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.
A new tropical threat may loom for the Caribbean and North America in the not-too-distant future, while eight more weeks remain in the Atlantic hurricane season.
The greatest danger of flooding across the central United States will unfold in western Texas, where downpours will be most persistent into Monday.
Fall air has finally arrived in the northeastern United States and may yield the first frost of the season in parts of the region this weekend.
Typhoon Megi will continue to strengthen before threatening lives and property across Taiwan and eastern China this week.
The first windstorm of the season could blast the northern United Kingdom around Tuesday as Karl arrives.
Hot, dry and windy weather into Monday will lead to an increased risk of wildfires across Southern California.
El Cordnazo, CA (1939)
Greatest September rainstorm with 5.42 inches in 24 hours at L.A. Floods killed 45; $2 million damage.
Southern CA (1970)
Record late September heat wave seared Southern CA for a week. L.A. hit 105 degrees; San Diego hit 97 degrees.
New Jersey (1975)
4-day rains of 7.50 to 11.00 inches. Flooding in northern part of the state.