Start of the month brought thunder and hail. What happened? IPMA explains

O early October brought bad weather to mainland Portugal. According to the Portuguese Institute of the Sea and Atmosphere (IPMA), these episodes, associated with “a cold-drop depression, caused thunderstorms, hail drop and strong wind.

In a public note site, the IPMA states that “Portugal continental was affected, between October 2 and 6, by episodes of strong atmospheric instability, associated with a cold-drop depression, which originated strong showers, thunderstorm, hail drop and strong wind in several locations”. 

Now, “at the beginning of this period, the disturbance suffered a slight digging left to Galicia, giving rise to a depressionous core that slowly moved south-west”, having affected “progressively the continental territory and focusing, from day 4, to the west of the Portuguese west coast”. 

The IPMA explains that “the depression carried a mass of hot air, very unstable and with high content in precipitable water”. 

Thus, “the weak atmospheric circulation favored the slow movement of convective cells, allowing some areas to be affected for prolonged periods and, in some cases, for successive precipitation cells”. “These conditions contributed to the occurrence of sudden floods and hail drop,” he stressed. 

For example, in the Alentejo region, “during the morning of day 4, atmospheric conditions favored the formation of an organized set of storm cells, with continuous precipitation over at least 100 km, usually called the mesoscale convective system”. 

In Manteigas, in the Guarda district, “the episode at the end of day 2 was particularly significant”. “Between 9:30 p.m. and 10:30 p.m. UTC there was very strong rainfall, sometimes accompanied by hail, which lasted longer,” he says.

According to IPMA, “the radar images show the persistence of this episode. The storm was also accompanied by strong electrical activity, with the highest number of lightning discharges between 23:00 and 24:00 UTC.”.

In turn, the south of the territory, Santiago do Cacém, in the district of Setúbal, “was affected on day 4 by very strong precipitation and significant drop of hail, also associated with high frequency of lightning electrical discharges”.

In the end, the IPMA points out that “the numerical weather forecasting models allow identifying areas where there is a higher probability of occurrence of these phenomena, but have limitations in the exact prediction of the site, period and intensity of convection. Thus, not all sites covered by a weather warning will necessarily be affected by the expected phenomena.”.