Storm Forecast
Valid: Mon 26 Jul 2021 06:00 to Tue 27 Jul 2021 06:00 UTC
Issued: Mon 26 Jul 2021 02:26
Forecaster: BEYER

A level 3 was issued along the northern Alpine rim in Austria mainly for (very) large hail, (extreme) severe wind gusts, and excessive precipitation.

A level 3 was issued for parts of Northern Italy mainly for (very) large hail, (extreme) severe wind gusts, and excessive precipitation. A tornado is not excluded.

A level 2 was issued for the surrounding Greater Alpine region mainly due to the same threats but with less likelihood for extreme events.

A level 2 was issued for Northeastern Germany into Denmark mainly due to excessive precipitation.

A level 1 was issued for parts of Central Europa for locally excessive precipitation.

A level 1 was issued for Eastern Poland mainly for excessive precipitation

A level 1 was issued for Eastern Spain mainly for excessive precipitation and severe wind gusts.

A level 1 was issued for Northern Turkey and Caucasus mainly for excessive precipitation and large hail and to a lesser extent for severe wind gusts.

A level 1 was issued for parts of Northwestern Russia mainly for severe wind gusts.

SYNOPSIS

A long-wave trough is persistently situated over Western Europa that has an independent center over NW France and the North Sea. Central and Eastern Europe are lying on the forward flank of this through in a diffluent upper-level flow. Warm and humid airmasses are present in all of this area.

Several short wave features are traveling northeastward leading to maxima in the omega field, responsible for the different regions of interest.

The trough axis is situated over E Spain and travels slowly eastward throughout the forecast period.

Another but weaker trough can be found over SE Europe. NE Turkey and the Caucasus region is situated on its forward flank.

NW Russia is influenced by the rear flank of another trough. A cold front is passing this area from north to south. Thereafter cool airmasses spread that region with a northwesterly flow.


DISCUSSION

... Greater Alpine region ...

Ahead of the diffuse and almost stationary cold front, a plume of steep lapse rates (the remnants of an elevated mixed layer from the Sahara) are reinforced by diurnal heating over the Alps. As they overspread moist low-level air over the forelands with 2m dewpoints in the upper tens or locally lower twenties, about 500-1500 J/kg CAPE are created and overlap with moderate deep-layer shear (10-15 m/s) and veering wind profiles in the lowest layers. Even more CAPE and stronger shear are likely in N Italy, which is situated closest to the jet maximum, and more locally along the north Alpine rim in Austria, where moist NE-erly upvalley circulations undercut the SW-erly mid-level flow. In these two regions, an overlap of ~2000 J/kg CAPE and ~20 m/s deep-layer shear will likely be realized according to high-resolution forecast models.

With the help of various mid-level vorticity maxima and the rather strong SW flow storms will initiate on the leeward flank of the N Apennines and E Alps in the afternoon and evening hours. A high degree of storm organization into strong multicells and supercells is expected given the high shear values. High resolution models simulate several long-tracked supercells.
Later some of these storms may grow into a few MCSs. While at the beginning large hail and severe wind gusts are a main, clustering will enhance the possibility for excessive precipitation. Rather high mid-level shear (0-3km) also enhances the risk for bowing segments that then again pose a threat for severe wind gusts.
Isolated to scattered extreme events are foreseen, especially in the mentioned regions of best overlap of enhanced CAPE and vertical wind shear.
In the plains of N Italy, low cloud bases and locally enhanced 0-1 km shear also point to a possibility of one or two tornadoes.

... NE Germany into Denmark ...

A maximum in LL moisture overlaps with instability. As a result, 1000 to 1500 J/kg of MLCAPE are forecasted. Along a LL convergence zone storms are forecasted to initiate during the day. Precipitable water is high with values around 35 mm. Having this rather persistent convergence zone and only moderate storm movement, high to excessive amounts of precipitation are expected that may result in local floodings. Even a few extreme events with more than 40 l/qm in less than 1h or more than 60 l/qm in a few hours with repeated convection are possible.

The LVL 1 surrounding the LVL 2 also has the risk of excessive precipitation. Lower PWAT values and lower coverage minimize this threat, though.

... E Poland ...

A vorticity maximum can be found that is also visible as a short wave trough in the 500 hPa isohypse field. This vorticity maximum is a remanent of the MCS that was active the night before over the Czech Republic. It influences this region in the afternoon and should initiate convection in a very moist environment. Precipitable water goes locally up to 45 mm. DLS instead is only weak. Given the high amount of liquid water clustering convection can lead to excessive precipitation.

... E Spain ...

This region is influenced by an UL trough moving eastern into the W Mediterranean Sea. LL moisture is advected inland from the Sea with an easterly surface flow and can further inland overlap with steep lapse rates. That results in CAPE values between 500 and 100 J/kg. Shear is rather weak.
Given high values of precipitable water locally excessive precipitation is possible. On a relative basis, the low-level troposphere is rather dry. Forecast soundings show some inverted V-shapes and DCAPE rises, especially further inland. Thus there is also an enhanced threat local downbursts

... NW Russia ...

A cold front is moving southward and moisture is forecasted to accumulate along this boundary that also is marked by a significant wind shift line. Overlapping with instability, a few hundred up to 500 J/kg of CAPE should develop during the day, mainly concentrated in the lower to middle troposphere.

Having mid-level shear values of up to 15 m/s and a rather strong mid-level flow, there is a certain chance of severe wind gusts. However, this is only a low-end LVL 1 threat.

... N Turkey and the Caucasus...

Low-level moisture is advected from the Black Sea and can overlap with steep lapse rates further inland. As a consequence, several hundred up to 1000 J/Kg of CAPE are forecasted. Given the high values of precipitable water, an enhanced threat of excessive precipitation is present.

Over the mountainous Caucasus area, orographic pumping will take place during the day and will lead to moisture convergence. Thanks to the orography models forecast rather strong DLS values of 15 to 20 m/s. Thus organized convection bringing large hail is possible. Also, severe wind gusts are on the list having inverted-V profiles with forecast soundings and enhanced DCAPE values.