Valid: Sat 13 May 2017 06:00 to Sun 14 May 2017 06:00 UTC
Issued: Fri 12 May 2017 05:15
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 the tornadoes.
A level 1 was issued for S Romania, Bulgaria and Serbia mainly for the large hail, severe wind gusts and excessive precipitation.
Severe thunderstorms with a potential of producing large to very large hail, severe wind gusts and excessive precipitation may develop in the high shear / high 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.
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 and cooler polar air at the north. 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 (evapotranspiration advection) will develop up to 2000-2500 J/kg CAPE over Serbia/S Romania/Bulgaria .
Thanks to the position of the jet, around 25 m/s 0-6km shear will be available in this area, giving an impressive value of 1600-1800 m2/s2 WMAXSHEAR. Such values suggest that any thunderstorm developing in this environment, may have the potential of becoming severe to extremely severe and produce all kinds of thunderstorm phenomena. 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.
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. Strong shear in 0-3km layer as well as high instability involves also severe wind threat (especially if the squall line will evolve). Excessive precipitation and local flash flooding is also possible given PW ~ 30-35mm.
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 line 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 supress developing towers. Low and mid-level clouds associated with morning convection over Serbia may be also an issue.