Weather Encyclopedia

Severe Thunderstorms

Thunderstorms that precede a cold front are generally stronger and more likely to reach severe proportions than localized thunderstorms associated with tropical air masses.

A few things must happen in the earth's atmosphere in order for a thunderstorm to become severe.

By definition, a severe thunderstorm is a thunderstorm that contains any one or more of the following three weather conditions:

• Hail that is 3/4 of an inch or greater in diameter
• Winds 58 miles per hour or greater
• Tornadoes

Supercell thunderstorms are fierce and can sometimes discharge a number of tornadoes. They are tremendously powerful and well-organized, containing rotating columns of rising air.

These storms are capable of maintaining severe thunderstorm strength for hours. They can also produce dangerous straight line winds, large hail and torrential rain. Sometimes these storms spawn particularly strong tornadoes.

Frequently,severe thunderstorms develops as part of a line along or ahead of a cold front associated with strong jet stream winds in the upper levels of the troposphere. Spurred on by the jet stream, this line of severe thunderstorms is called a squall line. Bow echoes associated with squall lines or mesoscale convective systems can produce widespread damage.

Downbursts And Microbursts

A downburst is a severe localized wind blasting down from a thunderstorm. These strong downward currents are classified by meteorologists as microbursts if the downburst covers an area less than 2.5 miles in diameter and as macrobursts if the down burst covers an area of at least 2.5 miles in diameter.

Wind shear is any sudden change of speed or direction in wind flow.  Microbursts have strong wind shear horizontally from their front to back edges and vertically from the surface to near cloud base.

Check how thunderstorms form for more information.