Storm Forecast
Valid: Wed 28 Jul 2010 06:00 to Thu 29 Jul 2010 06:00 UTC
Issued: Wed 28 Jul 2010 05:07
Forecaster: ESTOFEX

A level 2 was issued for Estonia, Latvia, Belarus and the N-Ukraine mainly for large hail (hail greater 5 cm in diameter well possible), severe/damaging wind gusts and tornados.

A level 2 was issued for parts of Poland, Lithuania and parts of Latvia mainly for excessive rainfall.

Both level 2 areas are surrounded by a level 1 for all kind of severe (more isolated in nature), including parts of Finland and SE-Sweden.

A level 1 was issued for N-Morocco mainly for large hail.


SYNOPSIS

Large-scale upper trough still covers most parts of central Europe. Downstream advection of a very hot and moist air mass still affects NE-Europe with life threatening conditions continuing for the foreast period. Heat also remains in place over th Iberian Peninsula with cooler conditions in-between. The main synoptic story will be an evolving cyclonic vortex along the eastern fringe of the central-Europe trough. This low and attendant frontal boundaries serve as foci for organized
thunderstorms.

DISCUSSION

...Estonia to the NW-Black Sea ...

Vertically tilted votex is in full progress somewhere over E/S-Poland with tightening surface pressure gradient over Belarus and the Ukraine, as high pressure over W-Russia still remains in place. This configuration results in an extensive convergence zone, which ranges from Estonia all the way to the Black Sea. This concentrated swath of lift will be the focus for severe and long-lasting, deep moist convection. Initiation will last all day long, probably peaking around sunset and onwards with scattered multicells/supercells. Intense moisture pooling in the BL beneath western fringe of extensive EML yield a swath of 1 to 1.5 kJ/kg MLCAPE. During the daytime hours, the strength of the wind field increases from SE to NW with whopping shear magnitudes around sunset over N-Ukraine/parts of Belarus (25-30 m/s 0-6km bulk shear and 40 m/s 1-8km shear). We don't buy into LL shear, which seems to be modified by model "QPF bomb", but at least modest speed/directional shear will be in place. Repeated thunderstorm development is forecast during the forecast and thunderstorms will rapidly gain organization with well organized mid-level mesocyclones and an attendant large hail risk. Giant hail with 5cm or more in diameter can't be ruled out, especially over E-Belarus and N-Ukraine during the evening hours, although an isolated event will be possible anywhere from Estonia to the W-Black Sea (in the highlighted areas). Storms, which evolve along the eastern fringe of this convergence zone tap into substantially drier BL air mass, so severe downburst risk will be enhanced. This may also foster an upscale growth into numerous severe MCSes, which move rapidly to the NW, producing a more widespread severe wind gust risk. Finally, storms, which evolve along the western fringe of that convergence zone, ingest a very moist air mass. Backing along that boundary and good LL-CAPE support may yield a favorable environment for organized LL mesocyclone development with tornadoes. A broad level 2 was issued to reflect the significant risk with any storm, despite the more isolated nature of storm coverage in some parts of the level 2. Thunderstorms go on all night long with a gradual decrease in strength over the Ukraine.

... NE Poland, parts of Lithuania, Latvia and parts of Finland ...

Another surge of hot/moist air to the north/(north)west will be underway during the forecast, as warm front pushes westwards. The immediate prefrontal environment along the warm front reveals a favorable set-up for rotating thunderstorms with adequate deep and low-level shear. Warm front set-ups remain complex to forecast in advance, as thunderstorms need to ride along the warm side for an all kind severe risk, diminishing to a hail risk, when storms move off of the boundary to the cooler/elevated side of the front. We decided to stick with a level 1 over Finland for now, as forcing beneath anticyclonic shear remains a bit marginal, so thunderstorm coverage may be more isolated. Nevertheless, any developing thunderstorm is forecast to gain rapid organization at low/mid-levels with an attendant large hail/severe wind gust and tornado risk, the latter one maximized along the warm front itself. A level 2 was introduced for NE-Poland and areas to the north/northeast. Dependent on the final track of the deepening vortex, a long lasting deformation zone may set-up somewhere over the highlighted areas, setting the stage for excessive rainfall with training/slow moving storms. Shear remains a bit weaker compared to Finland, but at least isolated large hail/strong wind gusts and tornadoes may accompany that activity. Excessive rainfall risk gradually spreads to the north/northwest during the night hours, probably affecting SE-Sweden after midnight. The level areas were adjusted accordingly.

Thunderstorms over Germany, E-France, Switzerland and N-Italy remain sub-severe with modest CAPE and weak shear. A few funnel reports/spouts may occur (also including the coast of Belgium, the Netherlands, NW-Germany and Denmark), if LL CAPE indeed achieves magnitudes of latest GFS model output, but overall signals are not comparable to the past few days, so no level 1 was issued. Marginal/isolated large hail may accompany strongest updrafts with up to 800 J/kg SBCAPE.

A large hail risk my evolve during the afternoon/evening hours over N-Morocco with intermittent initiation.