Storm Forecast
Valid: Sat 22 Aug 2020 06:00 to Sun 23 Aug 2020 06:00 UTC
Issued: Sat 22 Aug 2020 00:00
Forecaster: PISTOTNIK

A level 1 and level 2 are issued from Poland to the eastern Alps for excessive convective precipitation, large hail and severe convective wind gusts.

A level 2 is issued for coastal areas of NE Turkey and Georgia mainly for excessive convective precipitation.

A level 1 is issued for inland areas of E Turkey, Georgia, Azerbaijan and N Iran for large hail, severe convective wind gusts and excessive convective precipitation.

SYNOPSIS

A mid-level trough and a strong but gradually filling surface cyclone affect the Norwegian Sea, the British Isles and the North Sea. A strong mid-level jet with an axis from the Bay of Biscay to Germany, the Baltic Sea and Finland circles around the trough. Further downstream, a pronounced mid-level ridge filled with very warm air stretches from Tunisia to NW Russia and moves very slowly eastward. Otherwise, the southeastern half of Europe is covered by weak pressure and temperature gradients.
The cold front of the North Atlantic cyclone moves slowly eastward across Poland, the Alpine region, S France and Spain and is the main focus for thunderstorm activity on Saturday.

DISCUSSION

... Poland, Czech Republic, eastern Alps ...

Hot air from the SW is advected into central Europe ahead of the low-pressure system. Friday's 12 UTC soundings uniformly showed a pronounced inversion in the range of the inactive warm front, whose height gradually dropped from ~2 km in Poland to ~1.5 km in the Alpine region and ~1 km south of the Alps. Steep lapse rates are present in the subtropic airmass above the inversion, gradually relaxing towards the north. In some higher Alpine valleys, strong daytime heating and/or slight downslope (Foehn) winds mixed the hot and dry air down to the surface and created extremely high maximum temperatures, most remarkably 36C at an altitude of almost 900m at Bourg Saint Maurice in the French Alps. However, mostly the capping inversion remained in place on Friday, and a lot of moisture could accumulate beneath it: 2m dewpoints rose to values in the upper tens in Poland, the Czech Republic and the major Alpine valleys and even in the lower twenties along the Alpine rims and in the adjacent forelands. Apart from an isolated evening storm in central Austria, the entire region stayed devoid of convection and the moisture reservoir remained untouched.
On Saturday, the cold front enters the region of interest from the west. Forecast models show a rather straightforward frontal structure and a supporting mid-level vorticity maximum over Poland, whereas synoptic lift is weaker but an early shift to westerly winds near the surface is foreseen from SE Germany to the Czech Republic and Austria. This forerunning convergence line will likely become stationary over the eastern parts of these countries. With similar low-level moisture and daytime heating to Friday but a gradual erosion of the cap and mid-level cooling, CAPE will rise to 500-1000 J/kg over Poland and to probably up to 2000 J/kg over Austria. Deep-layer shear around 15 m/s allows some storm organization, especially towards the north. Vertical wind shear across lower levels is rather weak.
Scattered to widespread thunderstorms will initiate from the early afternoon onwards along the cold front in Poland and mainly along the forerunning convergence line in Moravia and Austria. The expected storm mode is mainly multicellular, but high-resolution models show a few right-moving supercells, too. Large hail and severe downbursts may occur in early stages, though limited lapse rates towards the north and limited vertical wind shear towards the south keep these risks in check. With clustering storms and an increasing coverage, excessive rain soon becomes the main risk. The center of convective activity will likely emerge over Austria in the late afternoon and evening. Especially Lower Austria may be exposed to prolonged storms along the stationary convergence line with a risk of numerous flash floods, perhaps including a few high-impact events. Signals are mixed in how far convection will propagate NE-ward into Moravia and parts of Slovakia, but persistently converging winds under solid CAPE make the risk appear high enough to encompass the entire area into an elongated level 2.
Convection will gradually become elevated and weaken overnight.

... western Alps into S France and E Spain ...

CAPE on the order of 1000-2000 J/kg is expected in the upvalley wind regime over the Po Valley and in the sea breeze regime along the Spanish east coast, which are sheltered against the advancing cold front. In contrast, the shallow tail of the cold fron will soon push across France into the Mediterranean Basin and will likely bring a cool and dry change without triggering any convection.
Isolated afternoon storms should therefore be confined to the Lombardian and Piedmontese Alps, the eastern Pyrenees and the mountains of southern Aragon, but in the absence of any synoptic lift, the capping inversion will keep their lifetime short and their position attached to the high mountains. In case longer-lived updrafts could be maintained, this convection could organize well under moderate vertical wind shear, and large hail and severe downbursts would become a possibility. However, this scenario is too conditional for a level 1.

... NW and N Europe ...

Maritime polar air advances eastward behind the cold front. Mild sea surface temperatures, daytime heating over land and lift from various travelling mid-level vorticity maxima create a few hundred J/kg CAPE and allow scattered to widespread, partly thundery showers. While the overall shear magnitude is limited by deep vertical mixing, frictional effects can enhance low-level shear just enough to allow an organization into low-topped multicells and possibly a few briefly rotating storms. Short-lived tornadoes and marginally severe wind gusts are not ruled out, especially in the coastal belts.

... E Black Sea, NE Turkey, Georgia, Azerbaijan ...

At the SW flank of a mid-level trough, synoptic lift and diabatic heating support a shallow cyclone over the eastern parts of the warm Black Sea. Persistent onshore flow of very moist, unstable and weakly sheared flow creates a potential flash flood situation with repeated rounds of clustered and/or embedded storms in coastal areas of NE Turkey and Georgia. Further inland and further ahead of the mid-level trough, the SW-erly mid-level flow quickly increases and creates long and straight hodographs with up to 20 m/s deep-layer shear. Scattered inland storms, mostly in the afternoon and evening, can organize well and pose a risk of large hail, severe wind gusts and isolated heavy rain alike.
The complex topography, rather sparse surface observations and the lack of high-resolution models impose considerable forecast uncertainties in these areas.