Climate indicators - extreme areal precipitation

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 The climate indicator for extreme areal precipitation shows the number of cases with at least 90 mm precipitation over 1000 km² during 24 hours, since the 1930s. When as much as 90 mm falls over 24 hours it leads to high water levels in rivers and streams with a risk of landslides and flooding in the affected area.

Each bar in the diagram shows the number of cases over a 10 year period since 1930 in Sweden with at least 90 mm precipitation over an area of 1000 km² during 24 hours. During the current 10-year period (2010-2019) there has been one case in 2010, none in 2011, two during 2012, none in 2013 and one in 2014
Each bar in the diagram shows the number of cases over a 10 year period since 1930 in Sweden with at least 90 mm precipitation over an area of 1000 km² during 24 hours. During the current 10-year period (2011-2020) there has been two cases during 2012, one in 2014 and one in 2017. Enlarge Image

In order to calculate the cases of extreme precipitation in a uniform and comparable way, the precipitation is first considered over two days (48 hours) and then a value for the 24-hour period with the greatest precipitation value is extracted.

The analyses are based only on observations from SMHI’s official stations so that the data is comparable with time. This means that there will have been cases of extreme precipitation that have occurred between measurement stations, but which are not included in the climate indicator.

One such case occurred on 30-31 August 1997 in Fulufjället. A special study of this case gave a value of 172 mm, which is higher than any of the values included in the official series – the highest value there for that period was 154 mm.

How much precipitation feels like an extreme amount?

How much does it need to rain for it to be considered extreme? To simplify, a precipitation volume of 40 mm per day is often described as torrential (even if this is not SMHI’s definition of torrential rain). When as much as 90 mm falls during 24 hours it leads to high water levels in rivers and streams with a risk of landslides and flooding in the affected area.