The tornado frequency increases in the spring as the warm and cold seasons battle it out in the U.S.
Warm, humid air surges into the South during the spring months, while colder air remains in place across the northern tier of the country. The clash of warm, humid air and cooler, drier air to the north creates instability for thunderstorms to develop.
Another key ingredient for severe thunderstorm and tornado development is a powerful jet stream, or a corridor of fast winds around the altitude where planes fly. When there are powerful winds high in the atmosphere and weaker winds at the surface, a twisting motion in the atmosphere can help to produce rotating thunderstorms capable of spawning tornadoes.
During the spring months, the jet stream is often still potent across the U.S. before it shifts too far to the north during the summer to support violent storms with tornadoes touching down.
The sun's rays are also strengthening during the spring months, promoting increased daytime heating that further reduces the instability of the atmosphere.
Showers and thunderstorms will return to the Southwest late this week and could reach part of California.
A cold front swinging into the Northeast will bring the threat of severe weather to part of the region on Tuesday afternoon.
The southwest Gulf of Mexico has given birth to the Atlantic basin's fourth tropical storm of the season and will send torrential rain into northern Mexico.
Flooding is a concern across southwest Mexico through midweek as Norbert moves just offshore.
The Alaskan wood frog, which freezes itself during the harsh winter months, can remain in an extreme frozen state far longer than researchers originally thought.
An area of low pressure will bring a threat of heavy rain and flooding to parts of southern Europe through the middle of the week.
Mecca, CA (1950)
126 degrees - highest ever for U.S. in Sept.
East Coast (1775)
Matecumbe Key, FL (1935)
Labor Day Hurricane hit Florida. Pressure at Matecumbe Key dipped to 26.35"/892.3 mb. Most intense hurricane ever to hit the U.S. with 200-mph wind. Tide of 15 feet; 408 dead.