Weather Spotter Terms Defined

The term straight-line thunderstorms are used to describe any non-tornado winds produced by thunderstorms. In a thunderstorm, damaging straight-line winds are associated with the leading edge of the rain-cooled outflow (better known as the gust front). Normally these winds range from 30 to 50 mph - however, occasionally the winds can exceed 60 mph. The leading edge winds normally last between 5 to 15 minutes. Sometimes, the strongest winds are not associated with the gust front, but rather occur behind the gust front in close proximity to the area of heaviest rain.

Downbursts are winds which are potentially damaging to structures on the ground or to aircraft in flight. Following the mid-70's Easter Airlines Flight 66 crash in New York, the University of Chicago’s Dr. Theodore Fujita is credited with discovering downbursts. There are two types of downbursts: microbursts which produce strong winds less then 2.5 miles in diameter; macrobursts produce strong winds over an area larger than 2.5 miles.

Downbursts are typically produced within a single thunderstorm cell. Downbursts occur with all types of thunderstorms, from single cell to supercells. The most damaging downbursts may be produced by weaker, benign storms. These storms often give little advanced indication of imminent danger on weather radar or to the spotter - so warnings are difficult to issue. After the strong winds reach the ground, Doppler Radar can frequently detect the stronger winds, but the threat of additional damaging winds may be over.

Occasionally, many storms produce a squall line and produce damaging winds over a much larger area for a period of an hour or longer. This type of damaging wind is called Bow Echoes because a portion of the squall line accelerates or "bows" out in an easterly direction. Supercell storms also occasionally develop into Bow Echoes. In extreme cases, straight-line winds in a Bow Echo can approach 150 mph - stronger than 80 percent of all tornadoes! Since Bow Echoes produce distinctive radio echoes and last one to three hours or longer, the NWS can often provide considerable advance warning. Tornadoes sometimes accompany Bow Echoes. Although tornadoes in such instances are typically short-lived, the storm may be intense and considerable damage is possible.

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