Storm Forecast
Valid: Sun 30 Jun 2013 06:00 to Mon 01 Jul 2013 06:00 UTC
Issued: Sun 30 Jun 2013 04:18
Forecaster: VAN DER VELDE
A level 2 was issued for Moldova and southern Ukraine mainly for excessive precipitation and large hail.
A level 1 was issued for Turkey, Black Sea and eastern Balkan mainly for large hail.
A level 1 was issued for southern Sweden for spout-type tornadoes.
A level 1 was issued for central Finland mainly for marginal chances of excessive precipitation and large hail.
A level 1 was issued for the Norway-Finland border for a marginal chance of a tornado.
SYNOPSIS
An upper trough axis stretches from southern Scandinavia to Turkey. One surface level low slowly moves over southern Sweden and is sending a cold front through Poland into Belarus and Lithuania with marginal forcing and instability. An dynamically inactive old frontal boundary lies further to the east, over Russia into northern Finland. The warm airmass is squeezed into a 500 km wide tongue by advancing dry air from northern Russia. This airmass is very moist with rather weak mid level lapse rates.
In contrast to these stationary features, the situation is more dynamic over the Black Sea and surrounding regions. A surface low pressure system gets support from mid/upper vorticity advection and is fed by warm humid air with large CAPE from Ukraine.
DISCUSSION
...eastern Balkan, northern Turkey, Moldova, Ukraine...
GFS forecast hodographs are nicely veering with strong upper winds over northern Turkey and Black Sea. Along the east Balkan coast initiation should be more likely and forecast hodographs, although smaller, are reasonably curved with good mid level storm-relative winds. Circular hodographs with weak storm motion and storm-relative helicity of 100-250 mē/sē are predicted over southern Ukraine, which combine with 1500 J/kg MLCAPE or more. This is favorable for multi- and supercellular storms as well as mesoscale convective systems, mainly over southern Ukraine and eastern Moldova where a surface circulation and convergence line develop.
The main threats are large hail and excessive precipitation, given the slow storm and system motion.
...Finland, northern Sweden and northern Norway...
These are marginal level 1 areas.
Central Finland has an overlap of instability and 15-20 m/s deep layer shear, combined with low level convergence in the late morning. A cell producing large hail is not ruled out. The parallel storm motion and shear vectors to the frontal boundary may however destroy the environment for subsequent development and at best enhance the potential of training cells with some precipitation threat.
Vertical shear is stronger over the Norway-Finland border region, with circular hodographs indicating potential for rotating updrafts and more than 10 m/s 0-1 km shear (indicating some tornado potential) developing after 15Z. But the instability there is rather marginal, even with the presence of low-level convergence and advection of more unstable air from the southeast.
...southern Sweden...
Stagnant winds in the center of the barotropic low combine with some low level CAPE and enhanced low level vorticity. This often leads to cold air funnels and spout-type tornadoes.
