Valid: Mon 17 Mar 2014 06:00 to Tue 18 Mar 2014 06:00 UTC
Issued: Sun 16 Mar 2014 22:15
A level 1 was issued for Lithuania, parts of Belarus and N-Ukraine mainly for severe to damaging wind gusts and isolated tornadoes.
SYNOPSIS and DISCUSSION
A pronounced frontal zone bisects Europe as a deep/warm high pressure area over W/SW Europe faces low geopotential heights over NE Europe. Numerous impulses race along that deep baroclinic zone to the SE. The impulse of most concern leaves Norway/Sweden during the morning hours and crosses Lithuania and Belarus until sunset from NW to SE. Peak strength of that wave is forecast during the passage of Belarus. Thereafter a weakening but still very dynamic feature affects the N-Ukraine during the overnight hours.
A sharp frontal zone offers a confined area with marginal ingredients for sustained updrafts. Steep mid-layer lapse rates along the cyclonically sheared side of the 55 m/s mid-layer jet will be placed beneath modest 300 hPa divergence. Trigger of enhanced convection will be the spin up of a weak low/mid-tropospheric vortex, which enhances moisture advection towards the cyclonic edge of the frontal zone. The wave-like structure results in a broad warm sector, which is framed by SE-ward racing warm/cold fronts along its eastern/western fringes. An intense mid-level vorticity couplet accompanies that wave and results in strong forcing over the warm sector.
Low-end CAPE on the order of 200-400 J/kg is forecast with peak values over Lithuania and with a gradual decrease towards Belarus (although the spread in models remains large where to place the CAPE maximum .... dependent on the strength of the vortex and attendant moisture advection).
Shear will be strong to extreme with up to 25 m/s 0-1 km, 40 m/s 0-3 km and 50 m/s 0-6 km bulk speed shear. Directional shear will be strong with values well in excess of 200 m^2/s^2 for the 1 and 3 km layers. Actually, the directional shear increases during the afternoon hours in the warm sector as the cyclonic vortex crosses Belarus with peak strength.
The main focus for initiation will be along the sharpening and SE-ward racing cold front, but the weakly capped warm sector also offers moderate LL convergence signals and isolated to scattered warm sector storms are possible, too. Storm motion in excess of 60 kt (110 km/h) with rather straight forecast hodographs favor fast moving low-topped thunderstorms ... either supercells or bowing line segments. As forecast soundings show a well mixed 1-4 km layer with steep LL lapse rates, intense winds may easily be mixed down to the surface. Swaths of damaging wind gusts are forecast and any more organized line segment could cause quite long swaths of damaging winds all the way to the N-Ukraine. In addition, the tornado risk will be augmented with aforementioned parameters. Marginal CAPE/updraft strength and limited longevity of storms may lower the risk of long-tracked tornadoes, but localized higher BL moisture/LL CAPE may cause an isolated strong tornado event. Isolated large hail will be possible, too. Meager looking CAPE precludes a level 2 but this will be an high-end level 1 due to the wind gust risk.
Despite rapidly waning CAPE values after sunset, strong forcing could bring an organized convective threat all the way to the N-Ukraine.
No thunderstorms are forecast for the rest of Europe, excluding non-severe marine convection over the S-Norwegian Sea and NW/N of Ireland and Scotland. The same for the offshore area east of Crete.