Storm Forecast
Valid: Tue 12 Sep 2006 06:00 to Wed 13 Sep 2006 06:00 UTC
Issued: Mon 11 Sep 2006 22:41
Forecaster: TUSCHY

SYNOPSIS

Intense high pressure area continues to cover most parts of central and eastern Europe, suppressing any convective activity.
An intense upper-level trough over the eastern Atlantic should continue its eastward shift, reaching Ireland and the United Kingdom during the latter part of the forecast period....
Combined with another, although weaker upper-level trough over Portugal and Spain, strong WAA will continue downstream of this huge complex and will affect most parts of western Europe.

DISCUSSION

...Eastern Tunisia and Sicily...

A pretty diffuse synoptic constellation sets up for this part of the Mediterranean..... Although model pool is fairly consistent in showing broad area of convective precipitation, shear conditions are forecast pretty different..... While GFS wants to develop a weak low-level depression / an area of highly convergent flow SW of Sicily , models like NMM do not even forecast traces of any closed surface circulation...Upper-levels will be fairly conducive for scattered TSTM development under a broad and nearly stationary trough, lingering over the area of interest.... Upper-/mid - level wind field should be fairly weak but right entrance region of a streak, placed over S-Italy, will cause moderate upper-level divergence values to persist ( at least ) during the daytime hours over a broad area, running from eastern Tunisia to Sicily.

Latest sounding reports indicate a very humid bundary layer with steep low-level lapse rates and moderate instability release over Sicily ( Trapani ~970 J/kg ), but main negative factor will be the pretty warm mid-level airmass.... No significant change regarding the thermodynamic profile should occur during the morning and early afternoon hours, but a slight warm-up in the low-/mid-levels during the later afternoon hours should start to support a decline in convective activity.

GFS is very aggressive with very high SRH-3km values and up to 20m/s DLS....on the other hand, there are models like NMM with significant lower values.... Forecasting such small-scale disturbances that far out with little ground station/ship reports available is difficult and hence some uncertainties will remain, but main threat should be a severe wind gust threat, especially if storms line up.

...Balearic Islands, eastern Spain and S-France...

First area of concern will be CNRTL-E Spain and the Balearic Islands.
A weakening, but still active upper-level trough will be placed over the Iberian Peninsula and should be nearly stationary during the forecast period due to a still strong upper-level streak on its upstream side ( west of Portugal )....
A short wave trough is forecast to develop along the SE tip of the trough over extreme SE Spain, moving towards the north and crossing the area of interest from the south during the evening/night hours....
Shear won't be impressive, but DLS of 15m/s should be available.
Thermodynamic environment will be very favorable with instability release of up to 1500 J/kg ( due to steepened mid-level lapse rates and a still humid boundary layer).
Excessive rain should be the main threat, but won't exclude an isolated large hail/ severe wind gust report with more discrete cells.

NE Spain was also included in the level-1 area due to scattered TSTM development in a favorable environment for isolated large hail reports.

The other area of interest will be S/SW-France.... Main concern will be the quality of the northward returning airmass, but global models like GFS indicate 1-2°C higher dewpoints over extreme SW France, compared to yesterday.... A frontal boundary will reach the Bay of Biscay during the early night hours and a LLJ will develop over SW France, which should also support the advection of a more humid boundary layer airmass of the western Mediterranean ( SSTs around 22°C ).
Enhanced LL shear and low LCLs should pose a risk for an isolated tornado report along the coastal areas south of Montpellier and SW of Bordeaux, where veering profilers are forecast.

Areas over NE France, Belgium and the Netherlands and also over SW Germany, Switzerland and western Austria will again see isolated to scattered TSTM development, but lack of shear should suppress any significant organisation.... Nevertheless...storms over SW Germany ( yesterday) were responsible for a few hail reports and won't exclude a few updrafts, strong enough for producing isolated large hail.