Storm Forecast
Valid: Sun 12 Apr 2015 06:00 to Mon 13 Apr 2015 06:00 UTC
Issued: Sun 12 Apr 2015 03:06
Forecaster: TASZAREK
A level 1 was issued for S Norway, S Sweden, N Denmark, N Lithuania and Latvia mainly for severe wind gusts and to a lesser extent for tornadoes
A level 1 was issued for parts of Russia, Serbia, Romania, Bulgaria, Montenegro, Bosnia and Herzegovina mainly for large hail
SYNOPSIS
Weather in Europe is influenced by low over Norwegian Sea and extensive high covering most of the central and southern Europe. Relatively shallow long-wave with axis extending from central Sweden, through Estonia, Belarus and central Ukraine provide conditions for convection within cold atmospheric front that will westerly move over these regions during the forecast period. Advection of cold air of polar origin (behind the cold front) that will take place in the cold sector of low situated over central Scandinavia will create favorable conditions for convection that will occur in the highly wind sheared environment. Jet streak will extend during the forecast period from Ireland to southern Baltic Sea. Internal airmass thunderstorms that result from boundary layer's daytime heating and as usual occur in the weak kinematic field, are expected to occur over Balkan Peninsula, parts of Italy, Sardinia, and parts of Spain and Portugal. Lifting mechanism within these storms will come from orography and low-level convergence zones.
DISCUSSION
...S Scandinavia, N Lithuania, Latvia...
Instability that is mainly driven by advection of cold air and thus the occurrence of steep vertical lapse rates is predicted over most of the level 1 area to reach around 100-200 J/kg (CAPE). Its peak is forecast in afternoon hours when daytime heating will take place. Although instability is marginal, the overlap with strong mid and low level flow that provide 0-3km shear locally ~ 15 m/s, and 0-6km shear 15-35 m/s create risk that convection may be likely to organize into linear features and produce locally damaging wind gusts. NWP models simulate that maximum wind speed gusts can reach up to 25-30 m/s, especially in the SE Sweden. The occurrence of vertical directional wind shear in the lowest 1km AGL and 0-1km shear exceeding locally 10 m/s cannot rule out some isolated tornado phenomenon. However, due to relatively high LCL and LFC over these area (> 1000m AGL), this risk is strongly limited. Increased potential of lightning is forecast in the Norwegian coast where orographic lifting will support convection.
...parts of Russia...
Thunderstorms in this region will occur within cold atmospheric front that will move eastwardly. Since thermodynamic instability exceeding locally 800 J/kg will occur in weak kinematic wind field, the only severe weather threat that may be likely to occur within cumulonimbus clouds is a large hail. Overlap of strong vorticity advection and PW locally exceeding 20mm in front of the cold front may result in local hail up to 2-3cm.
...parts of Serbia, Romania, Bulgaria, Montenegro, Bosnia and Herzegovina...
Internal airmass thunderstorms that are forecast to occur in the parts of Balkan Peninsula may have the potential to produce hail up to 2-3cm due to overlap of moist boundary layer (7-8 g/kg) with steep vertical lapse rates (7.5 C/km) that will result in CAPE up to 1000 J/kg. Higher terrain elevation may be also supportive. Main sources of lift will come from orography and low-level convergence. This region due to relatively cold equilibrium levels and well developed free convective layer is expected to be electrically very active. Similar as in the Russian area, the lack of significant vertical wind shear will limit the potential for the other severe weather phenomena occurrence.
