Storm Forecast
Valid: Mon 16 May 2022 06:00 to Tue 17 May 2022 06:00 UTC
Issued: Sun 15 May 2022 20:49
Forecaster: ROSBERG

A level 1 was issued for eastern France, Benelux, Germany and the Alpine region for excessive convective precipitation and to a lesser extent hail and severe wind gusts.

A level 1 was issued for eastern Bulgaria, northeastern Greece and western Turkey for severe wind gusts, hail and to a lesser extent excessive precipitation.

A level 1 was issued for the Caucasus-region, eastern Turkey and northern Iran for severe wind gusts, hail and to a lesser extent excessive convective precipitation.


SYNOPSIS


A stationary longwave upper level trough covers large parts of western Russia and Finland, and is flanked by an upper level ridge over the Norwegian Sea. Upper level ridging is also occurring over the western part of the Mediterranean.

A shortwave trough is expected to pass the UK, northwestern France and Benelux from southwest, while another shortwave trough will move east- and southeastward across the Black Sea region and Turkey. A third shortwave trough/upper level low over the Caucasus-region, eastern Turkey and northern Iran is expected to move eastward towards the Caspian Sea.

At the surface an area of low pressure over northern Russia generates a north and northwesterly flow of cold air over northwestern and western Russia, Finland and the Baltic States with diurnally driven showers and a couple of thunderstorms.

An area of low pressure over northeastern Atlantic generates a southwesterly flow of warm and quite moist air across southwestern, western and central Europe. An associated frontal boundry is located from the UK to western Europe. This frontal zone is expected to move northeastward across central Europe and the North Sea, being the main trigger for deep convection over these areas.

A surface trough over the Caucasus-region and easternmost Turkey is moving eastward, and this surface trough will be the main focus for deep convection over these areas.

A cold front is expected to approach northwestern Spain in the end of the forecast period from west. Despite favorable forcing and lifting mechanisms combined with strong DLS, the mid-level lapse rates will be so weak that the convection will be so shallow that it barely genrates lightning. Thus, no severe weather is expected but a few strong wind gusts cannot be ruled out in connection with the shallow convection.


DISCUSSION


The UK, France, Benelux, Germany and the Alpine region

Showers and isolated elevated storms will be ongoing Monday morning along the frontal boundary across the UK, France, Benelux, southwestern Germany and the Alpine region. As the day progresses and the frontal boundary moves northeastward, shower and thunderstorm activity will increase in coverage and become at least partly surface based. However, storms will remain elevated across the North Sea due to the cold surface water.
The convection will be trigged by the lift from the frontal boundary combined with solar heating. Showers and storms will also form by mountain-valley circulations over the Alps.
Moisture pooling along the frontal boundary with mixing ratios locally over 10 g/kg will contribute to 500-1000 J/kg MLCAPE in connection with the frontal boundary, locally slightly higher. Low deep layer shear, moist sounding profiles with quite low LCLs and preciptable water locally in excess of 30 mm indicate a risk for very heavy rains with these storms with a flash flood threat.

In some spots DLS of 10-15 m/s will make it possible for cells to organize into clusters and/or lines with risk of hail and strong to severe wind gusts.

Behind the front some showers and storms are expected to form across England, Ireland and perhaps also France during Monday afternoon. Convection might struggle to form in France due to weaker lapse rates and some dry air at low- and midlevels.
A couple of hundreds of MLCAPE and 10-18 m/s could generate some multicells with risk of hail and strong wind gusts in these areas. However, the convection does not look to be vigorous enough and coverage to be extensive enough to warrant a level 1.

During the evening and night to Tuesday showers and storms will weaken and become elevated as the frontal boundary will continue eastward into southern Denmark, southwestern Poland, Czech Republic, Slovakia, Hungary and southwestern Romania, with a diminishing wind and flood threat.


Western and central Turkey and southeastern Bulgaria

Showers and thunderstorms are expected to develop across western Turkey, northeastern Greece and eastern Bulgaria due to a combination of solar heating, mountain-valley circulations and sea breeze convergence. A few showers and storms might also pop up over the Carpathian Mountains.

As the shortwave upper trough approaches and passes over the area, the synoptic scale forcing and slightly cooler temperatures aloft will increase the mid-level lapse rates. This combined with dewpoints in the range of 8-15 degrees Celsius will yield areas with locally 500-1000 J/kg MLCAPE. Some locally deep layer shear of 10-15 m/s could contribute with some organization into multicells with threat of hail and strong to severe wind gusts. The wind threat will be supported by quite high LCLs that will contribute with evaporational cooling strengthening the downdraft potential.

The convective activity will taper off after sunset due to lack of solar heating, but a few showers and perhaps a storm could linger during the night over Turkey.


The Caucasus-region, easternmost Turkey, northern Iran

Showers and a couple of storms will be ongoing Monday across the Caucasus-region, easternmost Turkey and northern Iran in connection with the surface trough. The thunderstorm activity is not expected to be that extensive due to weak mid-level lapse rates and low EL-heights. DLS will be mostly low, under 10 m/s, with a flash flood threat.

Over northern Iran very strong DLS of 20-30 m/s, in connection with strong upper level winds south of the upper level low, will make it possible for any storm that forms to evolve into supercells with risk of large hail and severe wind gusts. However, just isolated convection should be possible in this region due to dry air at low- and midlevels. Monday morning there is a short time window when there are some curved hodographs in connection with the area with strong DLS, before the hodographs become more straight later in the day. During this time storms tend to be elevated and have high LCLs, and thus tornado potential will be very low.

As the surface trough moves eastward showers and storms are expected to be out of the area in the late afternoon or evening.


Italy, the Balkans, Albania and Greece

Showers and thunderstorms will form across mountainous areas across Italy, the Balkans, Albania and Greece by solar heating, mountain-valley circulations and sea breeze collisions/convergence. The solar heating combined with mixing ratios up to 10 g/kg and 6.5-7 K/km mid-level lapse rates will yield up to 500-1000 J/kg MLCAPE.

Deep layer shear will be low, under 10 m/s, and thus heavy rains with a threat of flash flooding will be the main hazard from these storms due to the slow-moving cells. Isolated hail and strong to severe wind gust events could occur with stronger cells, but the activity does not look widespread enough to warrant a level 1.

The showers and storms will taper off and die out during the evening and night due to lack of solar heating.


Algeria and Tunisia

Isolated diurnally driven showers and thunderstorms are expected to develop Monday afternoon across northern Algeria and Tunisia. Very dry low-level air and quite weak mid-level lapse rates will prevent any extensive coverage.
Inverted V-profiles indicate a risk of strong downdraft potential in any storm that might form, but due to the sparse coverage no level 1 has been issued.


Western- and northwestern Russia, the Baltic States and Finland

Showers are expected to form within the cold airmass across large parts of Russia, the Baltic States and Finland by solar heating and along areas of surface convergence near the low-pressure area. Low EL-heights and lack of steep mid-level lapse rates will prevent widespread thunderstorm activity, but slightly higher coverage is possible over eastern part of Russia where the best lapse rates are expected to occur.

A strong mid-level jet, located south of the long wave trough over Russia, will be located from Scandinavia down across eastern Europe to southwestern Russia, generating strong deep layer shear over these areas. However, convection will not be deep enough to take advantage of the favorable shear conditions. That being said heavy rain, gusty winds and small hail might occur in stronger cells, especially over eastern Russia, but convection will not be vigorous enough to warrant a level 1.