Severe Thunderstorms expected over Bohemia, E Germany, and W Poland
Valid: Wed 12 Jun 2019 06:00 to Thu 13 Jun 2019 06:00 UTC
Issued: Tue 11 Jun 2019 21:53
A level 2 was issued across Bohemia, E Germany and W Poland mainly for large hail, severe wind gusts and excessive precipitation.
A level 1 was issued for Central Germany, Denmark and Southern Sweden mainly for excessive rainfall and to the lesser degree for large hail.
A level 1 was issued in a belt from the Baltic Sea into W Russia mainly for large hail.
A level 1 was issued for W Poland, Central Czech Republic, Austria, N Italy, Slovenia, Croatia and Bosnia mainly for large hail, severe wind gusts and excessive precipitation.
A level 1 was issued for Turkey mainly for excessive precipitation and large hail.
Main macrosynoptic feature, a deep cyclonic vortex will be centred over NW France and its associated short-wave trough is forecast to eject from Central France towards BENELUX and NW Germany during the forecast period. At the same time, the centre of the vortex will translate slowly towards the British Isles and the Northern Sea. With the zone of cyclogenesis moving from France over the Northern Sea, previously stalled frontal system will start moving. A warm front, stretching from S Sweden towards Estonia and W Russia will move towards N, while a cold front, located over Germany will move towards the Czech Republic and Poland. With abundant lower tropospheric moisture and steep lapse rates ahead of the front, scattered to widespread convective storms are forecast over a large area. Widespread storms are also forecast over parts of Balkans and Turkey underneath a shallow cyclonic vortex, centred over Turkey.
Germany to W Poland and Denmark
At the time of forecast writing (Tuesday 21 UTC), abundant convective storm activity is ongoing over E Germany and W Poland with a history of very large hail production. Should this activity persist into the late morning hours, it may have a serious impact on subsequent development over the area via the cloudiness and the outflow boundaries.
NWP is consistent regarding the moisture return over the area in SE-ly flow ahead of the convergence zone and the frontal zone that will cross the region in the late afternoon and evening hours. As the steep lapse rates are replenished by S-ly flow, another day with moderate to high MLCAPE values, locally exceeding 2000 J/kg, is anticipated. While vertical wind shear is forecast to decrease slightly compared to the previous two forecast periods, the probability of convective initiation will be actually more certain given the cold front passage across the area and weak synoptic-scale lift from the approaching trough.
Current thinking is that in the morning hours, one or two convective systems may be ongoing over Central and N Germany. These are forecast to move N-ward and pose a threat of excessive rainfall and isolated large hail. One of the systems may eventually cross Denmark into S Sweden. During the afternoon hours, initiation is forecast along the convergence zone, the cold front and along the mountains. High-resolution NWP suggests that several of these cells may start rotating, but with bulk shear around 15 m/s mostly concentrated in the bottom 3 km and expected clustering, linear segments will develop with time. Large to very large hail will be the initial threat, giving way to severe wind gusts and excessive precipitation in the later stages of development as storms form one or more convective systems and spread N and NE-wards. A broad Lvl 2 was introduced for the region, where more widespread severe weather is forecast.
Elevated thunderstorms capable of excessive rainfall and isolated large hail are forecast as the warm front lifts N-ward and lapse rates reach around 7 K/km in the mid-troposphere. While forecast hodographs reveal environment of high SRH, elevated nature of storms will likely preclude them from ingesting vorticity-rich airmass.
Baltic Sea into W Russia
Isolated to scattered thunderstorms are forecast along the advancing warm front. Decent vertical wind shear and steep mid-tropopsheric lapse rates may allow for isolated large hail with stronger cells.
Lack of steep mid-tropospheric lapse rates will limit CAPE values over the region and initiation is uncertain over the Po Valley and along the Apennines. Nevertheless, high resolution NWP suggests that one or two supercells may cross the lowlands with the threat of large to very large hail and some isolated severe wind gusts. Initiation is more likely along the southern Alpine rim, where isolated heavy rainfall event is not ruled out.
SE Austria, Slovenia, Croatia, Bosnia
On Tuesday, several severe storms formed over this region, at least one of them being capable of damaging hail. Environment should remain similar on Wednesday, albeit a belt of high CAPE will shift more eastwards. While background shear is moderate at best, local mesoscale circulations may enhance the DLS to around 15 m/s, which may prove enough again for some rotating updrafts. Combined with high CAPE, isolated large to very large hail and severe wind gusts are forecast over this region.
Slow-moving, widespread thunderstorms are forecast over the area. Stronger cells will be capable of excessive rainfall, due to the high RH throughout the low and mid-troposphere, and perhaps some isolated large hail events.