Historical damaging flood records for 1871–2011 in Northern Portugal and underlying atmospheric forcings

A long time series of damaging flood records in Northern Portugal for 1871–2011, gathered from a large number of documentary sources, is analyzed. The relationships between damaging floods (DFs) and relevant circulation weather types (CWTs) are also assessed. The DFs database has 1861 records and CWTs are identified using the 20th century reanalysis dataset v2. A coefficient of effectiveness (CE) is calculated for each weather type in order to assess DF–CWT relationships. Furthermore, conditions in the 10 days preceding a DF outbreak, type of flood and season were taken into account in CE calculations. The DF occurrences were responsible for 186 killed people, 59 injured, 29 missing, 1873 displaced and 15,924 homeless people. The monthly frequencies each CWT show that anticyclonic (A) and easterly wind (E) types are prevalent in winter, whereas R tends to prevail in the summer half of the year. However, the results show that the cyclonic (C) type has a positive frequency with DF occurrence (i.e. anomalously frequent), both on the DF day and on the nine previous days. The C type is commonly associated with southwesterly flow and unsettled weather conditions over Portugal, which are favorable to rain-generating mechanisms. The results also highlight some seasonal variation: in autumn, winter and spring, the C type is largely related to DFs, while the A and E types acquire higher preponderance in the summer. In effect, the latter two CWTs may trigger thunderstorms and heavy precipitation episodes in the Douro River catchment in summer.