Squall lines of severe thunderstorms can be dangerous even if they don't spawn tornadoes, and you should seek shelter even if tornado warnings aren't issued by the National Weather Service.
We're all familiar with the destruction tornadoes can cause, especially those of EF4 or EF5 intensity – capable of demolishing homes and other buildings – on the Enhanced Fujita Scale.
Severe thunderstorms alone are also dangerous and sometimes deadly, especially when they form into a squall line – a group of storms arranged in a line, often accompanied by high winds and heavy rain, which can be hundreds of miles long but are typically only 10 to 20 miles wide. A derecho is a type of squall line that meets certain criteria and can feature gusts over 100 mph in extreme cases – the equivalent of an EF1 tornado.
Here are four big reasons to take shelter when a squall line approaches your location.
1. Destructive Winds Can Still Strike
The most common type of severe weather in the United States is strong, often damaging, straight-line winds, which are not associated with tornadoes.
From 2002-11, an average of 16,254 reports of winds of at least 58 mph or wind damage were logged annually in the U.S., according to The Weather Channel severe weather expert Dr. Greg Forbes.
Winds this strong are capable of downing tree limbs and knocking out power at the very least, but they may also be strong enough to down trees onto homes, vehicles or anything in their way, as well as produce damage to homes and other buildings.
A recent example of damaging straight-line winds occurred when a squall line roared through the Dallas-Fort Worth Metroplex. Power poles were snapped in Fort Worth and several homes were damaged in Rockwall County, Texas, where the NWS estimated 90- to 95-mph straight-line winds caused substantial structural damage, including to the house pictured below.
If you have large trees surrounding your home, it is best to seek shelter in your basement or the lowest level when a severe thunderstorm warning is issued with the potential for damaging straight-line winds.
Should you see the word "derecho" in the forecast, or a severe thunderstorm warning which mentions "wind gusts over 70 to 75 mph," you should take shelter immediately as you would for a tornado warning.
In 2014 alone, 33 people were killed and another 240 injured in the U.S. by high winds from thunderstorms.
2. Destructive Hail Could Fall
Squall lines are often capable of producing severe hail, defined as 1 inch in diameter – roughly quarter-size – or larger. There were over 5,500 reports of severe hail in the U.S. in 2014.
According to a study of 1989-2004 hail reports by Jewell and Brimelow, 95 percent of hailstones are golf ball-size or smaller. Golf ball-size hail is roughly 1.75 inches in diameter.
It is important to keep in mind, however, 1-inch diameter hail is still capable of damaging roof shingles, while golf ball-size hail can put dents in vehicles.
More serious damage occurs if hail reaches baseball size, which could smash vehicle windshields, or softball size, capable of punching holes in roofs.
A recent example took place when softball-size hail, 4.25 inches in diameter, pelted a swath of the far north Dallas-Fort Worth Metroplex, from near Denton to McKinney. Hail from 1 to 3 inches in diameter was also reported across other parts of Texas and central Oklahoma that evening.
If thunderstorm winds are strong enough to drive the hail, a structure's exterior could be a total loss, with siding destroyed, numerous holes in the roof and windows smashed. The St. Louis metro area saw a pair of billion-dollar hailstorms in April 2012 and April 2001.
While extremely rare, deaths from large hail have occurred. For example, a pizza deliveryman in Fort Worth, Texas, was killed by baseball- to softball-size hail on March 28, 2000. Hail may have also been the main cause of a June 18, 2014 plane crash near Lehman, Texas, killing three. In addition, there were 23 people injured by large hail in the U.S. in 2014.
If an incoming squall line has a history of producing severe hail, move your vehicles into a garage, parking deck or some covered area, if possible. However, if you are driving, do not park under a highway overpass. This could cause a backup on the highway if many people attempt to do the same thing, placing many more people at risk of an incoming severe thunderstorm.
3. Tornadoes Can Form Along a Squall Line's Leading Edge
Sometimes, brief tornadoes form quickly along the leading edge of a squall line of severe thunderstorms with otherwise damaging straight-line winds.
These typically brief tornadoes may occur overnight or be rain-wrapped and difficult to see.
Along a squall line, thousands of cloud-to-ground lightning strikes per hour can occur. Even after the squall line passes, lightning can persist for a half-hour or longer behind the main line, though the highest winds will have already passed by this time.
With lightning nearby, avoid contact with electrical devices, corded phones and metal pipes while indoors. It is safe to go outdoors 30 minutes after hearing the last clap of thunder.
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