Storm Forecast
Valid: Wed 13 Jun 2018 06:00 to Thu 14 Jun 2018 06:00 UTC
Issued: Tue 12 Jun 2018 21:33
Forecaster: PUCIK

A level 2 was issued across SE Hungary, N Serbia and NW Romania mainly for large hail, excessive rainfall and to the lesser degree for severe wind gusts.

A level 1 was issued across Italy, Slovenia and SE Austria mainly for excessive rainfall.

A level 1 was issued across NW Croatia, Hungary, S Slovakia, W Romania, S Serbia mainly for excessive rainfall and large hail.

SYNOPSIS

At the mid to upper troposphere, a cyclonic vortex will progress from S France towards Corsica and Sardegna. Embedded within the vortex, a subtle short-wave is forecast to cross from the Adriatic Sea to Bosnia and Serbia. At the same time, a short-wave trough will move across N Germany to the Baltic States and a powerful impulse will translate towards the British Isles.

At the lower troposphere, a wavy frontal boundary will stretch from the Alpine rim through the Czech Republic, Slovakia into S Poland, Ukraine and Russia. As a low pressure develops south of the Carpathian mountains, a surge in the cold front is forecast along the eastern rim of the Alpine range.

South of the cold front, an airmass characterised by abundant lower tropospheric moisture will enable for scattered to widespread thunderstorms across much of south- and south-eastern Europe.

DISCUSSION

... SE Austria to NW Romania and Serbia ...

At the time of the forecast issuance, a large quasi-linear convective system is progressing through Hungary eastwards. As of now, it is unknown to what extent will the overnight convection influence the development during the day. Nevertheless, it seems likely that some storms will be ongoing over the area also in the morning hours and that numerous outflow boundaries will be laid out by previous activity. While outflow boundaries may become a focus for the storm development later in the day, lingering cloudiness and remains of the storms may inhibit solar heating.

Thanks to the rich lower tropospheric moisture, characterized by 2 m dew-point readings of 18+ deg C and mid-tropospheric lapse rates of 6.5 to 7.5 K/km, widespread CAPE values of 1000+ and locally more than 2000 J/kg are forecast. The highest values are simulated for N Serbia and NW Romania. Compared to the previous day, mid-tropospheric flow will weaken slightly, resulting in lesser shear and less storm organisation. Nevertheless, 10 to 15 m/s of DLS will suffice for multicells. Over Serbia, models simulate even pockets of stronger shear, so that transient supercells are not ruled out.

Forecast soundings suggest that over northern part of the area, running from Slovenia, through SE Austria, N Hungary and Slovakia, excessive precipitation will be the dominant threat given rather skinny CAPE and very moist profiles. The hail threat will increase towards south and east, as the buoyancy in the hail growth zone gets larger. Severe wind gusts are not ruled out with large clusters of cells and in strong cells due to the high precipitation loading, but this threat will be smaller compared to the latter two.

Storms will be initiated along the pre-existing outflow boundaries, developing convergence zones as the shallow low develops south of the Carpathians, and along the surging cold front in the evening hours. Thus, numerous rounds of storms are possible, which may exacerbate the heavy rainfall risk over some of the locations. Lvl 2 is issued for an area with the best overlap of ingredients and to the south of the outflow boundary that may be laid out by the ongoing convection. Morning analysis of the situation will likely allow for a more precise location of the highest threat area.

... Italy, Sardegna, Corsica ...

Heavy rainfall threat will be present with any storm that develops in the area, underneath the cyclonic vortex. Particularly over Central Italy and the islands, convection will be almost stationary and forecast soundings suggest high rainfall efficiency due to the moist profiles.