Flash floods and floods
■ A flash flood occurs within a few hours (usually less than six hours) of heavy or excessive rainfall, a dam or levee failure or the sudden release of water impounded by an ice jam.
■ A flood is the inundation of a normally dry area caused by abnormal high water flow. Floods develop more slowly than flash floods, normally greater than six hours.
■ Flash floods and floods on average are the No. 1 cause of deaths associated with thunderstorms, more than 90 fatalities each year.
■ More than half of all flood-related drownings occur when a vehicle is driven into hazardous floodwater. Turn around; don't drown!
■ Many flash flood fatalities occur at night.
■ Six inches of fast-moving water can knock you off your feet.
■ Two feet of rushing water can carry away most vehicles, including SUVs and pickups.
■ Avoid being in or near high places and open fields, isolated trees, unprotected gazebos, rain or picnic shelters, baseball dugouts, communication towers, flagpoles, utility poles, bleachers (metal or wood), metal fences, convertibles, golf carts and water (ocean, lakes, swimming pools, rivers, etc.).
■ Do not be the tallest object in the area. Lightning tends to strike the tallest objects.
■ Stay away from metal conductors such as wires or fences. Metal does not attract lightning, but lightning can travel long distances through it.
■ When inside a building avoid use of a corded telephone or computer, taking a shower, washing your hands, doing dishes or any contact with conductive surfaces with exposure to the outside such as metal door or window frames, electrical wiring, telephone wiring, cable TV wiring, plumbing, etc.
■ How far away is the lightning? Count the number of seconds between a flash of lightning and the sound of the resulting thunder. Divide this number by 5 to get an estimate of the distance in miles to the lightning strike.
■ Remember, if you are outdoors and can hear thunder, you are in danger of being struck by lightning. Wait 30 minutes after you see the last flash of lightning before leaving your shelter. More than one half of lightning deaths occur after a thunderstorm has passed.
Occasionally storms will organize into a squall line and produce damaging winds over many miles 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. In extreme cases, straight-line winds in a bow echo can approach 150 mph, stronger than about 80 percent of all tornadoes!
■ A derecho is a widespread, long-lived wind storm similar to the bow echo, only there are certain criteria that must be met for it to be labeled a derecho. A derecho can produce destruction similar to that of tornadoes. The damage typically is directed in one direction along a relatively straight swath ... hence the term “straight-line wind damage.” By definition, if the wind damage swath extends more than 240 miles and includes wind gusts of at least 58 mph or greater along most of its length, it’s a derecho.
■ Due to the threat of damage and injury, it is important when a Severe Thunderstorm Warning for straight-line winds is issued, that you seek cover in a fashion similar to what you would do during a tornado warning: Go to the basement or underground storm shelter or lowest floor of the building you’re in to a central room (like a closet) away from exterior walls.
■ If you are caught in a vehicle and cannot safely make it to a sturdy building, stay in your vehicle with your seat belt on and slouch down in your seat as far as possible, protecting your head from flying glass and debris.
■ Mobile homes, in particular, may be overturned or destroyed, while barns and similar buildings can collapse.
■ Downed electrical lines are also another hazard for anyone caught outdoors during the storm, or anyone who ventures out after strong winds have moved through.
■ Campers or hikers in forested areas are vulnerable to being injured or killed by falling trees.
■ People in boats risk injury or drowning from storm winds and high waves that can overturn boats.
■ Another type of a straight-line wind is a downburst. It is a small area of rapidly descending air beneath a collapsing thunderstorm. A downburst can cause damage equivalent to a strong tornado and can be extremely hazardous to aviation. Unfortunately, there is usually not enough time to detect the collapse of a thunderstorm to issue a warning.