SWAW--Thunderstorm Safety

THUNDERSTORM SAFETY

Hail remains one of the most costly severe weather phenomena observed in the United States every year. Damage from hail not only occurs to crops, but also to homes, vehicles and businesses.

Hailstorms produce around $2 billion worth of property damage each year. Damaging hail the size of golf balls or larger occur with the strongest of all thunderstorms, and hail will usually accompany tornadic thunderstorms. Remain alert for severe thunderstorm warnings from the National Weather Service, and use this valuable information to protect your property from the threat of hail.

If caught outdoors during a hailstorm, seek shelter in a reinforced building as quickly as possible. The key to personal safety in a hailstorm is to protect yourself from the falling hailstones. Hail rarely kills people, but it can become a killer if precautions are not taken. In China in May of 1986, intense hail killed 100 people, injured 9000, and destroyed 35,000 homes.

Damaging winds come in many forms, sometimes from squall lines of thunderstorms and other times in the form of downburst winds. The most frequently encountered type of damaging straight-line wind in a thunderstorm is that associated with the leading edge of the rain-cooled outflow, known as the gust front. Although most thunderstorm outflow winds range from 30 to 50 mph, on occasion these winds can exceed 60 mph. These outflow winds typically last 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.

When these winds are potentially damaging to structures on the ground, or to aircraft in flight, these winds are referred to as downbursts. Dr. Theodore Fujita of the University of Chicago, is credited with discovering downbursts in the mid 1970s, following investigation of the crash of Eastern Airlines Flight 66 in New York. There are two types of downbursts: microbursts produce strong winds less than 2.5 miles in diameter, while macrobursts result in strong winds over an area 2.5 miles in diameter or larger. Downbursts occur with all types of thunderstorms, from single cells to supercells. In fact, the most damaging downbursts often are produced by weaker, benign-looking storms. Downburst-producing storms often give little advance indications of the imminent danger on weather radar or to the spotter, so warnings are difficult to issue. Once the strong winds reach the ground, Doppler Radar can frequently detect the stronger winds, but the threat of additional damaging winds may be over.

While downbursts are typically produced within a single thunderstorm cell, occasionally many storms will organize into a squall line and produce damaging winds over a much larger area for a period of an hour or longer. Damaging winds of this type are known as Bow Echoes, since 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 about 80% of all tornadoes! Since Bow Echoes produce distinctive radar echoes and last 1 to 3 hours (sometimes longer), National Weather Service meteorologists can often provide considerable advance warning.

All severe thunderstorms have the potential to produce damaging winds and large hail. Count on your local National Weather Service office for the latest information regarding these weather hazards.