Storm Forecast
Valid: Fri 31 Oct 2008 06:00 to Sat 01 Nov 2008 06:00 UTC
Issued: Thu 30 Oct 2008 19:20
Forecaster: TUSCHY

SYNOPSIS

Positive tilted, major trough is placed over western Europe. As intense upper streak dives southwards along its upstream side, gradual amplification keeps going. A disturbance (the remnants of the last cut-off process over NW-Africa ) lifts northeastwards, embedded in the strong SW flow ahead of the approaching major upper trough. This impulse will cross the central Mediterranean during the forecast. A complex pattern of numerous surface depressions, decaying and intensifying, dictates the picture over most parts of Europe. Warm and stable conditions prevail over SE/E-Europe.

DISCUSSION

...Tyrrhenian Sea, parts of Sicily, parts of Italy...

MET-9 loop (until 16Z) has aforementioned potent disturbance over Western Sahara / N-Mauritania with a pronounced intrusion of lower stratospheric air present in WV image (24°N/9°E). Latest satellite-derived wind measurements indicate a good handling of GFS regarding timing of re-integration in cyclonic circulation to the north but the strength of the wind field is slightly underestimated by GFS, especially along the SE quadrant of the impulse. Global model data coverage is not optimal, so variations in following outputs still expected as seen in past few runs. However, strengthening impuls should curve along the SE-quadrant of the main vortex to the north, leaving the coast of Algeria after 18Z, entering the Tyrrhenian Sea during the night hours, while continuing northeastwards. Swiftly intensifying geopotential height gradients along its southeastern flank ensure a rapid build-up of an intense wind field, which in turn results in a rapid lift of the impulse to the NE. Abundant forcing will be available for widespread initiation as strong PVA couplet crosses the area from the SW.

At lower-levels the attention turns to the highlighted area, as upper impulse interacts with a rapidly stengthening baroclinic zone , placed just west of Sicily and a spin-up of the LL vorticity center can be expected wich moves along this zone all the way to NE Italy during the end of the forecast. Strength of this feature remains ambiguous as some models like GFS/GEM and JMA show an ill defined "open" wave whereas NOGAPS spins up a quite strong surface depression over the N-Tyrrhenian Sea after 00Z.

In addition to abundant forcing mechanisms, a rapidly strengthening baroclinic zone - which gets caught by the eastward racing cold front behind the NE-ward departing surface depression - also provides support for widespread initiation.

The thermodynamics change for the better during the day, as cooling at mid-levels starts. Strongest release will stay confined along a narror corridor, running from W-Sicily to the north and later northeast as strong thermal ridge just east of the baroclinic zone shifts to the NE ( readings at 850hPa above 15°C) and another surge of an EML allows for some moderate, elevated and capped instability east of the front. MLCAPE of 500-1000 J/kg along this baroclinic zone should foster updrafts, strong enough for organisation.

Shear at all levels improves already during the morning hours, as an intense 40m/s mid-level streak overspreads the area from the SW. 0-6km shear ranges from 20-35m/s, 0-3km shear barely weaker and as 850hPa winds ramp up to 25m/s, LL shear also increases to 15-20m/s. Global models like GFS show a sharp increase in directional shear just east of the baroclinic zone, constantly separated from the QPF signals, but right now we see no reason why thunderstorms, evolving along this boundary, should not ingest the high-helical flow (SRH1 above 300m^2/s^2 and SRH3 near 600m^2/s^2).The inflow layer of those thunderstorms exhibit a very high moisture content (SSTs south/west of Sicily still above 24°C), so LCLs below 800-1000m are likely.

Thunderstorms should evolve during the midday hours in the highlighted area with a rapid increase thereafter.The main question will be the storm mode and there are numerous ingredients, which point to a rapid clustering thunderstorm event (potentially evolving into a V-shaped MCS). Intense forcing and left exit region of strong mid-level jet, combined with the strengthening frontal circulation should yield widespread initiation. In addition, persistent and intense LLJ constantly points into the area of interest, supporting a strong moisture influx. So for now, we think that the severe wind gust risk will be the main issue, although more discrete storms during the onset of initiation are embedded in a favorable environment for mid-/low level mesocyclone development with an attendant tornado risk, especially those, which move along the boundary. Strength of shear could support strong tornadoes, so combined with the widespread severe wind gust threat, a level-2 was issued. This cluster of storms approaches central Italy during the early night hours with a constant risk for widepsread,damaging wind gusts (especially if the more aggressive version of NOGAPS and also GFS-12Z verifies) and tornadoes, so the level-2 was extended well to the northeast.

Impressive depth of the parcel inflow layer, strong storm-relative upper flow (ventilation effect), very moist inflow layer, prolonged period of a front-parallel 20-25m/s LLJ and potential back-building along this front indicate an high flash flood risk for central Italy.

... N-Adriatic Sea and the coastal areas along its eastside...

21Z onwards, aforementioned features also affect those areas. Strength of surface depression will be crucial, how far to the north BL moisture gets advected. Another uncertainty will be how strong subsidence in the lee of the mountainous topography of central Italy will influence thunderstorm development as intense flow, normal to the mountain range eases and backs slightly. Thunderstorms, moving in from the SW or evolve in the small warm sector over the N-CNTRL Adriatic Sea should gain rapid organisation with a severe wind gust/tornado threat, as shear below 5km remains very intense. Modifications of this level-2 may become necessary, if track of surface depression shifts in upcoming runs. Extreme N-Adriatic Sea was included into a level-1 despite very limited instability release, but convective nature below the base of upper impulse could still support the downward-mixing of intense winds (800-700hPa) next to showers / thunderstorms. The risk for thunderstorms will continue until 06Z mainly over the eastern parts of the highlighted area.

... Central France ...

Run-to-run consistency of most models is bad regarding the development of a surface depression over SW-France, moving to the NE during the night. Dependant on the final strength of this feature and attendant moisture advection to the north, thunderstorms are not ruled out,but will wait for new informations, before increasing probabilities. A general thunderstorm area was issued, but a level may become necessary, if thunderstorms materialize as shear would be impressive.

Another area with thunderstorm chances will be S-France, as rich BL moisture beneath a coupled jet structure provide some low-end instability for isolated storms. Overall strong isentropic lift over cooler airmass onshore indicates more elevated nature of storms, but an isolated tornado along the coast remains possible. Otherwise, strong to severe wind gusts will be the main risk.

... Spain ...

Diurnal driven thunderstorms are forecast beneath eastward shifting upper trough. Most of the activity will stay confined to the base of the trough, where shear remains very weak, so main risk will be marginal hail and strong wind gusts.