Storm Forecast
Valid: Fri 04 Aug 2017 06:00 to Sat 05 Aug 2017 06:00 UTC
Issued: Thu 03 Aug 2017 15:31
Forecaster: TUSCHY
A level 2 was issued for the N-Ukraine and far W-Russia mainly for large hail, severe wind gusts and an isolated tornado event.
A level 2 was issued for parts of Austria mainly for a few large hail, severe wind gust and excessive rainfall events.
A level 2 was issued for parts of the E-Czech Republic, S-Poland and extreme N-Slovakia mainly for severe wind gusts and large hail. An isolated tornado event is possible.
Level 1 areas surround all the level 2 areas with similar risks but less probabilities.
A level 1 was issued for parts of S-Norway, Sweden, Denmark, the CNTRL Baltic Sea and surrounding areas mainly for an isolated excessive rainfall and tornado event.
A level 1 was issued for SW-Turkey mainly for large hail and excessive rain.
SYNOPSIS
A zonal flow regime is present over most parts of Europe as numerous upper lows over N-Europe face a strong ridge over S-Europe. In-between, an intense mid-/upper-level wind field affects an area from the Bay of Biscay to Belarus.
An elongated wavy front extends nearly parallel to the frontal zone but remains displaced to the south. Due to the passage of small waves along the front, some overlap of unstable air with the strong wind field exists with an attendand severe risk. In general however, displaced strong winds from an CAPE-rich air mass keep the general severe risk confined to small regions. A much broader severe risk exists over the N-Ukraine and W-Russia.
Hot and stable conditions persist over the Mediterranean.
DISCUSSION
... The Czech Republic, S-Poland, N-Slovakia and the W-Ukraine ...
The main severe risk evolves in the eastern level 2 area, as a weak surface low traverses east along the boundary during the forecast. It pushes warm and unstable air north over the SE-Czech Republic and S-Poland and beneath a strong westerly flow regime. Attendant overlap of 500-1000 J/kg MLCAPE, 20 m/s DLS and similar 0-3 km shear magnitude create a favorable environment for organized convection. Forcing will be the main problem, but there are some hints on a short-wave crossing the region during the afternoon/evening. Combined with CI due to ongoing diabatic heating and some LL forcing along the eastbound moving cold front scattered thunderstorms should develop, which can organize into a mixed mode of multicells/isolated supercells. Given strong low-/ to mid-level flow, a few bow echoes with confined swaths of severe wind gusts are also possible. Beside the wind risk, large hail and locally heavy rain are forecast, too (although the rapid forward propagation should limit the overall rainfall risk). In case of more discrete storms during the late afternoon and evening hours, the tornado risk may increase a bit with strengthening LL shear despite LCLs around 1000 m. Storms also spread from the SE-Czech Republic and S-Poland into the W-Ukraine and may graze the N-parts of Slovakia (e.g. deviating storms). During the night, the severe risk gradually diminishes with decreasing CAPE and the departing short-wave.
... SE-Germany and Austria ...
The prefrontal air mass over SE-Germany features similar CAPE and shear profiles compared to the regions further east. However, earlier passage of the short-wave and the combination of a gradually weakening cap but only temporal diabatic heating lower the confidence in CI over SE-Bavaria (away from the orography). Nevertheless, CAPE/shear space is supportive for organized convection with a severe wind gust and large hail threat in case isolated storms can evolve.
Higher confidence in storms exists over parts of Austria, where CI over the mountains is forecast around noon and later-on. DLS remains in the 15-20 m/s range, overlapping with 500 to locally 1000 J/kg MLCAPE. This setup is supportive for longer-lived updrafts with a large hail and severe wind gust threat. Clustering of that activity is expected betimes with a localized excessive rainfall risk. A small level 2 area was issued where confidence in numerous severe weather reports is the highest. This acticity spreads east and weakens after sunset.
... Far W-Russia and the N-Ukraine ...
An eastward moving occlusion opens into a broadening warm sector towards the N-Ukraine. Despite rather weak mid-level lapse rates, the moist air mass in the warm sector supports 400-800 J/kg MLCAPE with peaks of 1000 J/kg along the border of N-Ukraine and Russia. Combined with 15-20 m/s DLS, multicells/isolated supercells with large hail and severe wind gusts area forecast. An isolated tornado risk exists, too, especially in the level 2 area, where increasing LL shear during the evening overlaps with still adequate CAPE and where weaker forcing increases the risk for more discrete activity (also valid for storms, which ride along the warm front to the E/SE). In general, thunderstorm coverage and intensity decrease during the night.
... S-Norway/S-Sweden, S-Finland, the CNTRL-Baltic Sea and surrounding areas and Denmark ...
Placed beneath a broad pool with rather cold mid-levels and with ongoing influx of moist air from the SE, a setup exists for scattered diurnal driven thunderstorms. Weak shear precludes organized convection, but pulsating storms may grow into numerous slow moving clusters. Effective PWs in excess of 20 mm and slow storm motion indicate a risk of numerous heavy rainfall events in the area of interest. Hail near severe limit can't be ruled out with an isolated more intense updraft. In addition, low LCLs and LLCAPE in excess of 100 J/kg may result in a few funnel/short-lived tornado events. The risk decays onshore after sunset with an ongoing risk over the CNTRL Baltic Sea and adjacent areas.
... SW-Turkey ...
Slow moving storms over the SW-Turkey beneath an upper low pose a risk of isolated large hail and excessive rain. The activity gradually weakens during the night.
