Storm Forecast
Valid: Wed 11 Feb 2009 06:00 to Thu 12 Feb 2009 06:00 UTC
Issued: Tue 10 Feb 2009 23:17
Forecaster: PUCIK

SYNOPSIS

An amplifying mid and upper level trough will influence the weather conditions over Europe as a cold airmass is being advected from northern Scandinavia to the Mediterranean. Strong flow is surrounding the base of the trough and several impulses are rotating around it. The strongest impulse is forecast to affect the eastern Mediterranean and develop into a closed circulation as it reaches the western coast of the Black Sea. At the surface, a large low pressure system is affecting most of Central Europe and as its cold front progresses across the Mediterranean to the east, a separate low pressure center will develop in the trough, travelling with the above mentioned impulse. This separate low will deepen significantly and its central pressure will fall under 992 hPa by Thursday morning.

DISCUSSION

...Central Mediterranean...

A cold airmass, characterized by high mid-level lapse rates will be advected over the sea. This will even increase the high vertical temperature gradient present in the airmass. Synoptic scale lift associated with the trough should result in the destabilization and few hundred J/kg of MLCAPE are anticipated over the large parts of this region. Low level wind field convergence should result in the thunderstorm initiation. Strong mid-level jet ( having speed of more than 45 m/s ) will yield high values of DLS over the southern half of the area. However, mid and low level shear should be insignificant, implying that the increase in the wind speeds will be mostly confined to the mid-levels. As storms should stay relatively low-topped, most of the shear will be probably realized in their upper levels, not aiding in the storm organisation.

Scattered thunderstorms are expected, with possibility of sub-severe hail. Moreover, high values of low level CAPE and lapse rates in the lowest km, low LCLs and surface convergence fields suggest that few waterspouts are possible across the area. Nevertheless, risk is too low to warrant a threat level.

... Aegean Sea, Turkey, Bulgaria...

As the cold front will approach the area, destabilization is expected due to the strong synoptic scale lift of the moist and warm air advected ahead of the front. Still, only few hundred J/kg of MLCAPE are expected. As the surface low will deepen thanks to the impulse, low level wind field is forecast to strenghten. Very high values of DLS are expected ( 25 - 30 m/s) thanks to the strong mid-level jet. Significant SWly flow will establish ahead of the surface low, with 20 - 25 m/s at 850 level. Shear in the lowest 3 km should increase to more than 20 m/s and LLS will reach 15 m/s ( especially inland, thanks to the frictional reduction of the wind speed close to the surface). Moderate values of SREH are also anticipated, ranging between 150 - 300 J/kg. The highest values of both the low level shear and SREH are forecast over Turkey.

Thunderstorms will be easily initiated by the cold front and a strong surface convergence. We expect mostly well organised multicells with tendency for back-building, as the mean flow will be roughly parallel to the boundary. Briefly rotating cells are also possible, thanks to the moderate values of SREH and high values of DLS. Marginally severe hail might occur, especially with the brief supercells, as overall vertical temperature profiles will be quite cold ( below -24°C at 500 hPa). Some severe gusts are also anticipated thanks to the strong low level wind field. A well pronounced overlap of LLS/SREH and instability will exist, especially in the coastal areas of Turkey. With such favourable environment supported by the low LCLs, an isolated tornado event can not be ruled out. Due to the combination of these threats a Level 1 is issued for this region. The severe weather risk should increase from 18Z and will cease with the front leaving the area by Thursday morning.