The month of October 2017 in Portugal mainland was extremely dry and exceptionally hot.
This was the warmest month of October in the last 87 years (since 1931), with the average value of average air temperature about 3°C above normal value.
The average value of maximum air temperature was the highest since 1931, about 5°C above normal value. The mean value of minimum air temperature was above normal at about 1°C.
It is worth noting the occurrence of new highs (records) of maximum and minimum temperature for the month of October, with special emphasis on the 15th (hottest of the month). In most of the territory, hot days (maximum temperature ≥ 30°C), very hot (maximum temperature ≥ 35°C), and tropical nights (minimum temperature ≥ 20°C) were recorded between October 1 and 15; from 24 to 29 October there were hot days (maximum temperature ≥ 30°C).
There were 2 heat waves, from 1 to 16 and from 23 to 30 October, which covered a large part of the mainland, except for the coastal regions. The first heat wave had a maximum duration of 15/16 days and is among the longest for the month of October.
Regarding precipitation, the month of October was classified as extremely dry, with a mean value of precipitation in Portugal mainland corresponding to 30% of normal value. This month has been the driest of the last 20 years.
The combination of persistence of much lower than normal precipitation values and values of temperature much above normal, in particular the maximum temperature, resulted in high evapotranspiration values and significant values of soil moisture deficit.
According to the Palmer Drought Severity Index (PDSI), at the end of October, the entire continental Portuguese territory is in severe drought (24.8%) and extreme (75.2%) drought.
In previous drought situations, there was a significant reduction in severity (severe and extreme classes) at the beginning of autumn. On the contrary, in the current situation there was a worsening of the severity classes.