EVENT ANALYSIS: Grignano, Italy tornado of September 17, 2013

An impressive tornado appeared in the Gulf of Trieste, Italy, just offshore Grignano, Trieste on September 17 between 7:10 and 7:20 am local (CEST). The tornado formed over the sea and then tracked NE past the Grignano marina and onshore, where it left a 2 km long damage track (OSMER FVG, personal communication).

The tornado was witnessed, photographed and videoed by many people, and was the first event Severe Weather Europe reported on. Data from nearby meteorological stations allow a detailed analysis of the event.

Grignano tornado, shot from just above Grignano marina. Source: Francesco Ripandelli.

A strong negatively tilted trough was crossing central Europe with a surface cold front extending across the southern Alpine region (see Fig. 1 and 2 below). Southerly low-level winds were present across the N Adriatic sea and Trieste gulf, maintaining moderately unstable and sheared environment across the warm sector.

Fig 1 – EUMETSAT IR satellite image 17/09/2013 05Z. Source: Sat24.com / Eumetsat / Met Office.

Fig. 2 – 500 mbar geopotential GFS forecast and ARW-12 km 500 mbar wind forecast for 17/09/2013 06Z. Source: Wetterzentrale.de / Consorzio LAMMA.
 

REGIONAL MODEL

 
Surface cold front was rapidly moving from southern Friuli region towards the gulf of Trieste, where tight pressure gradient and SFC wind convergence with tramontane wind coming immediatelly behind the front and SE winds over the SE parts of gulf of Trieste (Fig. 3,4).

Fig. 3 – ALADIN/SI model forecast for 925 and 850 mbar wind on 17/09/2013 06Z. The rectangle on the 925 mbar map shows the area covered by the ALADIN/SI 10 m wind forecast (Fig. 4). Source: ARSO.

Fig. 4 – ALADIN/SI model forecast for 10 m wind on 17/09/2013 06Z. Source: ARSO
 

LOCAL ANALYSIS

 
The tornadic supercell formed on a strong surface frontal convergence of SE and SW winds (Fig. 5) with 5 m/s SE and over 10 m/s SW, while the strong tramontane winds supported the tornadogenesis process in this supercell.

Fig. 5 – surface wind, Sept 17, 2013, 07:00 CEST. Source: OSMER FVG.
Modified 00Z Udine sounding (Fig. 6) with OSMER FVG Trieste station data yields notable shear and helicity parameters.

• DLS (sfc-6 km): 34 kt
• LLS (sfc-1 km): 21 kt
• SREH3: 190 m²/s²
• SREH1: 102 m²/s²

Fig. 6 – 17/09/2013 00Z Udine sounding modified with Grignano station sfc wind data and 925/850 mbar ALADIN/SI model wind and temperature. Source: OSMER FVG, ARSO.
Surface temperatures and dewpoints in the inflow region were 19/17 °C (OSMER FVG Stations Trieste, Muggia and Monfalcone). This resulted in significant CAPE:

• SBCAPE: 1253 J/kg
• 0-3 km CAPE: 192 J/kg
• EHI 0-2 km: 1.4

OSMER Fossalon Doppler radar shows a clear presence of a mesocyclone between 7:00 and 7:20 CEST located just offshore Grignano, tracking NE (Fig. 7).

Fig. 7 – OSMER Fossalon VMI / Doppler radar images, 7:10 CEST. Source: OSMER FVG.

Interestingly, a satellite waterspout briefly formed next to the tornado (Fig. 8).

Fig. 8 – satellite waterspout. Source: Francesco Ripandelli.
Damage track: 2 km long (OSMER FVG, personal communication). Damage mostly limited to broken trees, as the ground track missed populated areas. The tornado hit and damaged an amateur astronomical observatory on Campo Sacro/Božje Polje. Peak recorded wind speeds at Grignano station were in the 150 – 180 km/h (EF1) range, but the station’s anemometer failed during tornado passage (OSMER FVG, personal communication).