Storm Forecast
Valid: Thu 03 Aug 2017 06:00 to Fri 04 Aug 2017 06:00 UTC
Issued: Thu 03 Aug 2017 00:04
Forecaster: GATZEN

A level 2 was issued for north-eastern Germany and north-western Poland mainly for tornadoes and severe wind gusts and to a lesser extent large hail.

A level 1 was issued for northern Poland, Denmark, southern Sweden, and northern Germany, mainly for tornadoes and severe wind gusts and to a lesser extent large hail.

A level 1 was issued for southern Germany, the Alps, western Czech Republic, western Poland mainly for large hail and to a lesser extent severe wind gusts and tornadoes.

A level 1 was issued for western central Russia mainly for large hail and tornadoes.

A level 1 was issued for northern Ukraine into western Russia mainly for excessive rain.

SYNOPSIS

Strong mid-level flow has established across western, central, and north-eastern Europe. Embedded short-wave troughs move over north-western Russia, from Benelux to the Baltic Sea region, and across western Europe during the period. Broad ridging over southern Europe includes mid-level disturbances over the west Mediterranean, the Balkans, and over the Aegean and Turkey area. At lower levels, a wavy frontal zone extends from central France to southern Germany and Poland into western Russia. Rich low-level moisture is present along this boundary, whereas the elevated mixed layer is found to the south of it from the Mediterranean toward southern France, Alps, and northern Balkans, and locally advects northward where it overlaps with the rich moisture.

DISCUSSION

Germany into Denmark and southern Sweden, Czech Republic, and Poland

At the base of the short-wave trough spreading over central Europe on Thursday, a 40 m/s mid-level jet streak enters western Germany on Thursday noon, reaching central Poland late in the evening. Warm air advection ahead of this trough affects Germany during the day, and an elevated mixed layer can spread into parts of Germany, Czech Republic, and Poland. At lower levels, especially high-resolution models indicate that winds back to the south, allowing rich moisture to spread northward before drier air masses enter the area in the afternoon and evening behind a cold front.

Elevated storms may be possible along the warm air advection regime over south-western Germany, spreading north-eastward during the day. Additional showers and thunderstorms may be also ongoing over the North Sea, entering northern Germany and Denmark. While CIN increases from the south in the morning hours, large-scale lift and diurnal heating weaken the cap during the day, with larger CAPE development especially from southern Germany towards western Czech Republic and western Poland according to the position of the elevated mixed layer. It is possible that surface-based storms can form in the afternoon and evening from south-western Germany towards the Czech Republic and south-western Poland. Further storms are expected to initiate across central and northern Germany ahead of the cold front where CIN will be close to zero due to stronger lift.

Deep-layer vertical wind shear across the forecast area is strong, allowing for well-organized storms including supercells. Given higher CAPE values in the south-eastern portions, possibly up to 1000 J/kg, large hail is expected with any surface-based storm that can sustain in the capped environment. Additionally, severe wind gusts are not ruled out. Increasingly long, straight-line hodographs are expected to develop in the afternoon, with stronger low-level shear later on also over the southern portions, increasing the potential of severe wind gusts and a few tornadoes before drier air spreads east. Expected coverage of severe weather is larger in the northern portions where more widespread initiation is forecast due to the weaker CIN.

Curved low-level hodographs can evolve at the cyclonically sheared flank of the mid-level jet streak due to backing surface winds, in particular over northern and north-eastern Germany into north-western Poland. Latest models indicate quite large low-level buoyancy as well, posing a higher risk of tornadoes and severe wind gusts compared to the region further south. Up to 15 m/s 0-1 km bulk shear and 20 m/s 850 hPa wind speed point out that strong tornadoes are possible.

Storms will decay from the west as the cold front approaches. Convective activity shifts to western Poland in the evening and night where it weakens due to boundary layer cooling.

Alps

A few storms are expected over the Alpine range given upslope flow and rich low-level moisture. The cap is likely too strong to allow the storms to spread into the low lands. With deep layer vertical wind shear around 15 m/s, storms may produce large hail and severe wind gusts locally. Overall threat is limited.

Northern Ukraine into western Russia

Along a quasi-stationary frontal boundary, rich low-level moisture is present. Remaining lapse rates over the Ukraine allow for 1000 J/kg CAPE, but mid-level sinking is expected and low-level convergence may be rather weak. Additionally, weak vertical wind shear is present so that storms will be weakly organized, possibly causing locally excessive precipitation. Further east, increasing vertical wind shear can be sufficient to support multicells and a few supercells over Russia. With strong low-level vertical wind shear in the wake of the frontal boundary that spreads south during the day, and rich low-level moisture advecting from the north-west, a tornado is not ruled out in the afternoon and evening.

Northern Scandinavia into north-western Russia

A trough axis moves from Finland towards Russia during the day. In the base of the trough, a weakly capped air mass will be associated with showers and thunderstorms. Low-level vertical wind shear will increase to the east across Russia, where rotating storms are not ruled out, capable of producing tornadoes and large hail. Severe winds gusts are also possible. Main limiting factor is weak CAPE due to the weak moisture, and overall potential is expected to be too low for a risk level.

Turkey

Although moisture is limited, a few storms may form over the mountains in the afternoon. Vertical wind shear is limited, but a few large hail events are not ruled out.