Storm Forecast
Valid: Wed 15 May 2013 06:00 to Thu 16 May 2013 06:00 UTC
Issued: Wed 15 May 2013 04:34
Forecaster: PUCIK

A level 2 was issued for Algeria mainly for very large hail and damaging wind gusts.

A level 1 was issued for Sardegna towards Corsica and Gulf of Genoa mainly for excessive precipitation.

A level 1 was issued for Northeastern Spain and Southern France mainly for excessive precipitation.

A level 1 was issued for Central to Northern Germany and Southern Denmark mainly for marginally large hail, excessive precipitation and to the lesser degree for severe wind gusts.

A level 1 was issued for Russia mainly for marginally large hail and severe wind gusts.

SYNOPSIS

A deep long-wave trough will reside over Atlantic with strong southerly to southwesterly flow at mid to upper troposphere spreading over most of Western and Central Europe. Its base is forecast to amplify towards Iberia and further southeastwards with time. Several impulses in the form of the separate short-wave troughs are forecast - one moving from Northern Africa towards Italy, another, more subtle one will translate over Germany towards north during the forecast period. An interesting feature will be a cut-off low centered over Romania, slowly filling with slow motion towards east. Another shallow cut-off low is observed over Turkey while over the Southern Mediterranean and Eastern Europe, ridges of higher geopotentials will be situated.

In this complex scenario, several areas with thunderstorm development are forecast and shall be discussed in more detail below.

DISCUSSION

... Algeria...

A volatile setup is forecast for this area with very steep mid-level lapse rates (confirmed by Dar-El-Beida sounding) and at least marginal low-level moisture contributing to low to moderate CAPE build up. Likely, the highest values will be confined to the coastal areas, where low-level moisture will be the deepest. Towards south, strong vertical mixing might reduce the available moisture, so that instability might be less with very high LCLs. Furthermore, strong capping layer is forecast with high CIN values, at least in the beggining of the forecast period. However, with the arrival of the short-wave trough and an attendant frontal system (with surface cyclogenesis), strong forcing and pronounced convergence will likely sustain at least isolated DMC. Moreover, a very favourable vertical wind profiles for supercells and/or bow echoes will be in place with DLS values approaching or even exceeding 30 m/s and SREH locally up to 300 m2/s2 (strong mid - upper tropospheric jet over the surface easterly flow).

Supercells or well organised MCS will be a distinct possibility in this setup, posing risks of very large hail and damaging wind gusts. Near the coastlines, with better low level moisture and lower LCLs, tornadoes can not be ruled out either. Level 2 seems to be justified for this area.

... Sardegna, Corsica towards Gulf of Genoa ...

As the short-wave trough emmerges from N Africa during the evening and night hours, it is well possible that a large MCS will form near the Algeria coastline, heading northeast in strong southwesterly steering flow. Fed by high values of low-level moisture over the sea and with pronounced tongue of MUCAPE values over the area, locally very high precipitation rates might be observed. Over Sardegna, even an isolated large hail event is not ruled out as steep mid-level lapse rates could ovespread the island area with DLS values exceeding 25 m/s.

... Northeastern Spain, Southern France ...

Prolonged moist onshore flow thanks to the cyclogenesis over the Balearic Sea could support continuous regeneration of thunderstorms over these areas with attendant threats of local excessive precipitation. Instability should not be very high without steep lapse rates and with only moderate vertical wind shear, other threats will be limited, albeit a marginally large hail event is not ruled out for Spain in case that well organised multicell manages to form.

... Central to Northern Germany ...

With the southerly mid-tropospheric flow and the short-wave approaching from the south, it seems that a patch of steeper lapse rates will be advected towards north. With dew points expected to reach only barely above 10°C, high CAPE values can not be expected, but MLCAPE values reaching around 500 J/kg sound plausible in this scenario. Southeasterly surface flow will veer towards south - southwest with height with windspeeds reaching around 15 m/s from 700 to 500 hPa. Therefore, vertical wind shear will be only moderate, bulk values approaching 15 m/s in the 0-3 km layer especially in the western and northern extents of the area. Some well organised multicells or marginal supercells could form once convergence along the cold front initiates convection with several isolated reports of large hail and/or severe wind gusts. Toward the later afternoon, parallel-stratiform MCS especially in the northern part of the area might end up in the locally high precipitation amounts.

... N. Ukraine to Russia ...

Around the periphery of the ridge, EML has been advected from Caucasus, contributing to the widespread thunderstorm activity that has been observed over this area. Also for today, scattered to numerous thunderstorms are expected to initiate, with most of DMC staying in a "pulse-like" form. However, especially at the northern flank of the ridge, slightly stronger mid-level flow might contribute to the storm organisation and together with moderate instability, this might suffice for a few well organised, strong multicells that might be capable of marginally large hail. Moreover, dry subcloud layers and steep lapse rates might promote strong downdrafts, so isolated severe wind gusts in downbursts are possible as well.