The deadliest storm of the 20th century striking Portugal: flood impacts and atmospheric circulation

Abstract

The deadliest storm affecting Portugal since, at least, the early 19th century, took place on the 25 and 26 November 1967 causing more than 500 fatalities. This work aims to assess the most relevant aspects of this episode. This includes describing the associated meteorological conditions and key hydrological characterisation such as the level of exceptionality of the observed precipitation at different temporal scales, or the estimation of peak discharge values in 20 small river catchments affected. Additionally, from a human impact perspective we provide a full account of all the main socio-economic impacts, particularly the numbers and location of victims (dead, injured, homeless and evacuated). Based on the sub-daily time series of a representative station, and its Intensity–Duration–Frequency curves, we have found that the exceptionality of this rainfall event is particularly linked to rainfall intensities ranging in duration from 4 to 9 h compatible with return periods of 100-years or more. This range of time scale which are similar to the estimated concentration time values of the hydrographic basins affected by the flash flood event. From a meteorological perspective, this episode was characterised by strong convection at the regional scale, fuelled by high availability of moisture over the Lisbon region associated with a low pressure system centered near Lisbon that favoured the convective instability. Most victims were sleeping or were caught by surprise at home in the small river catchments around the main Lisbon metropolitan area. The majority of people who died or who were severely affected by the flood lived in degraded housing conditions often raised in a clandestine way, occupying flood plains near the stream beds. This level of destruction observed at the time is in stark contrast to what was observed in subsequent episodes of similar amplitude. In particular, since 1967 the Lisbon area, was struck by two comparable intense precipitation events in 1983 and 2008 but generating considerably fewer deaths and evacuated people.