eastern Poland and further groups of storms have formed upstream over Serbia.

On Thursday, convective debris will be situated over most of the area, with outflow boundaries extending well into Bulgaria, Romania, and Ukraine. During the morning and noon hours, new initiation can be expected especially across the W Ukraine with pronounced warm air advection from the south and associated low-level convergence. An MCS may form that spreads north-west and could also affect south-eastern Poland later on. Further south, additional storms are forecast in the broad frontal zone where rich low-level moisture overlaps with steep lapse rates that advect north. CAPE can be as high as on Wednesday.

The potential of severe thunderstorms will be lower compared to Wednesday given weaker warm air advection over most places as well as weaker vertical wind shear. However, large hail is again possible as multicells can be expected with 15 m/s deep layer vertical wind shear. Clusters of storms may also produce isolated severe wind gusts and excessive precipitation, especially along the south-western slopes of mountains. Convective activity will slowly decay during the night hours, when severe potential also decreases.

Alpine region and surroundings from eastern France into northern Italy, parts of Switzerland and Austria and southern Germany, Czech Republic

Cold mid-level air is associated with steep lapse rates that allow for weak CAPE during the day, except for the Alps where snow layer will limit low-level heating. Showers and thunderstorms are expected to form. Weak vertical wind shear will limit storm organization, but low wet-bulb freezing level and CAPE in the hail growth zone pose a threat of large hail with stronger cells, especially over south-eastern Germany and north-western Austria as well as the Czech Republic where CAPE will be highest and also some better vertical wind shear can be expected in the wake of the Alps.

Best storm organization is expected across northern Italy due to stronger vertical wind shear at the cyclonically sheared flank of a mid-level jet streak that moves east. Multicells are forecast and supercells are not ruled out, capable of producing large hail. A tornado is not ruled out but weak low-level vertical wind shear limits the potential. Storms may cluster late in the evening and spread east with a potential of isolated severe wind gusts.

Western Turkey

In weak flow but cold mid-level air mass and associated steep lapse rates, thunderstorms are expected to form. Although weakly organized, these storms could produce large hail locally, as well as excessive precipitation due to the slow storm movement. Storms will weaken after sunset.