Why You Shouldn't Ignore Severe Thunderstorm Warnings

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Do severe thunderstorm warnings grab your attention as much as tornado warnings? What actions do you take when you hear a severe thunderstorm warning has been issued for your area?

We all know about the destructive power of tornadoes, particularly those violent tornadoes of EF4 and EF5 intensity on the Enhanced Fujita scale, capable of demolishing homes, leaving only a pile of rubble or a foundation swept clean.

However, severe thunderstorms are also dangerous and occasionally deadly.

Here are reasons why you should never disregard a severe thunderstorm warning issued by the National Weather Service.

Destructive Winds

By far the most common form of severe weather in the United States is from strong and/or damaging straight-line winds, those 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 in the U.S., according to Dr. Greg Forbes, former severe weather expert at The Weather Channel.

These winds may be strong enough to down tree limbs or knock out power at a minimum, or may be strong enough to down entire trees onto homes, parked cars or whatever is in their way, and produce structural damage to homes and buildings.

If you have large trees around your home, you may want to seek shelter on the lowest floor, preferably in the basement, if a severe thunderstorm warning is issued with the potential for damaging straight-line winds.

Sometimes in late spring or summer, thunderstorms may form a long-lived, fast-moving complex of high winds known as a derecho. These are capable of widespread tree damage and power outages, as well as some structural damage from wind gusts that can top 100 mph in the extreme, the equivalent of an EF1 tornado.

If you hear the word "derecho" mentioned in any forecast, or see "wind gusts over 70 to 75 mph" noted in a severe thunderstorm warning, take shelter immediately as you would for a tornado warning.

In 2014 alone, 33 people were killed and another 240 injured by high winds from thunderstorms in the U.S.

Before thunderstorms threaten, keep healthy trees trimmed and remove any trees, particularly dead ones, leaning toward your house.

Destructive Hail

There were over 5,500 reports of severe hail – at least 1 inch diameter – in the U.S. in 2014.

A study of hail reports from 1989 to 2004 by Jewell and Brimelow showed that 95% of hailstones are golf-ball-sized or smaller.

However, 1-inch diameter hail, roughly the diameter of a quarter, can damage roof shingles. Golf-ball-sized hail, roughly 1.75 inches in diameter, can put dents in your vehicle.

Damage quickly becomes more serious with more rare baseball-sized (smashes vehicle windshields) and softball-sized (punches holes in roofs) hail. 

Add thunderstorm winds driving the hail, and your home's exterior could be a total loss, with its siding destroyed, numerous holes in your roof, even windows smashed. The St. Louis metro area had a pair of billion-dollar hailstorms in April 2012 and April 2001.

(MORE: Large Hail Is an Underrated Danger)

While exceedingly rare, deaths from large hail have occurred. Most recently, a pizza deliveryman in Fort Worth, Texas, was killed by baseball- to softball-sized hail on March 28, 2000. Twenty-three people were injured by large hail in the U.S. in 2014.

So if a severe thunderstorm warning for your area mentions the threat of large hail, move your vehicle(s) into a garage, parking deck or some covered area if you can.

Do not, however, park under a highway overpass. If many try to do the same thing, the highway may become blocked. Furthermore, highway overpasses are a terrible option for taking shelter in a tornado if caught outdoors.

(MORE: Don’t Do This When Severe Weather Strikes and You Are Driving)

Tornadoes Still Can Form

Occasionally, you'll see the following wording in National Weather Service severe thunderstorm warnings:

"Severe thunderstorms can and sometimes do produce tornadoes with little or no advance warning."

That sounds unnerving, doesn't it? Let's explain.

This wording may be in severe thunderstorm warnings when a tornado watch is already in effect, yet the severe thunderstorm does not yet show sufficiently strong rotation on Doppler radar or via storm spotter reports.

Therefore, the NWS meteorologist may first issue a severe thunderstorm warning for a developing supercell based on the threat of large hail and perhaps damaging straight-line winds.

However, in the span of minutes, the intensifying thunderstorm may quickly develop low-level rotation that, under the right conditions, could lead to a tornado.

Sometimes, brief tornadoes form quickly on the leading edge of a long line of severe thunderstorms with otherwise damaging straight-line winds. 

These typically brief tornadoes may occur during the night, or may be wrapped in rain and hard to see.

In general, if an NWS severe thunderstorm warning mentions the wording above, you would be wise to seek shelter as if it was a tornado warning, based on this potential for a tornado to quickly develop. 

Frequent Lightning

Don't forget the threat from lightning in a severe thunderstorm.

While any ordinary midsummer thunderstorm can produce lightning, severe thunderstorms can produce more frequent cloud-to-ground lightning.

This is particularly the case with long squall lines of severe thunderstorms capable of damaging straight-line winds, as well as classic late-spring, summer and fall clusters of severe thunderstorms in the South, Midwest and East.

In these cases, thousands of cloud-to-ground lightning strikes can occur in the thunderstorm cluster over one hour's time or less. Lightning strikes can persist for a half-hour or slightly longer after the main line of thunderstorms with the highest wind gusts passes.

Avoid contact with electrical devices, corded phones, metal pipes, etc. while taking shelter indoors during a severe thunderstorm. Stay inside for at least 30 minutes after hearing the last clap of thunder.

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