Storm Forecast
Valid: Fri 21 Jul 2023 08:00 to Sat 22 Jul 2023 06:00 UTC
Issued: Fri 21 Jul 2023 07:53
Forecaster: PUCIK
A level 3 was issued across N Italy for very large to giant hail and severe wind gusts.
A level 2 was issued in a belt from N Italy to W Romania mainly for large to very large hail and severe wind gusts and to the lesser extent for tornadoes, especially over Slovenia.
A level 1 was issued from S Germany to Czechia and Poland mainly for severe wind gusts and large hail.
A level 2 was issued for extreme NE Spain mainly for large hail, heavy rainfall and severe wind gusts.
A level 1 was issued for E Ukraine and W Russia mainly for severe wind gusts and to the lesser extent for tornadoes.
SYNOPSIS
A deep cyclonic vortex resides over the Norwegian Sea with strong mid-tropospheric flow extending from Iberia through much of S and central Europe. Closer to the surface, a complex wavy frontal boundary is situated over central Europe. Abundant low-level moisture is located S of the boundary, yielding a pronounced overlap of CAPE and strong vertical wind shear and yet another day with potential severe weather outbreak S and SE of the Alps.
DISCUSSION
... N Italy ...
Elevated storms are ongoing over N Alps at 07 UTC. Surface observations from 06 UTC show already very warm airmass with dewpoints ranging from 18 to 22 deg C over the area. Basically all models agree on the overlap of 1500+ J/kg CAPE and 25+ m/s of 0-6 km bulk shear, yielding environment favorable for intense, well-organised convection. ICON-based models show locally 4000+ J/kg of CAPE, but that may be a bit too high given the fact that they show widespread 22+ deg C dewpoints, which is currently not supported by the observations.
High-resolution convection-allowing models don't agree on the exact placement and coverage of storms. For example, ICON-D2 is more conservative concerning the initiation along the Appenines. Nevertheless, enough confidence exists that an upslope flow combined with high RH around the LFC will yield conditions favorable for initiation and numerous isolated supercells will form, moving towards the lowlands. Supercells will, similarly to the previous days, pose risk of very large to perhaps even giant hail and severe wind gusts. A potential for upscale growth will exist and a convective system may cross from N Italy towards Istria.
... S Austria, Croatia, Slovenia, S Hungary, N Serbia, W Romania ...
Similarly to N Italy, an overlap of abundant CAPE and 25+ m/s of 0-6 km bulk shear is forecast. The scenario over this region is complicated by a number of storms already existing over the domain and considerable cloudiness and stratiform rainfall over S Austria. Over Slovenia, as of 07 UTC, several both left- and right- moving elevated supercells have initiated. These will already pose risk of (very) large hail. Anvils of the supercells advected east may inhibit solar heating over the adjacent areas. It is also possible that some of these morning storms will become eventually surface-based and move far towards east.
A lvl 3 was considered also for this region, but uncertainty concerning the exact corridor of the most severe weather remains. Nevertheless, a boundary moving S/SE across the area, plus the upslope flow, will initiate numerous storms that spread across the area especially in the late afternoon to evening hours. Expect a combination of supercells and linear segments, posing very large hail and damaging wind gust threats. As the S-moving cold front undercuts the E-moving storms, they will become elevated especially towards the overnight hours.
A tornado risk is forecast to develop over Slovenia and N Croatia during the late afternoon to evening hours with 0-1 km bulk shear exceeding 10 m/s in some models, combined with curved hodographs and rather low LCLs. If isolated supercells exist over the area at this time, low-level rotation trends need to be monitored in the storms to detect a potential development of a tornadic supercell.
... Central to N Hungary ...
Much of Hungary will be N of the main frontal zone in the late afternoon to evening hours. High values of "effective shear" above the slable layer will result in elevated linear segments and supercells. While the boundary layer will be stable, severe wind gusts can't be completely ruled out. Also, risk of large hail will be present given steep-mid tropospheric lapse rates and enough shear to sustain supercells.
... S Germany to Czechia, S Poland and W Slovakia ...
Disagreement exists in the models concerning the CAPE profiles. ECMWF shows very marginal CAPE, while ICON-driven models show around 1000 J/kg of MLCAPE. All models show vertical wind shear sustaining well-organised storms, including supercells. Current surface observations show dewpoints mostly between 10 to 14 deg C, supporting more the
models that show more modest CAPE values. Nevertheless, a potential for severe wind gusts and large hail will exist as well-organised storms form and a Lvl 1 is warranted over the area.
... NE Spain ...
A small Lvl 2 is introduced over a Pyrenees, where all models show abundant convective precipitation in an environment of 1000+ J/kg of MLCAPE and around 25 m/s of 0-6 km shear. A supercell may form with threat of large hail and severe wind gusts. Storms will likely quickly decay as they move away from the mountains towards the sea. A lvl 1 was issued further S, where very strong shear will also be present, but initiation is much more questionable.
... E Ukraine, Russia ...
A frontal system crosses the region with 500 to 1000 J/kg of CAPE in the warm sector and around 15 m/s of 0-6 km bulk shear. Especially in the N part of the Lvl 1, around 10 m/s of 0-1 km bulk shear will combine with low LCLs, yielding slightly elevated tornado risk. Otherwise, severe wind gusts may be the main risk along the widespread storms along the cold front.
