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 stretch of higher-than-average temperatures will continue across a large portion of the Western U.S. this week.
A dominant storm track featuring storms moving west to east across Europe will result in a stark contrast between cold air building across Scandinavia and milder air masses entrenched near the Mediterranean.
An El Nino-fueled October will feature more rainfall and storms for Southwest beginning this week.
After waves of cool air progress through the Midwest and Northeast this week, some areas will be cold enough for the first snow showers of the season by this weekend.
A "blob" of abnormally cold water in the North Atlantic, located near Greenland, has the potential to put enough drag on the ocean current to impact weather conditions in the years to come.
Tropical Storm Nora moved into to the Central Pacific Basin on Sunday, where unusually warm waters have already led to a record 13 tropical systems this hurricane season.
Denver, CO (1982)
Wet snow - 6 inches foothills; slush in city. Power lines down, as well as trees.
Early-season snows: Jay Peak 6 inches Warren 5 inches
New England (1990)
Remains of Tropical Storms Klaus and Marco brought torrential rains and flooding. Parts of Connecticut had 6 inches of rain or more. Stafford, CT, had 4.20 inches.