Valid: Sat 15 Jul 2017 06:00 to Sun 16 Jul 2017 06:00 UTC
Issued: Sat 15 Jul 2017 06:44
A level 1 was issued for S Italy mainly for excessive precipitation, severe wind gusts, and in lesser extent for large hail.
A level 1 was issued for Greece mainly for excessive precipitation, severe wind gusts, and in lesser extent for large hail.
A level 1 was issued for parts of W Russia mainly for large hail, severe wind gusts, excessive precipitation and in lesser extent for tornadoes.
Westerly flow regime continues over W Europe. An eastwardly moving broad longwave overspreads parts of CNTRL and E Europe. In the end of the forecast period it evolves into cut-off over E Belarus. A weakening vortex is situated over N Norway. Ridge extends from Azores to France. Jet stream stretches from N Atlantic through British Isles, France, N Italy and with difluent flow weakens over Balkan Peninsula. Very warm and humid conditions are present over Mediterranean, but mostly under strong cap. High theta-e values overspreads parts of W Russia in WAA where a widespread storm activity is expected in the afternoon hours.
Under deep moist BL (delivered within Eerly LL inflow from the sea), moderate lapse rates and strong diurnal heating, ML CAPE develops up to 1500-2000 J/kg. Although relatively strong cap overspreads this area, storms should develop in the afternoon hours over S Italy thanks to convergence zone resulting from sea breeze. In the environment of weak vertical wind shear (~ 10 m/s) and weak airflow, almost stationary pulse thunderstorms locally clustering into multicells will be most likely. Within them, heavy precipitation (PW > 30mm) events with a flash flooding potential will be the main threat. Few severe downburst type severe wind gusts are also possible given high delta theta-e values. Large hail risk is limited mainly due to poor lapse rates, high elevation of 0C isotherm and lack of sufficient vertical wind shear. However, some marginal event cannot be ruled out given decent instability. In the late afternoon hours thunderstorms should move into Tyrrhenian Sea and receive PVA which will maintain their elevated activity during night-time.
Environment over Greece features lower instability (due to weaker BL moisture) but higher shear (~ 15 m/s) compared to that of S Italy. Stroms should be triggered in the early noon hours thanks to difluent flow in upper levels and sea breeze activity along with strong diurnal heating. Slowly moving multicell clusters should pose a risk of excessive precipitation and local flash flooding. Due to enhanced steep lapse rates, marginally large hail events cannot be ruled out. Similar as over S Italy, these storms may produce downburst type severe wind gusts given enhanced delta theta-e values. Although storm severity will drop in the late evening hours, thunderstorms may maintain until nighttime hours.
More confidence regarding severe weather potential concerns storm activity over W Russia. East of the large long-wave overspreading CNTRL and E Europe, rich in moisture WAA extends from E Black Sea to Kola Peninsula. NWP models indicate LL trough stretching along W edge of warm sector and its cutting-off in the late afternoon hours, which may help to trigger storms. Under 12-13 g/kg MIXR, ~ 7 K/km lapse rates and strong diurnal heating, instability develops up to 2500-3000 J/kg (ML CAPE). Although mid-level airflow is weak and results in low DLS values (~ 10 m/s), a local lobes of enhanced 850-700 hPa flow overlaps instability zones. CI should take place in the afternoon hours along with strengthening convergence zones and diurnal heating. Multicell clusters and pulse thunderstorms with a heavy rain and severe wind potential are the most likely scenario. However, given locally enhanced LL-flow and 0-3km SRH, a 2-3 supercells capable of producing large hail cannot be ruled out. Within the same conditions, bowing segments of multicell clusters with a severe wind potential are also involved in the scenario. Increased LLS provides some probabilities for tornadoes, but with lack of substantial shear in mid troposphere the overall chances are rather low. Storms should receive more pronounced QG-lift in the late afternoon hours (eastwardly travelling cut-off over E Belarus) and thanks to this maintain during night-time, but with lower severe weather potential.