Storm Forecast
Valid: Thu 24 Sep 2015 06:00 to Fri 25 Sep 2015 06:00 UTC
Issued: Wed 23 Sep 2015 19:18
Forecaster: GATZEN
A level 2 was issued for the south-eastern Adriatic mainly for excessive precipitation an to a lesser extent large hail.
A level 1 was issued for the Adriatic and southern Italy mainly for excessive precipitation and to a lesser extent tornadoes and large hail.
A level 1 was issued for eastern Poland to the eastern Baltic States mainly for excessive precipitation and a lesser extend large hail.
SYNOPSIS
Broad low geopotential and relatively cool mod-levels are present across Europe. An embedded, quite intense cut-off will travel from central Italy to the southern Adriatic region. A strong mid-level jet streak curves around its southern periphery. Downstream, a weak short-wave trough crosses Turkey within a broad south-westerly flow. Northern Europe is affected by an intense long-wave trough over the North Sea. A lifting short-wave trough moves over the Baltic Sea to Finland. It is followed by a strong vort-max that affects the North Sea and southern Scandinavia late in the period. At lower levels, a tongue of subtropical air situated across the Ukraine advects into western Russia, Belarus, and the eastern Baltic States. Relatively warm air is also present across the Balkans, whereas cool air had spread into most of Europe.
DISCUSSION
Adriatic Sea region
Strong low-level convergence is expected across the Adriatic Sea on Thursday. QG forcing ahead of the cut-off trough mostly due to DCVA at the cyclonically sheared flank of a strong mid-level jet streak will result in mid-level cooling. Steepening lapse rates are indicated by latest GFS model run above the warm sea surface overlapping with rich maritime moisture at low levels.
Thunderstorms are forecast to develop through-out the forecast period. Most active area will be the southern Adriatic given weaker lapse rates and moisture to the north where storms could be elevated. Vertical wind shear is strong over most of the Adriatic. The best overlap with surface-based CAPE is expected over the southern Adriatic, with deep layer bulk shear exceeding 20 m/s in the afternoon. A parallel stratiform MCS is forecast across the southern Adriatic where strong flow is nearly parallel to the low-level convergence line. This MCS can be capable of producing excessive precipitation that affects western Greece and the western Balkans. Additionally, large hail is not ruled out, especially when embedded supercells can develop. Further north, severe potential decreases due to weaker CAPE, but given strong low-level vertical wind shear, a tornado cannot be ruled out with surface-based storms.
Ukraine and Belarus to eastern Poland and eastern Baltic States
Within the warm air advection regime across eastern Europe, low-level moisture convergence is expected along a line from eastern Poland to the eastern Baltic States. Steep lapse rates present across eastern Europe will overlap with this moisture. Although low-level air mass is rather cool, diurnal heating may result in uncapped CAPE along the convergence line.
Current thinking is that thunderstorms can form along this line. These will move parallel to the line due to south-westerly flow. As a consequence, rather strong storms may form that can merge along the convergence line. Vertical wind shear increases to the north, where 15 m/s deep layer bulk shear overlap with CAPE. Locally, large hail is not ruled out. Main threat will be excessive precipitation due to numerous storms across the same area until the evening.
Further storms are expected across the Ukraine where diurnal heating is stronger. Steep lapse rates and CAPE in the hail growth zone is shown by latest soundings, and large hail is forecast. Overall threat is quite weak given the weak vertical wind shear and a threat level was not issued. Storms will move into Belarus later on where they weaken.
