Storm Forecast
Valid: Mon 15 Oct 2012 06:00 to Tue 16 Oct 2012 06:00 UTC
Issued: Sun 14 Oct 2012 23:33
Forecaster: TUSCHY

A level 3 was issued for Sicily and far S-Italy mainly for excessive rainfall amounts, very large hail, damaging wind gusts and tornadoes (a signifcant event is possible).

A level 2 was issued for NE Tunisia, Malta, the Tyrrhenian Sea, parts of S/C Italy, parts of the Ionian and Adriatic Sea mainly for excessive rainfall amounts, large hail, strong to severe wind gusts and tornadoes.

A level 1 surrounds the level 2 mainly for similar hazards with less coverage and intensity.

A level 1 was issued for far NW Spain mainly for isolated heavy rain.

SYNOPSIS

A potent upper trough enters the W-Mediterranean and approaches Italy during the end of the forecast period. At lower levels, an extensive LL depression evolves over the C-Mediterranean and steers a cold front from the NW to the SE. A complex synoptic pattern is present ahead of this front, including a warm front/moisture boundary over Sicily and a prefrontal convergence zone ahead of the actual cold front.

During the end of the forecast, a rapidly strengthening zonal flow regime starts to affect W-Europe.

DISCUSSION

A major and potential life-threatening severe thunderstorm outbreak is forecast, including excessive and flash flood producing rainfall amounts and strong tornadoes over Sicily and parts of S-Italy.

... NE-Tunisia, Malta, Sicily, NW Ionian Sea and S-Italy ...

The forecast by most models, showing a pronounced intrusion of drier air all the way to Malta can be verified by the latest MIMIC-TPW analysis, where TPW values were cut into halves over the area of interest. Past ASCAT scan also showed a 5-10 kt northerly flow regime, enhanced by a small-scale anticyclonic eddy just east of N-Tunisia. A diffuse region of enhanced confluence south of Malta probably indicates the dissolved synoptic boundary. However, falling surface pressure over the W-Mediterranean should result in a strengthening southerly flow regime, which results in a slowly northward moving moisture boundary/warm front just south of Sicily. EZ and GFS agree well in the placement of that boundary and also in a complex interaction between the approaching cold front and a prefrontal convergence zone from the NW during the forecast. The warm front should affect Sicily around noon, the prefrontal convergence zone during the afternoon hours and the cold front during the overnight hours.

There is no concern about a potential limited CI due to the strength of the approaching and still southward digging upper trough and aforementioned synoptic boundaries. The air mass north and south of the warm front remains weakly capped and we would not be surprised to see additional residual outflow boundaries from the convection of the pervious night, which could serve as foci for CI.

In response to the upper trough, a pronounced plume of deep and well mixed N-African air spreads NE-wards atop a very rich low-tropospheric air mass. This assists in strong to extreme MLCAPE values in the order of 2000-3000 J/kg with a peak magnitude along the warm front itself. As an additional information, LCLs lower to 1000 m or less in the level 3 area.

Shear will be on a constant increase as wind field strengthens at all levels. When smoothing the forecast shear values (as some convective feedback problem might exist given impressive QPF "bombs" in the model data), we expect up to 25 m/s shear in the lowest 3 and 6 km. The final magnitude of the wind field in the lowest 2 km remains a bit unclear due to uncertainties, how strong the Italian LL depression will be, if a second surface low evolves west of Sicily and how strong the backing and veering will be next to the synoptic boundaries. However, global models forecast a broad zone with intense LL shear, both speed and directional, along the W fringe of the CAPE axis.

During the forecast period, we expect repeatedly initiation, first along the warm front and later on along the synoptic fronts from the west/northwest. Explosive development is forecast with severe and long-lived DMC over NE Tunisia and Sicily. The final storm mode could be twofold, starting with training and back-building (southwards building into the CAPE plume) activity and transforming into a more progressive mode betimes, as fronts and mid/upper forcing approach. Sicily can expect a prolonged period of favorable conditions for excessive and life-threatening, flash flood producing rainfall amounts, very large hail (isolated hail diameter in excess of 5 cm), severe wind gusts and tornadoes. Any storm, which rides along the boundary to the NE will be capable of producing damaging winds and a signficant tornado. Convection spreads to the NE, affecting S-Italy and the NW Ionian Sea during the overnight hours. A similar risk with that activity can be anticipated.

Despite the fact that similar set-ups proved difficult to forecast in the past (due to bad handling of models regarding extensive thunderstorm clusters), both the consistency of past models runs and the magnitude of the parameters convinced us to issue a rare level 3.

... Tyrrhenian Sea and C-Italy ...

A lot depends on how robust the prefrontal moisture return in the following hours will be (during the overnight hours of the 14th-15th October), but GFS and EZ showed a more robust CAPE build-up atop the Tyrrhenian Sea for the past few runs, so confidence in severe convection constantly increases. As the trough draws near from the west, the upper flow becomes increasingly diffluent and CI might occur already during the morning hours. For any storm development ahead of the cold front, conditions remain supportive for training thunderstorm activity with excessive rain, large hail, strong to severe wind gusts and tornadoes. This also includes the coasts of W/SW Italy.
A prefrontal convergence zone and the cold front itself should start to affect the area of interest during the afternoon hours onwards. An extensive and severe MCS (probably bound to the prefrontal convergence zone) races east/southeast and again all kind of severe is likely. With the slower SE-ward progression of the cold front, expect a decrease in thunderstorm strength and coverage first during the end of our forecast period from NW to SE.

... E-coast of the Adriatic and Ionian Sea ...

The past trend of GFS was to deepen the Italian surface depression on a stronger rate, which also caused a slow down of the shift to the east. Hence, the main activity first starts during the overnight hours.

During the morning hours, gradually decaying thunderstorms - on an isolated scale - along the coastal areas will still be capable of producing heavy to isolated excessive rainfall amounts due to ongoing training activity. However, strength and coverage of those storms gradually decreases until noon with probably isolated and non-severe activity until sunset.

Thereafter, a favorable pattern for large and back-building MCS events evolves over the Adriatic Sea and especially for its E-coasts. A 15-20 m/s LLJ advects a very moist and unstable air mass to the north beneath a diffluent upper level flow regime. Despite potential impact of anvil cirrus from the activity further upstream, CI is likely. Excessive rainfall with enhanced flash flooding is possible with strong to severe wind gusts, isolated large hail and an isolated tornado. Given the position of the moisture and LLJ axis, Montenegro might be the area of most concern regarding the rain risk. This hazard continues beyond our forecast period.

... S-Alps ...

Moderate to temporarily heavy rain occurs south of the Alps, but probably below our level criterion. However a non-thunder level 1 was issued for NE Italy where a better defined deformation zone could evolve as the Italian depression moves east. The rainfall should cease during the end of the forecast period from west to east.

... NW Spain ...

The interaction of a frontal wave with a plume of subtropical air creates an area with intense rain over far NW Spain during the night with a gradual motion to the south. Heavy rain will be the main hazard. Some MUCAPE is present, so a sporadic thunderstorm event can't be ruled out.