Storm Forecast
Valid: Sat 21 Apr 2018 06:00 to Sun 22 Apr 2018 06:00 UTC
Issued: Fri 20 Apr 2018 20:32
Forecaster: TASZAREK
A level 1 was issued for NW France and S United Kingdom mainly for large hail, severe wind gusts and excessive precipitation.
A level 1 was issued for Portugal and SW Spain mainly for large hail, severe wind gusts, excessive precipitation and in lesser extent for tornadoes.
A level 1 was issued for parts of W Russia mainly for severe wind gusts.
SYNOPSIS
Slowly weakening broad high dominates over most of CNTRL Europe and provides sunny and warm weather. A series of troughs and jet streaks are circulating around it. Almost stationary cut-off is located SW of Portugal. Eastwardly traveling longwaves are located over N Atlantic and Scandinavia. Another overturned long-wave covers Turkey. A deep low is traveling through W Russia. Rich boundary layer's moisture, mostly due to strong diurnal heating and evapotranspiration originates over France, S United Kingdom and parts of SW CNTRL Europe. High low-level moisture due to advection also covers SW Iberian Peninsula and parts of CNTRL Mediterranean. Advection of steep lapse rates from NW Africa is also taking place over Iberian Peninsula. The best chances for deep moist convection include N France, S United Kingdom and SW Iberian Peninsula where the overlap of boundary layer's moisture and mid-level lapse rates will be the most prominent.
...N France and S United Kingdom...
Under strong diurnal heating, ~ 10 g/kg mixing ratios and ~ 7 K/km mid-level lapse rates ML CAPE develops up to 1500-2000 J/kg. Although lifting mechanism is not prominent, a belt of weak PVA should help to trigger thunderstorms in the afternoon hours. Within the support of moderate instability and ~ 15 m/s DLS, convective cells should evolve into multicell clusters capable of producing severe wind gusts and excessive precipitation. Small chances for supercells may also provide few large hail events, especially given steep lapse rates in mid-levels. It is expected that thunderstorms will maintain their lightning activity until nighttime hours and move NEwardly.
...SW Iberian Peninsula...
Almost stationary cut-off SW of Portugal provides S-erly advection of 8-9 g/kg mixing ratios and 7.5 K/km lapse rates from NW Africa. Aforementioned combination results in ML CAPE up to 1000-1500 J/kg. A small jet streak on the eastern flank of the trough enhances vertical shear up to 15 m/s in 0-3km layer and 30 m/s in 0-6 km layer. Thunderstorms developing in this environment may quickly evolve into multicell clusters and supercells, especially given favorable vertical wind profile (SRH ~ 200-400 m2/s2). Main threat within these storms should include heavy precipitation, severe wind gust and large hail. Enhanced LLS cannot rule out also tornado event, especially given isolated supercells. Although environment looks prominent for severe thunderstorms, it has two limitations. First is uncertain stratiform precipitation and cloud cover during the day, which can negatively modify environment. Second is very late convective initiation of surface-based storms which by NWP models is expected in the late afternoon hours. Thunderstorms activity should continue during nightime hours.
...W Russia...
A deep low with strong forcing and moderate boundary layer's moisture (6-7 g/kg) travels SE-wardly. Within the process of increasing lapse rates due to prominent lift, a small instability up to 200-600 J/kg ML CAPE develops in the warm section of the trough. Low-topped convection is likely to develop in the afternoon hours under the support of diurnal heating and PVA. Due to strong vertical wind shear and 20 m/s flow at 850 hPa, thunderstorms may be capable of producing severe wind gusts. A convective line promoting widespread wind events cannot be ruled out. Convection should fade in the evening hours.
