The first four months of the year
brings risk for tornadoes in the southern US. From April through
June, the biggest tornado threat shifts to the Plains, Upper
Midwest, and Great Lakes. The main tornado risk then stays along the
northern tier of the country through much of summer, while tropical
storms and hurricanes increase back in the South as they move
inland. These are followed in November and December with more
chances of tornadoes moving back to the South.
About ninety percent of US twisters occur in a 300-mile wide
corridor extending from West Texas to Canada. Warm, moist surface
winds blow up from the Gulf of Mexico, while cool high-altitude
winds blow over the tops of the Rockies. The cool air wants to sink
while warm air wants to rise. However, the mountain air causes a
temperature inversion, which prevents the warm surface air from
rising. It is like clamping the lid on a pressure cooker. The
surface weather systems build up a big head of steam until they
break through the inversion and shoot up to towering heights and the
violent updrafts and downdrafts lead to form tornadoes. Tornadoes
occur most frequently in the central plains of the US. Australia
has the second most tornadoes each year.