Storm Forecast
Valid: Tue 20 Jun 2023 06:00 to Wed 21 Jun 2023 06:00 UTC
Issued: Mon 19 Jun 2023 22:51
Forecaster: BEYER

A level 2 was issued for S and E France, N Switzerland, and S Germany mainly for large to locally very large hail, severe to locally extreme wind gusts, and to a lesser extent for excessive precipitation (the latter especially for S France).

A level 2 was issued for NE France and parts of BeNeLux mainly for large hail, severe wind gusts, and heavy precipitation. A tornado cannot be excluded.

A level 1 surrounds the two level 2 area and includes the Pyrenees and the Alps. This level 1 was issued for the same reason but with less likelihood or coverage.

A level 1 was issued for NE Germany and parts of Poland.

A level 1 was issued for parts of E Spain mainly for large to very large hail as well as severe wind gusts.

A level 1 was issued for parts of E Europe (mainly Belarus and Ukraine) mainly for excessive precipitation and for an enhanced risk of landspouts.

A level 1 was issued for E Bulgaria, parts of Turkey, and S of Caucasus mainly for excessive precipitation.

SYNOPSIS

A complex synoptic situation on the forward flank of an amplified long wave trough influences large parts of SW, W, and C Europe. Several short-wave features can be anticipated in the prevailing upper-level SW flow.

On significant short-wave trough was responsible for the convective activity over France on Monday. Its remnants are crossing Germany NEward in the morning hours. These remnants and the weakening short-wave trough may influence the development in NE/E Germany and Poland during the afternoon.

Another prominent short wave feature will influence N France, Great Britain, and BeNelux in the second half of the day and the night hours. It is accompanied by an UL wind maximum (UL jet). In lower levels, a cold front can be analyzed that is followed by CAA.

Finally, several short-wave features are forecasted to develop N of the Pyrenees, Massif Central, and Alps. These features are induced by the named mountains (given the SW flow) and previous convection. They are a bit difficult to find in the weather maps but can be anticipated by IPV maxima. However, models differ (not surprisingly) in their positioning and timing.

E and SE Europe are influenced by another long wave trough. On its S flank, two weak UL lows can be found. One is moving from Belarus to Ukraine and another one is active over Turkey, leading to unstable conditions.


DISCUSSION

... S and E France, N Switzerland and S Germany ...

Steep lapse rates can overlap with LL moisture (specific moisture 10 to 14 g/kg) leading to high CAPE values of 1000 to 2000 J/kg. This high potential energy can overlap with strong DLS of 20 to 25 m/s (0-6 km).

The main difficulty is to anticipate the region and timing of initiation. This is mainly because there is no clear trigger but several short-wave features as described in the synopsis. ICON is forecasting the main trigger starting over S France around noon and moving NE ward reaching Germany during the evening. However, orography can also help to initiate storms (e.g. N of the Alps).

Developing storms have the chance to become supercells given high DLS. Rotating storms are also forecasted by UH tracks from ID2 EPS. These storms a capable of producing large to even very large hail given the good overlap of high CAPE and high shear.
Another prominent threat will be severe wind gusts. Besides the argument that supercells alone are capable of producing severe wind gusts LAMs also give hints for small bowing segments that may develop. In addition, LCLs are quite high with inverted-V profiles. This is also supported by high DCAPE of 500 to 1000 J/kg. This is mainly true for S Germany where some LAMs forecast some extreme wind gusts. Coverage, however, remains questionable and this is the reason we stayed with a LVL2.

During the evening and night hours, storms may develop into one or two MCS that may travel eastward. That is the reason the surrounding LVL 1 was extended so far E.

For parts S France persistent convective activity is forecasted throughout the night accumulating to some high amounts of precipitation.


... NE France and parts of BeNeLux ...

On the forward flank of the approaching shot wave trough CAPE values of 1000 to 1500 J/kg are forecasted to develop over E France and parts of BeNeLux. CAPE can overlap with DLS values of around 15 m/s. During the afternoon hours, storms are forecasted to develop. They may become supercells as supported by ID2 UH tracks and may bring large hail and severe wind gusts.

Later in the outlook period storms are forecasted to organize into a line given the linear forcing (LL convergence along the slowly E ward moving cold front). This reduces the chance for large hail but still, severe wind gusts may play a role when bowing segments will develop.

Another threat is excessive precipitation, especially over NE France where the movement of the storms is rather parallel to the boundary. This may lead to repeated and persistent convection.

Finally, tornados cannot be ruled out during the phase of discrete storms. LCLs are forecasted around 1000 m and LLS will have values around 10 m/s.

During the late evening storms will weaken when the short wave feature is moving off to the N and CAA is flooding the area.

... NE Germany and parts of Poland ...

This LVL1 is a bit questionable due to remnants of overnight convection that will reach the area in the first half of the day. Some model forecast that the airmass can recover quickly and CAPE values of 1000 to even 2000 J/kg may develop. DLS values are around 15 m/s. If storms indeed develop they are capable of producing large hail and excessive precipitation given the high ppw values (up to 40 mm).

... E Spain ...

High CAPE values of 1000 to locally 1500 J/kg can overlap with high DLS of 20 to 25 m/s. Due to the lack of synoptic scale forcing orography is the focus of initiation. Developing storms can produce large to locally very large hail. Also strong to severe wind gusts can accompany the storms having a rather dry BL (inverted-V).

... E Europe (mainly Belarus and Ukraine) ...

Steep lapse rates are induced by the UL low that is slowly moving S-ward. In addition, moist airmasses are present (12 g/kg specific moisture). The result are CAPE values of around 1000 J/kg. Shear is only weak and so is the storm motion. Having in mind ppw values of 30 to 35 mm slow moving or even persistent storms bring a threat of excessive precipitation.

In addition, there is an enhanced threat for landspouts. Weak UL winds and LL convergence are present as well as low LCLs (<1000 m).

... E Bulgaria, parts of Turkey and S of Caucasus ...

Another UL low influences this area. CAPE values are forecasted between 500 to 1000 J/kg during the afternoon. Having weak UL winds storms will move only slowly and also orography influenced their development. The main threat will again be excessive precipitation with persistent storms and the inland advection of moist LL airmass from the Black Sea.