Extreme value analyses

Return periods of the 5-day precipitation for 13-17 November 2002

This event brought extraordinary precipitation amounts over the central Alpine north slope, Ticino and the Grisons. Between 13 and 17 November 2002, the stations located in these regions measured precipitation with return levels corresponding to return periods of up to and more than 100 years.

The precipitation amounts recorded during these 5 days were the rarest at stations Campocologno (252.9 mm) and Avers am Bach (267.8 mm) in the southern Grisons with return periods of more than 100 years.

In the region of Geneva, precipitation corresponding to a 10-20-year event were also measured. The precipitation that affected that region mostly took place on 14 November. It will not be described in this section but is presented as another event for which the description can be found here: 14-15 November 2002.

The 5-day precipitation is measured between 13 November 2002, 05:40 UTC and 18 November 2002, 05:40 UTC. The return periods are based on the generalized extreme value distribution estimated from the annual 5-day precipitation maxima during the period 1961-2010. The size of the circles is proportional to the value of the return period. The filling of the circles indicates the reliability of the statistics: good (filled circle), questionable (circle with cross) and poor (empty circle).

Return periods of the largest 4, 3, 2 and 1-day precipitation

Since the precipitation was particularly high over the interval of 3 days (14-16 November 2002), the return periods are even higher when considering the largest 4-day and 3-day precipitation than for the 5-day precipitation. Most stations in the affected regions measured precipitation with return levels corresponding to return periods of 50 to more than 100 years, in particular the stations located in the south-western Grisons.

Return periods are also high but a little lower for the largest 2-day precipitation. Very rare precipitation sums were measured in the south of the Grisons at stations Brusio-Piazzo (200.7 mm), Bivio (210.1 mm) and Zervreila (254.9 mm) with corresponding return periods of more than 100 years.

The largest 1-day precipitation, on the other hand, was not very rare. A few stations in the affected regions measured precipitation corresponding to a 10-50-year event and only two stations recorded return levels having a return period of 50-100 years (Brusio-Piazzo: 112.8 mm) or more than 100 years (Bivio: 123.8 mm).

Hence, while for most of the stations the daily precipitation amounts were not that extraordinary, the addition of such amounts over two and especially three days led to very rare precipitation sums which had consequences on the central Alpine north slope, in the Grisons and in Ticino.

Extreme value analyses for 3-day and 1-day precipitation

This section provides the extreme value analyses for the stations presented in the section Hyetographs. The statistics are based on the period 1961-2010. Return level plots of 3-day and 1-day precipitation maxima are shown when the statistics are reliable. When the statistics are not reliable or when the length of the records is too short, tables of the ten highest precipitation maxima for the period 1961-2014 or between the beginning of the measurements and 2014 are shown instead. In addition, PDFs of the extreme value analyses are given when the reliability of the results is good or questionable.