From 6 to 9 november 1999 an intense cyclogenesis event characterized by the rapid development of an orographic cyclone south of the Alps and the passage of an intense cold front, coming from the north-west, over the range, took place and seriously affected Greece, Italy, Croatia and Spain, producing strong and damaging winds (modulated or caused by the orography) plus heavy precipitation (partly orographic as well) leading to surface conditions as summarized below:

On 6 November, a trough was elongating meridionally between the North Sea and the Mediterranean Sea. A potential vorticity streamer was strongly narrowing between Denmark and northern Italy, forming a cutoff low at its southern end above Italy, Corsica and Sardinia. ECMWF analysis showed significant tropopause folds at the western and eastern edges of the streamer. These folds transported stratospheric air down to a height of less than 6 km.

ECMWF analized fields for 06/november/1999 at 18 UTC. Left image: potential vorticity over 325 K surface (PVU). Right image: 500 hpa geopotential height plus temperature field. Figure adapted from Balearics Islands Meteorological Center (Mediterranean Meteorological Studies Unit), INM.

The strong cold-air advection associated with the trough reached the Mediterranean in the morning of 6 November, followed by rapid cyclogenesis over the south-western Po Valley. The strength of the MISTRAL peaked in the late afternoon (check slide 3).

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3 hours interval infrared satellite loop from 6 november 1999 at 00 UTC to 8 november at 11:30 UTC. Images in loop provided by Ana Sánchez, from the remote sensing INM department.

Hand-analysis of surface pressure with cold front on 6 November 1999 12 UTC. Contour interval is 1 hPa. Alpine topography exceeding 1000 m MSL is shaded. Indicated are Innsbruck (IBK) and Milan (MIL). From Gohm et Al, Foehn flow in the Austrian Alps interrupted by a cold front passage: part II.

Wind at 850 hpa after 12 hours of high resolution simulation (BOLAM model) verifying 6 november at 18 UTC. Wind scales is in m/s. Taken from Buzzi et al. (MAP 2002).

Time traces of mean wind speed and wind gusts for meteorological station Maslenica Bridge, downstream of the mountain pass on the Velebit Mountains (the longest mountain range in Croatia, southwest of Zagreb) during the period 6 to 9 November 1999, 00UTC. The time series show that this was indeed an extreme Bora case, as classified in MAP IOP-15 (a MAP Intensive Observing Period). Bora = Cold, northeast katabatic wind flowing through mountain passages into the Adriatic Sea east of Italy.