Storm Forecast
Valid: Thu 14 Jul 2016 06:00 to Fri 15 Jul 2016 06:00 UTC
Issued: Wed 13 Jul 2016 23:28
Forecaster: GATZEN

A level 2 was issued for eastern Poland and western Belarus mainly for severe wind gusts and to a lesser extent tornadoes and exsessive precipitation.

A level 2 was issued for the western Ukraine, eastern Slovakia, eastern Hungary, western Romania, and northern Serbia mainly for large hail and severe wind gusts.

A level 1 was issued for Belarus mainly for tornadoes.

A level 1 was issued Latvia, Lithuania, and central Poland mainly for excessive rain and tornadoes.

A level 1 was issued for Slovakia, Hungary, the Balkans, and western Romania and Ukraine mainly for large hail and severe wind gusts.

A level 1 was issued for north-western Italy mainly for large hail.

SYNOPSIS

A trough is centred across western Europe. Two intense vort-maxima rotate around its periphery. A digging short-wave trough ejects from France into the north Mediterranean and a negatively tilted one lifts from the eastern Alps to the Baltic Sea. Regions outside the strong jet curving around the trough are affected by mid-level ridging.

At lower levels, seasonably cold maritime air mass floods most of western Europe in the wake of an intense low pressure system and its cold front that continues to move eastward over eastern Europe. Latest soundings indicate an overlap of steep lapse rates and rich low-level moisture over the Pannonian Basin that can be included in the sharpening warm sector, where strong shear will overlap with CAPE.

DISCUSSION

Eastern Poland into western Ukraine, Belarus, Lithuania, and Latvia

Ahead of the deepening cyclone over Poland, warm air advection spreads northward over the region. To the north of the warm front, stratiform precipitation is expected with some embedded thunderstorms. Severe potential is weak given the cool and stable boundary layer.

Due to diurnal heating of the warm air mass in the wake of the warm front, surface-based storms become more likely along the warm front and in the warm sector. Given the rich boundary layer moisture with dewpoints of 16 to 20°C and steep lapse rates, CAPE of 1000 J/kg, locally also more than 1500 J/kg is expected. Models indicate that storms spread from south-eastern Poland northward and also develop along the warm front that extends over Belarus in the noon and afternoon.

Especially over eastern Poland and western Belarus, a favourable set-up for severe storms seems to be possible. Strong vertical wind shear exceeding 15 m/s in the lowest 3 km together with curved low-level hodographs near the warm front could support rotating updrafts. Main limiting factor is the weak surface heating given high cloud coverage and rain in the morning hours over most places across Poland. If daytime heating is strong in the moist air mass behind the warm front, a strong tornado is possible with any isolated supercells that forms.

Furthermore, excessive rain is forecast given a moist profile up to mid levels. Advancing north, low-levels tend to become more stable late in the day. Over Lithuania and Latvia as well as over central Poland, severe potential is limited due to stable low levels. However, excessive rain and tornadoes are not completely ruled out especially when some sunshine can increase the low-level buoyancy.

Ahead of the rapidly approaching cold front, storms can cluster and severe wind gusts are possible. More straight-line hodographs can support supercells, and right-moving storms have the potential of producing large hail as they stay some time ahead of the front.

Along the cold front, a narrow cold frontal rain band is expected with a further wind threat. Given weak SRH, tornado potential is expected to be lower compared to the warm front region.

Hungary and Slovakia into the central Balkans and Romania

The cold front is pushed east in the wake of the low pressure system. Moist air masses overlap with steep mid-level lapse rates over Slovakia and Hungary in the morning hours, where first storms are expected to develop. Long straight line hodographs are expected and right movers could stay some time ahead of the cold front. Large hail is forecast in these regions. As the activity spreads east, it will move into an air mass characterized by inverted-v profiles and weaker moisture. Storms may weaken and new initiation is most likely along the slopes of the Dinaric and Carpathian mountains. Severe wind gusts are the main threats, along with locally large hail in association with supercells. Storms will go on until the night hours when convective activity is expected to decay along the cold front.

Northern Italy

An intense short-wave trough moves into the north Mediterranean in the evening hours. In the wake of the Alps, cyclogenesis is forecast. Although low-level moisture is limited, steep lapse rates will form in the wake of the Alps and thunderstorms are forecast to develop and spread south-east in the night hours. Vertical wind shear is limited, but steep mid-level lapse rates can support strong updrafts in the hail growth zone. Large hail is therefore forecast with the stronger storms.