Dossier sur les orages violents de Sicile et d'Italie 1er partie
En cette journée du 15 septembre 2006 la situation est extrêmement orageuse sur le sud de l'Italie notamment la Sicile ainsi que au Nord-Est de la Tunisie. A 6 H00 du matin le 15 septembre Estofex déclenche une alerte niveau 2 sur 3 et envisageait une évolution au niveau 3 ci la situation prévus se dégrader. Cela ne fut pas le cas Heureusement.
Valid: Fri 15 Sep 2006 06:00 to Sat 16 Sep 2006 06:00 UTC
Issued: Thu 14 Sep 2006 21:24
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.
.... 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.
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.
Depuis le début de l'après-midi du 15 septembre des cellules isolées se sont formées sur l'Italie mais deviennent plus généralisées au cours des heures suivante avec de violents orages. Ces orages ne son pas ceux de la perturbation attendue. Les orages attendus devraient se formaient dans la soirée voir même en cour de nuit. les radars de la région montrent que les orages qui affectent l'Italie font parties de l'alerte niveau 1 de Estofex. Sur les images satellites on peut apercevoir des forts jets d'altitudes du coté de l'afrique. Ce sont eux qui vont en partie contribuer ses prochaines heures à produirent ces phénomènes violent prévue par Estofex. Vient se rajouter à cela une dépression qui fait basculer un flux de sud-sud-est sur l’Italie faisant ainsi tourner les cellules orageuses sur la zone concernées.
Voici quelques animation radars des cellules orageuses ainsi que l'intensté électrique.
La situation orageuse semblait se calmer au Nord de l'italie dans la soirée du 15 septembre. Mais l'épisode orageux attendu ne devait plus tardé avec l'arrivé d'orages par le sud sud est de la Tunisie en directions de l'italie et de la Sicile.D'après les dernier runs des models du 15 septembre la situation semblait effectivement très propice à une grosse dégradation orageuse vers la Sicile.
Animations et carte techniques :
Le run de 18 Z des models envisagaient une dégradation orageuse moins forte que prévue sur les régions de Rome. Après avoir frappé Rome elle devait se déclarer surement vers le Nord Est de l’Italie ainsi que vers la croatie. Cependant les régions côtières comme Rome devaient subirent l'assaut d'orages dans la nuit jusqu'à landemain matin.
Apres la nuit agitée que a passait l'Italie, la situation se calme mais restera toujours très orageuse avec un risque encore présent de supercellule orageuse le 16 septembre. La cause est encore des jets présents dans la zone qui pourront accentuer un mouvement giratoire des orages. Estofex à enlevé son état de vigilance niveau 2 et place toute la région en vigilance 1.
Valid: Sat 16 Sep 2006 07:00 to Sun 17 Sep 2006 06:00 UTC
Issued: Sat 16 Sep 2006 06:39
High-over-low upper flow configuration existing over Europe ... with vigorous vort maxima present at periphery of the upper low over the central Mediterranean. Quite moist low-level air is present E of and beneath this feature. Until Sunday 06Z ... the upper low is progged to weaken some ... though shear should remain supportive of potentially severe evolution.
S Italy ... Adriatic Sea ... Ionian Sea ... W Balkan States
Focus for potentially severe TSTM development will persist along the cold front currently stretching from the north Balkans southwards into southern Tunesia. Shape of the thermodynamic profiles is somewhat uncertain as no 00Z ascents are available from the pre-frontal air mass ... but it does not seem likely that lapse rates have steepened substantially compared to Friday's 12Z ascents ... which in addition is not supported by model fields. CAPE should be on the order of 1000 J/kg ... though it might locally be somewhat higher where BL moisture is richer than what was revealed by Friday's 12Z launch from e.g. LIBR.
Current activity should continue across the Adriatic Sea and the W Balkans over the next couple of hours ... though shear will gradually weaken to 20 m/s (0-6 km) until late afternoon/early evening ... which should still promote scattered severe TSTMS.
Expect a few mesocyclones ... capable of large hail and severe wind gusts ... and maybe also a brief tornado though threat is somewhat uncertain given essentially no information about the LL thermodynamic setup of the pre-frontal air mass.
Another ... potentially severe MCS should form ahead of peripheral vort max over the Ionian Sea early Sunday morning ... which may graze southern Italy.
central and northern Italy ... Tyrrhenian Sea ... W/NW Mediterranean Sea
Interesting thermodynamic setup is present in the wake of the front ... where steep LL lapse rates coinside with very moist (10 to 16 g/kg mixing ratios) and essentially saturated boundary-layers across the W abd central Mediterranean. Shear should be rather weak ... providing favorable conditions for waterspouts. Storms developing close to the upper jet at the cyclonic shear side of the E periphery of the upper low ... may benefit from sufficiently strong DLS to become supercellular ... with an enhanced hail ... severe wind-gust and tornado threat.
Activity over France will develop in similar thermodynamic environments conductive to non-mesocyclonic tornadoes ... and a couple of events are expected.
Coverage of waterspouts in such situations is unknown ... but indications are that it is sufficiently high to warrant a categorical risk.
L'alerte niveau 2 etant passer le risque de gros phénomènes etaient ecarté. Cependant l'italie a subit quand même l'assaud d'orages modérés à fort toute la journée.
Image sat :