Storm Forecast
Valid: Mon 03 Jun 2019 06:00 to Tue 04 Jun 2019 06:00 UTC
Issued: Mon 03 Jun 2019 02:40
Forecaster: BEYER

A LVL 1 was issued for parts of Northern Germany and Southern Sweden mainly for large hail and severe wind gusts and excessive precipitation.

A LVL 1 was issued for Northern Turkey mainly for large hail and severe wind gusts.

A LVL1 was issued for parts of the Balkan Peninsula mainly for excessive precipitation

A LVL1 was issued for Southern Ukraine mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts

A LVL1 was issued for parts of the Caucasus (Georgia, Southern Russia) mainly for excessive precipitation and to a lesser extent for large hail and severe wind gusts


SYNOPSIS

A long wave trough is situated over Western Europe. A short wave feature is traveling northeastward during the forecast period, influencing France, Benelux, Germany, and parts of Southern Scandinavia. At the same time, the main trough axis is forecasted to amplify just west of Ireland. The constant southwesterly flow advects warm and moist air masses into parts of Western and Central Europe. While the steering surface low is forecasted to remain over the Norwegian Sea, a surface pressure trough with an embedded low-level convergence is forecasted to travel eastward on its southern flank.

For most parts of Eastern Europe, an amplifying ridge will bring stable conditions. Further downstream another upper-level trough is traveling eastward during the first half of the forecast period. As a result, unstable conditions will influence parts of Western Russia.

Finally, an only slowly northwards moving upper level low is active over most parts of Southeastern Europe. On its northern flank, a strongly diffluent upper-level flow can be anticipated.


DISCUSSION

... Parts of Northern Germany and Southern Sweden ...

Night time convection is still active over parts of Western and Northwestern Germany. This convection will decay during the morning letting behind a cool and cloudy airmass. The cold front of the surface low over the Norwegian Sea is entering Benelux and far Northwestern Germany in the second half of the day leading to further stabilization. We do not expect the cold front to be active due to the aforementioned lack of insolation. The time frame until the cold front arrives seems to be too short.

A low-level convergence zone embedded in a strengthening surface trough is moving eastward during the day. This surface trough is supported by a short wave feature in the mid- and upper troposphere that is moving from southwest to northeast on the forward flank of the main upper-level trough. While instability in the form of steep lapse rates is present within the surface trough, the highest values of specific humidity can be found further westward.
There is a region of best overlap of both ingredients further to the west of the surface wind shift line, which is rather unfavorable. However, CAPE values in that area will be around 800 J/kg, locally up to 1500 J/kg are possible.

These CAPE values will become mostly uncapped during the day and can overlap with shear values of 15 to 25 m/s of DLS. The further north, the better the shear conditions get. This good CAPE-Shear environment opens the possibility for organized convection and large hail events. In addition, low levels look rather dry with inverted-V-like prognostic soundings. Thus there is also an enhanced threat of severe wind gusts. Local area models also point to that risk.

It is thought that storms will initiate in the late noon/early afternoon hours. Having DLS shear vectors rather parallel to the LL convergence line, storms should rather quickly organize into a line. Having values of 30 to 35 mm and the mentioned parallel flow will enhance the threat of excessive precipitation, especially during the later stage of convection.

In the late evening and night hours, the convection is forecasted to leave the synoptic forcing when moving further east, since the short wave trough is then moving to the north.

Although a few severe events are forecasted, they are not expected to be widespread due to a not so good of the ingredients and a rather fast development into a line with parallel mid-level winds. This prevents from issuing a LVL2.

Further to the south over Southwestern Germany and Switzerland, a lack of synoptical forcing prevents from better-organized convection. However, convection is forecasted to develop in a similar airmass with moist and unstable conditions. Local excessive precipitation events are not excluded but the overall threat is not high enough to justify a LVL1.

...Northern Turkey...

During the day moisture coming from the Black Sea is advected inland and can overlap with steep lapse rates that are present due to strong diabatic heating. As a result, CAPE values of 1000 to 1500 J/kg can develop along the northern coast of Turkey. DLs is forecasted to reach values of 10 to 15 m/s, locally even 20 m/s are possible, depending on the low-level flow.
Since the area of interest is situated under a diffluent upper-level flow, synoptical forcing is present which additionally can be supported by local lift mechanisms due to orography.

The main threats due to the good CAPE-Shear overlap are splitting supercells (long straight hodographs) that may bring large to isolated very large hail and severe wind gusts.

... Parts of the Balkan Peninsula...

Nighttime convection is already ongoing when the forecast period starts. This activity makes the forecast of daytime convection somewhat uncertain. After a weakening of convective activity during the morning hours, new widespread convection is forecasted throughout the day.

Daytime heating will lead to a steepening of the Lapse Rates that can combine with enhanced moisture to CAPE values of a few hundred to locally 1500 J/kg of CAPE. At the same time, the area of interest is situated under a diffluent mid- to upper-level flow. These ingredients will, as a result, lead to widespread initiation of convection.

Due to a lack of DLS, this convection will stay rather unorganized but will have a pulsating character. PPW values of 30 to 35 mm and a rather weak storm motion thus result in an enhanced threat of excessive precipitation. Before clustering of convection, there is a chance for a local large hail event.

The LVL1 area covers the regions where ingredients of excessive precipitation are best. However, a local excessive precipitation event surrounding that area cannot be excluded. Especially if one has in mind the aforementioned uncertainties due to the leftovers of night time convection.

... Southern Ukraine...

Very moist air masses are present in that area with specific humidity values of up to 14 g/kg and ppw-values of up to 40 mm. Combined with steep lapse rates, CAPE values of up to 2000 J/kg are forecasted to develop in that area. At the same time also Southern Ukraine is on the forward flank of the upper level low in a diffluent flow regime.

Developing convection may bring excessive precipitation as the main threat mainly due to the high amount of moisture and only moderate mid-level wind (20 kn).

In addition, DLS is locally enhanced with values of up to 15 m/s. Therefore, a few better-organized storms are possible that then may bring severe wind gusts and large to locally very large hail.

...Caucasus (Georgia, Southern Russia)...

Again high CAPE values and high amounts of moisture are present. The area of interest is just to the west of the axis of the downstream situated strong ridge and thus under a somewhat diffluent upper-level flow regime.

Having ppw-values of up to 40 mm and only moderate mid-level winds (about 20 kn) excessive precipitation can be expected. DLS locally reaches values of up to 15 m/s. Thus locally better-organized convection may bring severe wind gusts and large hail to very large hail.