dimanche 4 mai 2014

European Storm Forecast Experiment

Storm Forecast
Storm Forecast
Valid: Sun 04 May 2014 06:00 to Mon 05 May 2014 06:00 UTC
Issued: Sun 04 May 2014 04:00
Forecaster: VAN DER VELDE
A level 2 was issued for western Turkey mainly for tornadoes, large hail and severe wind gusts.
A level 1 was issued for parts of Bulgaria, Romania and Greece mainly for hail and non-mesocyclonic tornadoes.


Low sea level pressure is centered over Romania and near Albania. There is a long-wave mid level ridge over western Europe, a large trough over Scandinavia with cold air behind a cold front over Ukraine and a large trough over central/eastern Europe with cool air with a mid-level cold front stationary over western Turkey. Ahead of that cold front very steep Saharan mid-level lapse rates are advected. As result of the jetstream rounding the trough, deep layer shear is strong over the cold front region. At ground level, a zone of relatively high theta-w extends behind the mid level cold front over Bulgaria and surroundings, contributing to unstable conditions.


...western Turkey...

The area can be divided into two parts: NW Turkey where surface-based parcels are present with little capping, and SW Turkey where those parcels are capped, but some elevated parcels may reach their LFC mainly from 06-12Z and lifting over the terrain may help in this. Classic large curved hodographs with 25-40 m/s deep layer shear will aid supercell formation which raises large hail and tornado threats as well as severe wind gusts. The tornado threat is supported by strong SREH (>200 m²/s²) and 0-1 km shear (>12 m/s) while the gusts arise from fast storm motion in the strong ambient wind (>20-25 m/s with Bunkers or Corfidi vector methods). There is some doubt for SW Turkey due to capping, but the good low level lifting may overcome this.
In NW and N Turkey more than 500 J/kg MLCAPE should exist and much weaker cap. With predicted large curved hodographs and LCLs above 1500 m the chance of supercells with large hail is significant, as well as severe wind gusts. After 15Z/18Z low level shear and SREH soar to 12 m/s and 250 m²/s² as result of decoupling from the surface, which enhance tornado potential although parcels start to become elevated-only. The threat should therefore decrease to level 1 during the night hours.

...parts of Bulgaria, Romania, Greece, SE Italy, Albania, FYR Macedonia...

In an axis from Albania to eastern Romania and from Bulgaria to western Turkey some weakly capped 250-750 J/kg MLCAPE develops and should immediately lead to widespread deep convection. Low-level CAPE is on the high side and along with weak low-level winds (especially around Albania) should help local isolated landspout tornadoes, while deep layer shear is moderate over much of Bulgaria and adjacent areas, particularly in the east (10-20 m/s over 0-6 km) which can help generate organized multicells and updraft rotation. The cooler airmass may favor locally large amounts of small to marginally large hail (2-3 cm).

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