Storm Forecast
Valid: Sat 13 May 2017 06:00 to Sun 14 May 2017 06:00 UTC
Issued: Sat 13 May 2017 06:17
Forecaster: TASZAREK

A level 2 was issued for N Bulgaria and S Romania mainly for the large to very large hail, severe wind gusts, excessive precipitation and in lesser extent for tornadoes.

A level 1 was issued for S Romania, Bulgaria, Serbia, Bosnia and Herzegovina mainly for the large hail, severe wind gusts and excessive precipitation.

A level 1 was issued for NE Romania and Moldova mainly for the excessive precipitation.

A level 1 was issued for Germany, Czech Republic, W Slovakia and N Austria mainly for the excessive precipitation and in lesser extent for marginally large hail and severe wind gusts.

SYNOPSIS

On Saturday, a strong jet stretching from N Africa, through Balkan Peninsula up to Turkey will be dividing warm and moist tropical airmass at the south from a cooler polar air at the north. Central Europe will remain under the influence of a warm air advection with a mid-level ridge extending from Alps up to Norway. Long wave trough will be placed over N Atlantic and influence only W parts of British Isles.

Severe thunderstorms with a potential of producing large to very large hail, severe wind gusts and excessive precipitation may develop in the high shear / moderate instability environment around afternoon hours on the border of Romania and Bulgaria. Small chances for tornadoes will be possible in the late afternoon hours along with increasing 0-1km shear and the lowering of cloud base. Severe thunderstorms are also possible over parts of Bosnia and Herzegovina and Serbia where a locally enhanced 0-6km shear will be available. Rest of the European area will remain under thunderstorms forming in a weakly sheared environment and with a main potential of producing heavy rain.

DISCUSSION

...Romania, Serbia, Moldova, Bulgaria, Bosnia and Herzegovina...

Advection of steep mid-level lapse rates from Africa (elevated mixed layer) together with a strong diurnal heating and an increase in low-level moisture develops CAPE up to 1500-2000 J/kg over Serbia/S Romania/Bulgaria. Thanks to the position of the jet, around 25 m/s 0-6km shear will be available in the instability areas, giving an impressive values of 1400-1600 m2/s2 WMAXSHEAR. Such environment suggest that any thunderstorm developing in this area, may have the potential of becoming severe to extremely severe and produce all kinds of severe convective phenomena. The main risk justifying level 2 is associated with favourable conditions for large to very large hail. Given strong instability, steep lapse rates and strong shear, supercell thunderstorms may be capable of producing >5cm hail. Local event with hailstones reaching 6-7cm also cannot be ruled out given a good configuration of instability and shear. However, due to mostly parallel flow, not much helicity will be available for the storms, and together with a rather weak 0-1km shear, the risk of tornadoes will be limited. Small window for possible tornadoes (if the isolated supercells will remain) will show up in the late afternoon hours when the 0-1km shear will increase and LCLs will drop.

Strong shear in 0-3km layer (> 15 m/s) as well as high instability involves also severe wind threat (especially if the forming supercells will evolve into band or a squall line). Straigth hodographs should favour splitting right and left-moving supercells. Excessive precipitation and local flash flooding is also possible given PW reaching ~ 30mm. Some of the models take into possibility CI also over NE Greece, but due to higher CIN, this scenario is less likely.

Thunderstorms should overcome CIN and develop on the border of Bulgaria and Romania in the afternoon hours along with the passage of the shortwave trough. Although CI according to various NWP scenarios is uncertain, any thunderstorm developing in this environment may quickly becomes severe. Given 25 m/s 0-6km shear, supercells clustering in the evening into band are the most likely. Some limitations of this forecast are also associated with the model overestimation of low-level moisture and the magnitude of CIN which may suppress developing towers. Low and mid-level clouds associated with morning convection over Serbia may be also an issue.

Slowly moving thunderstorms in a weakly sheared environment but with high PW values (~ 35 mm) may occur over NE Romania and Moldova, and pose a threat of excessive precipitation with a local flash flooding even (although instability will be rather small).

...Germany, Czech Republic, W Slovakia, N Austria....

Thanks to diurnal heating and evapotranspiration in the warm section of the polar airmass (ridge stretching from Italy up to Norway) CAPE will build up to ~1500, locally 2000 J/kg. Thanks to the weak cap, convection should start to develop in the classic afternoon hours and due to lack of a strong synoptic-scale lift, fade in the late evening. Weakly sheared environment (0-6km shear ~ 5-10 m/s) should favour slowly moving pulse thunderstorms (locally convection may cluster into a weakly organized multicells) with the potential of producing heavy rain, marginally severe large hail (~2 cm) and local microburst type severe wind gusts. Level 1 refer to areas where a simulated ML CAPE will be the highest.