During the afternoon and evening of March 9, 2013, the weather in mainland Portugal was conditioned by a depression centered northwest of the Iberian Peninsula, with high expression levels in the troposphere and shift southeast. This depression was carrying in his movement a maritime polar air mass, moderately humid, with successive lines of instability embedded. The movement was intense, generally from southwest at various levels. A jet stream with general northeast-southwest orientation, stood on the south-central territory, forcing the runoff at high levels particularly on the north. In this context, and on the coast of the region, the atmosphere had some instability. Surface observations show that the wind was weak to moderate from the south, but in between it and the layer above 1000m, and ran strongly intensified southwest. This type of variation in vertical wind, known in meteorology as "veering" (rotation of wind direction vertically in a clockwise direction), accompanied by this intensification, have been crucial to organize the convective activity.
Indeed, during the afternoon and evening of the same day, several documented reports showed the occurrence of various phenomena like tornado on the north coast in association with some convective cells that affected the region. The damage which could be verified by analysis of many documents, it is assumed that the intensity of these should be made in the range of T0-T1 (Torro scale) equivalent to the level F0 (Fujita scale), ie associated with winds instantaneous maximum up to 119 km / h.
Although atmospheric instability was not substantial variation in a layer of very low wind and other forcers mechanisms at higher levels favored the formation of such phenomenon.