Storm Forecast
Valid: Sat 19 Aug 2017 06:00 to Sun 20 Aug 2017 06:00 UTC
Issued: Sat 19 Aug 2017 01:38
Forecaster: TASZAREK
A level 2 was issued for NE Italy and W Slovenia mainly for large hail, severe wind gusts, excessive precipitation and in lesser extent for tornadoes.
A level 2 was issued for E Latvia and E Lithuania mainly for large hail, severe wind gusts, excessive precipitation and tornadoes.
A level 1 was issued for SE Switzerland, N Italy, S Austria, Slovenia and NW Croatia mainly for large hail, severe wind gusts and excessive precipitation.
A level 1 was issued for a corridor from E Poland to parts of Estonia/W Russia mainly for large hail, severe wind gusts and excessive precipitation.
SYNOPSIS
Jet stream extends from N Atlantic, through S France, N Italy, W Poland, up to Baltic Sea. Broad ridge covers SW Europe. Mid-level high is located on SE Europe. Between them, an eastwardly moving long-wave with a polar marine airmass covers British Isles, North Sea, W Scandinavia and parts of N-CNTRL Europe. On its eastern flank it pushes warm and moist tropical airmass and triggers deep moist convection. At 18 UTC cold frontal boundary is expected to stretch from low located over N Sweden, through Baltic Countries, E Poland, and up to SE Austria. Severe thunderstorms within the support of a moderate shear are expected along this zone. Deep moist convection is also likely to develop in the Alpine area where a peak of a long-wave and shallow surface low will be located. Thanks to a good overlap of instability and shear, organized severe thunderstorms are likely to occur in this area.
DISCUSSION
...NE Italy and surroundings...
Under strong diurnal heating, EML from N Africa and surface S/SE LL moisture advection, a narrow area of instability up to 1500-2000 J/kg ML CAPE develops in the N Italy and Alpine area. Within the support of the jet stream, almost entire aforementioned zone is covered by > 20 m/s DLS. Along with a synoptic-scale lift, multicell clusters and supercells capable of producing large hail, severe wind gusts and heavy rain are possible in such environment. Due to lack of considerable LLS tornado chances are rather limited, but given vertical veering wind profiles and a possibility for an isolated convective mode in the early stages of CI, a local tornado event cannot be ruled out. According to NWP scenarios, surface-based storms with a potential of producing severe weather should develop in the afternoon hours in N Italy and cluster into MCS in the late afternoon and evening hours. The system should move eastwardly and enter Slovenia during nighttime. Due to moist vertical profile (PW > 40mm), excessive precipitation and local flash flooding event is possible within this system.
...corridor from E Poland to parts of W Russia...
Moisture convergence, strong diurnal heating and rather moderate temperature lapse rates create a narrow zone of instability ranging from 500 to 2500 J/kg ML CAPE. Peak values are expected over E Latvia, E Lithuania, E Estonia and NE Poland. Periferic zone of a jet stream from the west provides DLS of around 15 m/s, but in general it looks like a strongly displaced from the main instability zone. Low-level jet over E Latvia and E Lithuania results in 10 m/s LLS. Strong vertical veering wind profiles (SRH) are expected along cold frontal boundary over E Poland, and in the warm sector over E Lithuania and E Latvia. A line of storms along convergence zone from SE Poland to Estonia/W Russia is expected to develop in the afternoon hours. Isolated supercells may be capable of producing large hail, severe wind gusts and heavy rain. Tornadoes are also not ruled out over Lithuania, Latvia and Estonia where a low-level jet will be apparent. Later, thunderstorms should cluster into multicells and form MCS, most likely over Latvia and Lithuania where instability and QG-lift will be the best. Thunderstorms should weaken and fade during nighttime hours along with decreasing instability.
