Storm Forecast
Valid: Mon 19 Feb 2007 06:00 to Tue 20 Feb 2007 06:00 UTC
Issued: Sun 18 Feb 2007 23:05
Forecaster: TUSCHY

SYNOPSIS

Slightly inclined omega-like weather pattern evolves over SW Europe and will support an unsettled weather period for most parts of the western Mediterranean.
Very cold winter conditions can be found over N / NE Europe, where frosty conditions prevail with 850 hPa temperatures of -20 to -25°C.
Main focus for convective activity will be a weakening upper-level trough over the central Mediterranean, which is forecast to open up during the forecast period.

DISCUSSION

...Sicily and surrounding areas...

Main story will be the upper-level trough which will shift towards the ESE while weakening.
A 50m/s upper-level jet will place the area around Sicily under the favorable left exit region where some upper-level divergence is forecast.
A surface depression will form somewhere between Tunisia and Sicily but strengthening should only be marginal because of a constant displacement of the assisting jet, which ranges towards the SE.
SSTs of 17-18°C and a rapidly cooling atmosphere look favorable for a more robust instability release with locally up to 800 J/kg SBCAPE possible.
UKMET and GFS agree on indicating an increase of convection near the center, recognizable at the changing thermodynamic profile of the core structure.
Like it is often the case with depressions, developing in this region, there remains an uncertainty .
GFS indicates what can be already seen on latest WV / IR images.... a pretty dry airmass on the cyclonic site of the UL - jet.
Substantial drying over Sicily is forecast with only marginal convective precipitation signals.
Right now 2 areas with enhanced convective activity look reasonable:

An area between Tunisia - Sardinia - Sicily which will be placed under the base of the eastward shifting trough and

the region E / SE of Sicily which can be found under a field of constantly strengthening UL-divergence.

All those areas will be well displaced from the best shear and therefore no significant storm organisation can be expected.
There are signals in the global models that a loosely organized cluster of storms could evolve east of Sicily during the night hours, but the main threat looks like to be heavy rain .

A level-1 for Sicily was issued because of the over-all fine instability and shear conditions. LCLs stays very low, DLS will be strong and even LL shear is forecast to be slightly enhanced so that there exists a chance for an isolated tornado and a severe wind gust / marginal hail threat .
Beside the pretty dry airmass, temporarily capping at 800-700 hPa during the afternoon hours should also suppress a more robust thunderstorm threat over Sicily.
In fact, only a few storms are expected over the level-1 area but conditions will be favorable for each developing storm to become severe.


....Portugal and NW / SW Spain...

A cold front will reach the area of interest during the latter part of my forecast period. Instability does not look that impressive and so does the forcing along this front. Don't want to exclude a few storms as a result of increasing moisture at lower levels and a cooling atmosphere.
DLS would be significantly enhanced and an isolated severe wind gust can't be debarred but coverage of thunderstorms will be too intermittent for issuing higher probabilities.