Storm Forecast
Valid: Sat 23 May 2020 06:00 to Sun 24 May 2020 06:00 UTC
Issued: Fri 22 May 2020 21:20
Forecaster: PUCIK

A level 2 was issued across the northern Alpine rim mainly for damaging wind gusts and large hail.

A level 1 was issued in a belt from SE France through N Italy, Switzerland, SE Germany, Austria and SW Czech Republic mainly for severe wind gusts, large hail and excessive rainfall.

A level 1 was issued for NW Spain mainly for large hail.

A level 1 was issued in a belt from Syria and SE Turkey through Caucasus into Russia mainly for large hail and severe wind gusts.

SYNOPSIS

Weather pattern over Europe will be dominated by two deep troughs. The first one will rapidly move from the U.K. to central Europe and the second one, stagnant over E Russia will amplify towards the south, its base translating from W Turkey towards Cyprus and Syria. Both of these features will result in one or more belts of convective activity, which may become severe due to the strong low to mid-tropospheric flow. At the same time, none of the highlighted areas will be under high CAPE due to the lack of overlap of both abundant lower tropospheric moisture and the steep mid-tropospheric lapse rates.

DISCUSSION

... SE France to SW Czech Republic ...

In conjunction with the approaching deep mid- to upper-tropospheric trough, a strong cold front is forecast to cross the region during the forecast period, providing widespread lift for the development of deep-moist convection. Along and ahead of the cold front, MLCAPE between 200 and 1000 J/kg will develop as the dewpoints reach around 15 deg C. Lack of steep mid-tropospheric lapse rates will preclude large values of buoyancy, as evidenced by skinny CAPE profiles in forecast soundings.

Increasing mid and low-tropospheric flow ahead and along the frontal boundary will increase the 0-6 km bulk shear values to 15 - 25 m/s, while the 0-3 km bulk shear will reach 10 - 20 m/s. Deep-layer shear will increase towards towards the rear side of the boundary. Due to the strongly sheared environment, well organised multi and supercellular convection is forecast to develop that may be capable of large hail and severe wind gusts. Both of these threats will be limited to some degree by the lack of steep lapse rates and low LCLs. The highest chances of hail will exist over NW Italy, where the highest CAPE values are simulated whereas the highest chances of wind gusts will fall to the northern Alpine forelands, where the strongest 0-3 km shear is simulated and where high res models indicate development of line segments.

Overall severe weather threat could be limited by the fast moving nature of the front that will have tendency to "undercut" developing convection and turn it into elevated form. Due to this fact and skinny CAPE profiles, much of the area is kept under a Lvl 1.

A Level 1/2 deliberation was made for the northern Alpine rim, where supercells and line segments are simulated by several convection allowing NWP models in the environment of SBCAPE > 0 J/kg and 0-3 km shear of around 20 m/s. It is also likely that 850 to 700 hPa flow will reach up to 30 m/s as it is channelled along the Alps ahead of the incoming front. Any sustained convective storm would be capable of damaging wind gusts in such environment. That said, there is also a possibility that the forming convective system will lag the surface front with SBCAPE = 0 J/kg and in such case it would not contribute much to the strength of wind gusts at the surface. Nevertheless, decided to issue a Lvl 2 for the corridor of the highest severe wind gust risk.

... Northwestern Spain ...

15 - 20 m/s of 0-6 km bulk shear are simulated over the area along with several hundreds J/kg of MLCAPE and high LCLs. While the models are not certain regarding the CI in the evening hours, it is likely that isolated, well organised storms will form, stronger of which may be capable of large hail.

... Syria, SE Turkey to Caucasus ...

A quite complex scenario is forecast over this region on the forward flank of the deep trough. Strong vertical wind shear will promote high level of storm organisation, but the number of storms and the intensity of the primary updrafts is questionable. It looks like at least isolated storms will form in the afternoon hours over the highlands of Turkey and Caucasus and these may be capable of large hail and/or severe wind gusts. Forecast soundings suggest that the storms will become elevated as they move towards lowlands or will decay as the surface parcels will need to overcome high values of CIN.

Increase in coverage of storms is forecast across the southern half of the area in the late night to early morning hours of Wednesday as the base of the trough approaches the area, providing synoptic-scale lift. While the coverage of storms increases, their level of organisation will decrease over the western part of the area due to the weakly sheared conditions in the core of the trough.