Storm Forecast
Valid: Fri 15 Sep 2006 06:00 to Sat 16 Sep 2006 06:00 UTC
Issued: Thu 14 Sep 2006 21:24
Forecaster: TUSCHY

SYNOPSIS

An intense upper-level trough will finish a cut-off process over SW Europe during the morning hours.... This upper-level system should rapidly move towards the east, crossing the western Mediterranean and reaching Sardinia / Corsica during the latter part of the foecast period.
Cool and stable conditions over NE/E Europe should prevail under the influence of a broad low-level high pressure area.

DISCUSSION

.... Tunisia, Sicily, Tyrrhenian Sea, CNTRL and S-Italy...

Conditions will become increasingly favorable for an outbreak of severe storms over the level-2 area during the 16Z - 06Z ( 15.Sept.) time - frame !

An intense upper-level trough over SW Europe should have finished its evolution into a closed upper-level system, crossing the western Mediterranean during the forecast period from the west towards the east....Comparison between Wednesday 18Z WV image and model outputs indicate , that models like GFS and NMM have a good handling with this system....Impressive 135kt upper-level jet, which supported this cut-off evolution, will continue to race southward ( crossing Portugal / SW Spain ), surrounding the base of the upper-level low and crossing N-Africa during the forecast period....A jet, orbiting the tip of an upper-level trough without getting split up into 2 pieces is also a sign of a very dynamic upper-level system.
This increasingly negative tilted upper-level low should start to affect the level-2 area during the evening hours, when the wind in all altitudes will become stronger.
An interesting point is, that models want to develop another streak on its downstream side ( covering Sardinia and Corsica ), which should produce a coupled jet configuration and hence a broad area of very impressive UVM values during the 18Z - 06Z time frame over most parts of the Tyrrhenian Sea.
The critical point in such synoptic weather constellations is often the possible infiltration of drier and more stable piled Saharan air, but this should be no problem in this case.
Latest synop reports over N-Africa indicate an already pretty humid boundary layer as a result of yesterday's activity.
Latest calculation of NAAPS indicate the evolution of a well developed EML downstream of the arriving trough ( spreading over Tunisia NE-ward )..... also superimposed with steep mid-level lapse rates( forecast by GFS ), which should cover the level - 2 area during the night hours....As a result, still 24-25°C warm SSTs, an high boundary layer moisture content and steepening lapse rates should be fine for up to 1500 J/kg SBCAPE release over Sicily and the Tyrrhenian Sea.

The cold front and numerous convergence zones are forecast to cross the W-Mediterranean, slowing down significantly due to a more parallel alignment regarding the background flow and should reach the area of interest during the later morning hours.....No major activity should occur with this pretty diffuse frontal boundary, but 20-25m/s DLS, increasing UL divergence and already plenty of instability would yield a favorable environment for a few TSTMs to form with an enhanced severe wind gust risk.
This front is forecast to get re-activated during the later afternoon / early evening hours due to a broad region of pressure fall east of Tunisia ( and model pool is pretty consistent in developing a closed circulation, which should race rapidly towards the NE, reaching CNTRL-Italy during the latter part of the forecast period ).

Initiation should finally occur over Tunisia during the late afternoon hours along the re-activated frontal boundary and should rapidly develop towards the NE.
An environment of up to 1500 J/kg instability, DLS up to 35m/s ( GFS 12Z with outstanding 35m/s at 700hPa, 25m/s at 850hPa ) would be very favorable for storms to organize pretty fast with an attendant risk for severe-damaging wind gusts.
Main concern right now will be the development of an organized line of storms with a widespread damaging wind gust risk over the Tyrrhenian Sea, but there are still some inconsistencies left with each model run and between the different models, but if the more agressive run of GFS seems to be on track, an upgrade (level-3 )may become necessary later-on!

There will be also an enhanced tornado threat due to strongly veering profilers, but strength of low-level depression is still uncertain....Each more discrete developing storm in the level-2 area would have low LCLs and significantly enhanced SRH values and a few tornadoes, one or two of them even strong, can't be excluded.
Isolated large hail can be expected with each more discrete strom, mainly over Sicily and S / CNTRL Italy due to steepend mid-level lapse rates.

....Extreme western Mediterranean...

Pool of < -20°C 500 hPa temperatures will
cross the 23-25°C warm Mediterranean and hence a tight temperature gradient should develop.... Excluded the coastal areas of SE Spain due to the expected long time frame, the pretty dry and stable airmass should need to get well mixed.... The main storm activity should be well seperated from the strongest shear and hence only expect an few severe wind gust / large hail reports, still justifying a level-1.

...France...

A broad low-level depression will be placed over France and combination of wrap-around moisture in the lower-levels and cool mid-levels should be enough for low-end instability release and a few TSTMs can be expected....

During the day-time hours, W / SW France should be covered by a frontal boundary, which is forecast to intensify during the evening / night hours due to strengthening frontogenetic forcing as a result of the finally SSW-ward shifting low-level depression... Won't exclude a few embedded TSTMs in a broad stratiform area but storms should stay sub-severe due to lack of any significant shear.