Storm Forecast
Valid: Sat 16 Apr 2016 06:00 to Sun 17 Apr 2016 06:00 UTC
Issued: Sat 16 Apr 2016 06:25
Forecaster: TASZAREK
A level 2 was issued for S France, mainly for large hail, severe wind gust and tornadoes.
A level 1 was issued for S France, Switzerland, S Germany and NE Austria, mainly for large hail, severe wind gusts and in lesser extent for tornadoes.
A level 1 was issued for SE Poland, mainly for severe wind gusts, large hail and in lesser extent for tornadoes.
A level 1 was issued for NW Spain and N Portugal mainly for the excessive precipitation, severe wind gusts and large hail.
SYNOPSIS
NW and N Europe is covered with cold polar air mass and broad low over Scandinavia, while the S and SE Europe stays within a warmer and moister air within high over CNTRL Mediterranean. On its borderline, a jet streak extends from Iberian Peninsula up to E Europe, and almost on its entire path covers an unstable air mass, creating good conditions for the development of DMC with severe convective phenomena. Small shortwaves within this boundary are forecast by NWP and will help to initiate convection.
DISCUSSION
...NW Iberian Peninsula...
A passage of the shortwave trough will initiate storms in the afternoon hours. Within a mixing ratios of around 7-8 g/kg and ~ 7 K/km lapse rates a CAPE up to 600-800 J/kg will develop. Due to overlap with > 20 m/s DLS, supercells capable of producing severe wind gusts and hail up to 3-4cm cannot be ruled out. Thanks to DMC superimposing with a large-scale precipitation, local excessive precipitation cannot be ruled out. Thunderstorms should fade in the evening hours when the thermodynamic instability will drop.
...NE Spain up to S Germany...
Along the jet axis where a DLS ~ 30 m/s overlaps with few hundredths of CAPE, a meso-low will develop in the afternoon hours over S France. Thanks to this, a shortwave will amplify and result in a stronger QG-lift and higher SRH values on its right flank. A clustering supercells capable of producing severe wind gusts and large hail are expected over S France, with the lowering threat shifting to SE Germany and Switzerland in the late evening hours. A favorable wind field in the lowest part of the troposphere (strong LLS ~ 10-15 m/s, 100-150 m2/s2) and a weak dew point depressions resulting in low LCL, create threat for tornadoes that may occur within supercells over S France. In the environment of 20-25 m/s MLS, also a storm clustering into bowing squall line cannot be ruled out. Within such line, a threat for severe wind gust will considerably increase. Due to the available QG-lift associated with meso-low and thus highly probable CI in a such dynamic kinematic environment, a level 2 may be justified. The limiting factor may be the amount of environmental thermodynamic instability that in a current understanding is rather low and will quickly drop after 18 UTC. The highest threat for severe weather falls between 16 and 19 UTC. After 19 UTC a thunderstorm cluster will move to the Switzerland and Germany and is likely to weaken in late evening hours.
...SE Poland...
Within a passage of a weakly pronounced short wave, a CI is likely in environment of 8-9 g/kg mixing ratios and ~ 7 K/km lapse rates in the afternoon hours in S Poland. A CAPE of few hundredths of J/kg overlapping with a strong airflow and helicity in mid and low-levels may evolve thunderstorms into supercells capable of producing large hail, severe wind gusts and in lesser extent tornadoes. It cannot be ruled out that thunderstorm will cluster into bowing line and create a more pronounced threat for severe wind gusts in the SE-CNTRL Poland. In the evening hour a convection should transform from surface-based into elevated and lower the threat for severe convective weather phenomena. A convective cluster will move northeastwardly and may reach Belarus in the evening hours.
