Storm Forecast
Valid: Thu 09 Jun 2016 06:00 to Fri 10 Jun 2016 06:00 UTC
Issued: Wed 08 Jun 2016 22:41
Forecaster: GATZEN
A level 1 was issued for Italy, Corsica, Sardinia, Sicily, and western Balkans mainly for excessive rain and to a lesser extent large hail and tornadoes.
A level 1 was issued for south-eastern Poland, north-western Ukraine into western Russia mainly for excessive rain and large hail.
A level 1 was issued for north-eastern Austria, eastern Czech Republic, and Slovakia mainly for excessive rain.
A level 1 was issued for north-western Spain mainly for large hail.
SYNOPSIS
Supported by ridging into north-western Europe and a polar trough across north-eastern Europe, a north-westerly mid-level flow has evolved over most of Europe. Short-wave troughs embedded in the flow travel from eastern France across the western Alps into the Tyrrhenian and Ionian Sea. The north Mediterranean including the Balkans will be affected by these troughs and thunderstorms are expected. Downstream of the short-wave troughs, only weak mid-level flow will remain over eastern Europe.
At lower levels, cold air advection will affect France and central Europe in the wake of a cold front. Ahead of this cold front, a band of steeper lapse rates (7 K/km in the 900 600 hPa layer) will spread into eastern Europe. Although low-level moisture will be relatively weak in this area, about 500 J/kg MLCAPE is indicated by latest models. During the day, thunderstorms are forecast ahead of the cold front.
DISCUSSION
Italy, Corsica, Sardinia, Sicily, and western Balkans
Rich low-level moisture is present across the north Mediterranean. Although lapse rates are not very steep, diurnal heating will allow for CAPE due to relatively cool mid levels. Storms have already formed on Wednesday, focussed along sea breeze convergences.
On Thursday, greater storm coverage and intensity is expected due to the approaching mid-level trough that provides DCVA. Weak flow will allow the storms to stay some time near mesoscale boundaries mostly forming in response to the sea breeze. These boundaries provide low-level convergence what increases the threat of excessive rain. Stretching of vertical vorticity could also result in mesocyclones capable of producing large hail as well as tornadoes. The main threat is forecast over Italy where latest models also indicate falling surface pressure during the day.
Vertical wind shear will be weak over most places, increasing to the west. Storms forming over north-eastern Italy, Corsica, Sardinia, and Sicily can become well-organized multicells or supercells with the main threat being large hail.
Storms will spread south together with the trough late in the period.
North-eastern Austria, eastern Czech Republic, Slovakia, south-eastern Poland, north-western Ukraine into western Russia
Ahead of the cold front, storms are expected to re-intensify in the noon and afternoon hours. CAPE around 500 J/kg is expected, together with relatively moist profiles. Additionally, the flow will become more parallel to the cold front, and training storms can develop. Excessive precipitation is forecast to be the main threat, together with large hail given increasing deep-layer vertical wind shear over the Ukraine and surroundings.
Over the western portions of the front, vertical wind shear will be weaker. Upslope flow will nevertheless support intense storms, capable of producing excessive rain. Main reason to stick to a level 1 is that low-level moisture is limited across Austria.
North-western Spain
Late in the forecast period, a sharp short-wave trough approaches over northern Iberia. Strong DCVA is expected in the evening hours over the region. Moist low-level air masses spreading onshore with upslope flow during the day is forecast to overlap with steeper lapse rates ahead of the approaching trough. Additionally, strong vertical wind shear evolves in the evening hours. Strong capping inversion may suppress initiation over most places. Expect a few storms to develop in the afternoon and evening, these could quickly organize and pose a threat of large hail.
