We all know the real storm season in the USA starts in April, peaks in May, remains very active in June and then declines fast in July, with less activity in August.
What about Europe?
I have made a quick research using ESSLs (European Severe Storm Laboratories) severe weather database (ESWD). I divided the months in half for a better sample recognition and searched the number of reports for each period for years 2002 until 2011. I searched reports for tornadoes, large hail, heavy rainfall, funnel clouds and severe wind gusts. The number of reports for the first years are very scarce. I got the next graph :
It is evident that the storm season does not start before late April/early May. In the summer the activity is evenly distributed until August when the activity drops. Convective weather gets very rare in late September. Coastal cities around the Mediterranean profit from the warm sea feeding the energy in those months. Of course you can find reports of severe weather in non-storm season months too. Very large hail reports in March and tornado outbreaks in April are not that uncommon! The pattern in the graph above represents the whole Europe, with northern countries having shorter and southern countries longer storm seasons.
Reports in the winter months are mostly from severe wind gusts and heavy rainfall (non convective weather), but I did not filter out every such event for obvious reasons.
