An Atmospheric River during the 1966 flood

GFS analysis, 04 Nov 1966, 00 UTC

A deep trough deepened over western Europe and progressively extended towards the western Mediterranean in the first days of November 1966. Since the evening of 2 November and on 3 November, an Atmospheric River progressively elongated from the Atlantic coast of Africa to the Mediterranean basin, making landfall over central Italy first, and then reaching the north-eastern Alps. On 4 November the synoptic wave amplified and the 500-hPa geopotential isolines were almost perpendicular to the Alpine chain, denoting strong meridional transport across the Mediterranean, toward the orography. The intense large-scale forcing is also highlighted by a PV streamer progressively elongating towards Italy behind the cold front.

48 h rainfall, 3-4 Nov 1966
(ArCIS dataset)

Intense precipitation occurred over the northern Apennines due to the strong orographic uplift of the moist air associated with the Atmospheric River, whose IVT reaches a maximum intensity of about 1200 kg/(ms) over the Tyrrhenian coast of central Italy at 09 UTC, 4 November 1966. Intense rainfall was also caused by deep convection. At the same time, Sirocco winds further intensified, channeled in the Adriatic basin between the Apennines and the Dinaric Alps. This intense warm and moist flow impinged on the Alps leading to strong orographic lifting, and north-eastern Italy becomes the area affected by the heaviest precipitation.

The 48h accumulated rainfall from 08 UTC, 03 November, to 08 UTC, 05 November 1966 shows that the most affected areas were north-eastern Italy (Friuli Venezia Giulia and partly Veneto regions), with peaks exceeding 600 mm, and Tuscany, where local peaks (mainly convective) attained values higher than 350 mm. The rainfall maxima were clearly oriented in an almost meridional direction from Tuscany to the north-eastern Alps, and this pattern is ascribable to the characteristics of the Atmospheric River.

A dramatic flood affected the city of Florence, while the Adige river in Trento and the Tagliamento river in the Friuli Venezia Giulia plain caused extensive floods. The North Adriatic coast was struck by storm surges and high sea waves. Venice was deeply affected with the maximum sea level ever recorded (194 cm) since the beginning of sea level records in 1872. The most affected areas were the eastern Alps and Pre-Alps, where rainfall attained 751 mm in 48 h in Barcis (Friuli Venezia Giulia region).