Storm Forecast
Valid: Mon 03 Sep 2012 06:00 to Tue 04 Sep 2012 06:00 UTC
Issued: Sun 02 Sep 2012 22:06
Forecaster: TUSCHY

A level 2 was issued for most parts of Corsica and Sardinia mainly for excessive rainfall amounts.

A level 2 was issued for parts of W-Italy mainly for excessive rainfall amounts.

A level 2 was issued for parts of NW-Italy mainly for excessive rainfall amounts and an isolated tornado risk.

A level 2 was issued for parts of Sicily and S-Italy mainly for excessive rainfall amounts, large hail, strong to severe wind gusts and an isolated tornado event.

A level 1 surrounds the level 2 areas mainly for heavy rain, isolated large hail and strong to isolated severe wind gusts.

SYNOPSIS

Isolated from the active westerlies over N-Europe, a cut-off low affects most parts of the W/C Mediterranean with no real net motion to the east. With the progressively more zonal mid/upper level flow over N-Europe, geopotential heights over C-Europe increase and support stable conditions. The same for SE Europe, where hot and dry weather continues.

Side-note:

At 19Z (2nd Sept.), a 568 gpdam upper low was situated just to the west of Sardinia, featuring an healthy mid/upper vortex in model data. However, buoy and surface observations show a broad and ill defined surface circulation atop the W/C-Mediterranean, with near surface streamline analysis indicating two well structured convergence zones (one west of Sardinia and the other atop the Tyrrhenian Sea). However, coastal synop data locally and temporarily support the evolution of small scale vortices, which envolve within the broad cyclonic circulation, probably spawned by local orography or regionally enhanced baroclinity (e.g. Tyrrhenian Sea). One of those vortices should evolve east of Corsica during the night hours (until my forecast starts), which however is not easy to verify due to a thick cirrus shield of an active warm conveyor belt (WCB) and missing offshore data. Forecast development however seems plausible due to a favorable placement beneath the left exit of a 50 m/s 300 hpa jet and the deep baroclinic zone nearby.

During the forecast, the main scenario of interest will be the aforementioned small-scale vortex, which is forecast to loop cyclonically over Corsica and then towards Sardinia until the evening hours along the still existing N-S aligned convergence zone. Again, some models indicate rapid organization /a profound deepening rate of the depression's central pressure, which however will be challenged for the following reasons:

I) Forward calculated near surface trajectories still indicate ongoing offshore flow from S-France, where BL moisture remains meager (dewpoints in the lower tens). Also, I do not yet trust forecast rich BL moisture from parcels, emerging out of the Adriatic Sea towards the west, as this air has to cross the rough terrain of N-C Italy. In addition, active storm clusters in the inflow fetch may prohibit the advection of the highest moisture content to the west.

II) Any surface circulation evolves/moves atop the islands, which may inhibit or delay ongoing organization of the surface vortex.

III) Despite a gradual breakup of the dry mid/high level inflow from the NW and south, it takes some time for the mid-level moisture to wrap around the main vortex. However with the approach of a decaying front from the NW (from France), better moisture may infiltrate into the circulation. Of main concern will be any influx of an EML from the south.

IV) SSTs remain the coolest for the W-Mediterranean basin with readings in the lower twenties and a pretty shallow depth of the warm (near) surface layer.

Nevertheless, the convergence zone and the movment into a weakly sheared environment (with DLS decreasing to less than 10 m/s) next to the available enhanced background vorticity may assist in the spin-up and strengthening of such a vortex. In case of a gradually organizing convective structure (banding)/increasing longevity of DMC, we do not want to rule out the necessity of the use of the Herbert-Poteat technique and subsequently the classification of a subtropical cyclone.

During the night, this vortex continues to move eastwards atop the Tyrrhenian Sea, where conditions for further strengthening/organization seem more supportive (with higher SSTs and an ill defined warm eddy east of Sardinia). The evolving low will be monitored during the forecast and subsequent updates may be issued in case of organization. It has to be noted however, that none of the global/coarse models indicate any organized warm core development at this time.


DISCUSSION

... Sardinia and Corsica ...

Placed beneath the broad/well structured cyclonic vortex, DMC evolves all day long. A lot depends on smaller scale vortices and their development/placement, which could very well increase the risk of training storms and excessive rain. No surprise to see a tremendous spread in the QPF fields of available model data although most favor NE Corsica with high rainfall amounts. I do not want to be smarter than the models and hence a broad level 2 was issued. Next to the rain risk, waterspouts are possible due to good LL CAPE and forecast numerous convergence zones.

... Italy ...

East of the main vortex, a well structured WCB still affects areas like Sicily, the S-Tyrrhenian Sea and S-C Italy. Storms likely evolve along the wavy and leisurely eastward moving cold front with impressive BL moisture ahead (surface dewpoints in the mid twenties). Yesterday's soundings in this WCB featured PWATs in excess of 30 mm and this probably won't change during my forecact period. DLS of 15-20 m/s overspreads 1-2 kJ/kg MLCAPE which assists in well organized multicells and isolated supercells. History showed that similar "moisture streamers/WCBs" sometimes managed to support stationary V-shaped convection along convergence zones (on the meso-beta scale) with locally torrential rainfall amounts. We can't exclude this scenario and a level 2 covers that risk.

Any long-lived storms (especially those, which move along the wavy frontal boundary from Sicily to S-Italy) may temporarily acquire some better organization, posing an isolated tornado risk. Beside that, large hail accompanies thunderstorm activity.

Two level 2 areas were also added for W-Italy and N-Italy, as most models agree in the placement of active shower/thunderstorm activity with excessive rain. The one over W-Italy was merged with the one over Corsica.

The other one was issued as persistent onshore flow and long lasting and quasi-stationary convergence zone sets-up. I would not be surprised to receive an isolated tornado/waterspout report, as LL shear and LL CAPE create a favorable corridor with somewhat augmented tornado probabilities along the N-Adriatic coast. Isolated models also show another QPF maximum along the E-C coast of the Adriatic Sea, but confidence in this scenario remains too low right now to upgrade.

... Austria to W-Ukraine ...

A quasi-stationary boundary runs from the Alps to the NE and starts to transform into a southward moving cold front during the night. No forcing mechanism is seen although some divergent upper level flow persists during the day. Sporadic CI is forecast within a weakly sheared environment. DLS increases towards SE-Poland, where an isolated better organized mutlicell event with large hail and strong wind gusts is possible. Beside that, no severe risk is anticipated and overall activity rapidly diminishes after sunset.

... Finland ...

A sharp mid/upper trough affects the area of interest and some daytime driven cold-core convection is expected. Weak shear but moderate CAPE and a lowered WBZ may assist in marginal hail in strongest storms. Otherwise strong wind gusts and locally heavy rain will be the main hazard. Storms decay after sunset.