Storm Forecast
Valid: Mon 11 Jun 2018 06:00 to Tue 12 Jun 2018 06:00 UTC
Issued: Mon 11 Jun 2018 02:34
Forecaster: BEYER

A level 2 was issued for parts of Southern Germany into Western Czech Republic for severe wind gusts, large hail and heavy precipitation

A level 1 was issued for a broad area over Central Europe into Eastern Europe mainly for excessive precipitation but also large hail and locally severe wind gusts

A level 1 was issued for parts of Finland and Northwestern Russia mainly for severe wind gusts and tornados and to a lesser extent for large hail

SYNOPSIS

The general weather situation is characterized by an airmass boundary in zonal orientation over large parts of Western and Central Europe. This boundary separates cooler airmasses over Northwestern and Northern Europe from a warm and humid airmass south of it.

An upper level low is situated over Western France and is slowly moving eastward. On its forward flank warm airmasses are advected northward with a southwesterly flow. At the same time a short wave trough is moving from the British Isles to Denmark during the forecast period. In between both features the airmass boundary is reinforced during the day. In addition a significant short wave trough is forecasted on the forward flank of the upper level low. Its initiation is probably due to or at least supported by the southwesterly flow over the Western Alps.

Downstream another short wave trough is forecasted over Southern Norway moving eastward. On its forward flank the moist and humid airmass is advected northward. At the surface level a cold front is moving to the east and its activation is expected during the day due to the support of the short wave trough.

Looking further to the north another trough can be anticipated over Northern Scandinavia that is embedded in a jet maximum in 300 hPa. It is also moving eastward which is also true for a corresponding well defined surface low.

Finally an upper level low can be found over the Eastern Mediterranean providing lift for Turkey and further downstream.


DISCUSSION

... Central Europe...

The airmass in the LVL1 area is the same as it was the days before. It is characterized by high values of specific humidity (11 to 14 g/kg) and precipitable water (30 to 35, locally up to 40 mm). At the same time mid-level flow is mostly rather weak (5 to 15 kn in 500 hPa). Over Southern France the upper level flow is stronger but coming from the south supporting backward building of storms.
In addition the divergent upper level flow will provide enough lift to get widespread initiation during the day – along and south of the airmass boundary. Convection from the night is still ongoing and will thus initially weaken the risk of severe weather in the area where it is active. However its remnants together with the orography will dictate the region for new initiation during the day.
Humidity and instability should lead to CAPE values of 800 to 1500 J/kg in the afternoon. Mostly weak DLS values are present over central France. Thus the main severe weather risk in that area is the occurrence of excessive precipitation. Storms will be rather disorganized but may cluster throughout the day.

DLS values are higher over Southern France (around 15 m/s) and also towards Germany. In those areas better organized convection is possible and in addition to excessive precipitation also the risk for large hail and locally severe wind gusts is enhanced.


... Southern Germany and Western Czech Republic...

During the day it is expected that numerous storms may develop along and south of the cold front. Having DLS values around 15 m/s also better organized storms should be expected. Models also suggest rotating updraft so that a few supercells may develop during the day. Those storms can thus bring large hail and severe wind gusts as well as excessive precipitation.

And interesting setup is expected to develop during the late afternoon and evening hours when the pronounced short wave feature initiated over the Alps is moving northeastward. High resolution models suggest the development of an MCS in the lee of the Black Forrest that is then moving northeastward.

Such an MCS may bring large hail at its initial stage as well as severe wind gust. With a better developing cold pool in the major stage of the MCS severe wind gusts can become more widespread. High resolution models also suggest the development of a squalline. Up to hurricane force wind gusts are then possible.
Apart from that – having this humid airmass with high ppw’s – also excessive precipitation and local flooding should become a prominent threat, especially during the major stage of the event.

In second half of the night the MCS should weaken when moving further northeastward due to the upper level ridge and thus the lack of dynamic support.

... Eastern Europe...

The focus of that region will be the eastward moving cold front that should be activated during the day with the help of the upper level short wave trough. Wind vectors are not that strong with about 15 kn in 500 hPa. However they are mostly perpendicular to the cold front. With an activated cold front this enhances the possibility for severe wind gusts. Beside that large hail is locally possible, although DLS is forecasted to be only weak. High values of ppw support the risk for excessive precipitation events.
However the threat in that area is only on the low end side of a LVL1.

...Finland and Northwestern Russia...

Although the airmass in that area is by far not that rich of energy a LVL1 seems to be necessary due to the dynamic conditions.

Ahead of the short wave trough over Northern Scandinavia specific humidity value of up to 9 g/kg can overlap with steep lapse rates resulting in moderate CAPE values of about 500 J/kg.

On the southern flank of the surface low a nice zone of horizontal convergence is forecasted with southeasterly surface winds downstream and westerly winds upstream. Deep layer shear is forecasted to lie between 15 and 20 m/s and LLS values show a maximum of 10 to 15 m/s. In addition hodographs show a clockwise turn of the wind vectors, especially in the lower layers.

This altogether enhances the risk for the occurrence of tornados which is the main reason a LVL1 was issued in that region. In addition severe wind gusts and large hail can occur with possible developing supercells.