Storm Forecast
Valid: Thu 27 Aug 2015 06:00 to Fri 28 Aug 2015 06:00 UTC
Issued: Wed 26 Aug 2015 23:36
Forecaster: GATZEN

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

A level 1 was issued or northern Morocco, northern Algeria mainly or large hail and severe wind gusts and local flash flooding.

SYNOPSIS

Ahead of an intense trough over the North Atlantic, a strong jet streak spreads across northern Germany into the Baltic States. Ridging extends from Iberia to the north Mediterranean and further into eastern Europe. Turkey and the Aegean as well as southern Mediterranean and northern Africa are affected by a weak mid-level trough.

At low levels, a plume o moist air extends from the Bay o Biscay to north-western Germany to southern Scandinavia, where it partly overlaps with steep lapse rates currently associated with thunderstorms over northern Germany and Denmark. Another plume o low-level moisture extents from Finland to western Russia and further into the Ukraine and the Balkans, but lapse rates are pretty weak over this region. Rich moisture across the Mediterranean is advected onshore over Italy, northeast Spain and southern France, and especially across northern Africa.

DISCUSSION

Northern Germany, Denmark, southern Norway and southern Sweden

A strong vort-max enters north-western Germany in the morning hours and spreads into inland late in the period. A strong jet streak will curve around this vort-max, providing strong DCVA. Ahead of this, a 15 m/s southerly low-level jet advects warm and moist air into Scandinavia. Given steep lapse rates sampled by latest soundings across northern Germany, CAPE is expected over the region on Thursday.

Thunderstorms are quite likely in the strongly-forced situation. These are expected ahead of a cold front that enters northern Germany in the morning hours and crosses southern Scandinavia on Thursday. Strong low-level vertical wind shear overlapping with the unstable air mass will pose a threat of well-organized storms especially across southern Sweden. The main limiting factor is convective debris and stratiform clouds that disables strong diurnal heating. But even as chances o thunderstorms are low, tornadoes are not ruled out. Threat will spread into the Baltic Sea later on where it gradually decreases.

In the wake of the cold front, more showers and thunderstorms are forecast. Although vertical wind shear is weaker, isolated storms may become severe, capable o producing severe wind gusts and tornadoes. The overall threat is pretty weak, though.

Northern Africa

Upslope flow of Mediterranean moisture, a well-developed elevated mixed layer, and the mid-level trough will assist in thunderstorm formation in the afternoon and evening hours. Although vertical wind shear is only below 15 m/s in the lowest 6 km, these storms may become severe given the high coverage of storms and the potential of clustering and merging. Large hail is forecast and local flash floods are possible. Additionally, well-developed outflow boundaries may be associated with severe wind gusts spreading south in the evening.