Since we live in a country with a famous Tornado Alley right in its center, it is easy to forget that severe weather can occur, though with substantially less frequency, in other parts of the world.
A recent example of this is the flooding in Venice, Italy in early November and the strong tornado that roared through Teranto, Italy on Nov. 28.
In both cases, strong thunderstorms were involved. Just as in our Tornado Alley, the strongest thunderstorms result from instability in the atmosphere combined with upward vertical motion of the air. The particular form of instability that leads to severe thunderstorms is known as convective instability and it results from dry air sitting atop moist air.
In Tornado Alley the dry air comes off of the elevated Mexican Plateau and the moist air from the Gulf of Mexico. Along the Adriatic Coast of Italy, the dry air comes from mid-levels over the Sahara Desert and the moist air comes from over the southern Mediterranean Sea. When these ingredients are assembled by a strong cyclone in the central Mediterranean, it can produce extreme rains in northeastern Italy and even tornadic activity.
Because these circumstances are most likely to get organized in the late fall, the limited tornado season that does occur in Italy occurs in October and November. It would be interesting to know how many merchant vessels in the long history of Venice have gone down in such autumn storms.