Thunderstorms associated with approaching frontal systems are characteristically more severe than air mass type thunderstorms. Cold air is more dense than warm air.
As a cold front approaches a warm air mass, it rapidly lifts the warm air, creating an unstable situation in the troposphere.
Unstable air containing moisture is conducive to the development of severe thunderstorms, storms capable of producing winds greater than or equal to 58 miles per hour, 3/4 inch hail or larger, and tornadoes.
Frontal thunderstorms that precede a strong cold front often make a dramatic entrance into an area of warm air. You can face the storm and watch as the front approaches. It often appears as a line of dark, rumbling cumulonimbus clouds.
These clouds can tower up to the level of the jet stream, 30,000 to 45,000 feet above the ground. Clouds that vertically climb to such heights can be readily identified by their anvil tops, sheared flat by jet stream winds.
Frontal thunderstorms, if the cold front is strong enough, can occur any time, day or night. These storms affect weather in a much larger area than do air mass type thunderstorms and frequently produce torrential rains that can fall for several hours on end, for hundreds of miles.
During the winter months and into early spring, frontal thunderstorms are most likely to occur in Florida and the Deep South. As winter turns into spring, the upper jet stream generally begins to retreat from its position over the more southern regions of the United States.
As the jet stream migrates north toward Canada, severe weather conditions associated with cold fronts are more likely to develop in the northern regions of the United States.
From spring into summer, frontal thunderstorms can and often do turn severe east of the Rocky Mountains and into the Plains states. Sometimes these storms produce tornadoes.
During the summer months, frontal thunderstorms are common occurrences in the upper Midwest east to New England, giving this area of the country its fickle reputation for frequent changes in weather conditions.
The approach of a cold front in the U.S. Northwest and in California is quite different in intensity than cold fronts that bowl their way through warm air in areas east of the Rockies through to the Atlantic Seaboard.
West Coast cold fronts come in from the Pacific Ocean during the winter season, but almost never occur during the summer as far south as California.
At this time of year, the jet stream typically directs storms and their associated fronts well to the north. Cold ocean water tends to exert a stabilizing influence on the atmosphere, and inland air is usually too dry to support thunderstorms.
During the winter, however, vigorous weather systems are able to track farther south, and severe weather (including thunderstorms with small hail and mainly weak tornadoes) can develop across California.
Interestingly, this West Coast severe weather situation usually happens after a cold front passes, when sunshine and low level heating coincide with cold air aloft to produce an unstable atmosphere.