Introduction
The disastrous flood that affected mainly the southern Piedmont area in
November 1994 was associated with heavy rain that fell in a relatively
small area in successive episodes spanning the period from 3 to 6 Nov.
1994. However, as shown below, the most intense rain occurred over the
same region mainly from the second half of 4 to entire 5 Nov. Widespread
precipitation, though not as intense as on the previous day, occurred also
on the 6th in a large portion of northern Italy.
Occurrence of heavy rain over the same area for a sustained period usually requires the existence of favourable large and meso-scale situations that allow for strong upward motion and moisture convergence persisting over the same region. Fig. 1 shows synoptic charts, including the 850 hPa wind field, derived from ECMWF analyses, for the period 4-5 Nov. 1994. The 500 hPa maps show the slow approach towards the Alps of a deep trough. The ridge to the east, associated with an anticyclone over eastern Europe, remains almost in place, implying scale contraction and intensification of geopotential gradient on the eastern side of the trough. The flow at lower levels maintains a southerly direction over the western Mediterranean, with pronounced confluence between the post-frontal south-westerly flow and the pre-frontal south-easterly flow more to the east. Both currents converge over the western Alpine region, where the highest precipitation intensity was observed. In particular on the 5th, the 850 hPa wind field shows a pre-frontal jet, having the characteristics of a "warm conveyor belt" that flows directly towards the Ligurian Apennines and the western Alps.
Precipitation data
Figs. 2 and
3 show 24-hour cumulated precipitations
at available stations (data from different networks have been merged: see
the acknowledgments) for 4 and 5 Nov., respectively. On the 4th (
Fig. 2), precipitation peaks around and
north of Genoa, with an area with values consistently exceeding 100 mm. On
the following day (Fig. 3), the
wettest area, where values larger than 200 mm are reported, is displaced
some 50 km to the west, corresponding to the highest part of river Tanaro
and Bormida basins (Maritime Alps). A secondary, distinct maximum is
visible over the Alps north-west of Turin, with some stations reporting
more than 200 mm. The 100 mm contour line has an arc shape that follows
closely the eastern slope of the western Alps. On the 6th (not shown),
cumulated values over the same areas did not generally exceed 100 mm but
still contributed to the flooding that was at its apex early this day.
In order to appreciate the strong space-time variability in the precipitation, let's consider in Fig. 5 the time series of hourly cumulated precipitation observed in the stations of Ponzone (A in Fig. 2) and Priero (B in Fig. 2); the former located in the north downwind side of the Ligurian Apennines (30 km north of Savona), the latter at only 40 km south-west from the former, between Tanaro and Bormida rivers. It is possible to notice that in the first station a sharp precipitation maximum is present between the evening of the 4th and the early morning of the 5th, while at Priero station the precipitation has several peaks spread over the entire 5 Nov.
MAP Newsletter. October '00