Severe thunderstorms can happen any time of year but are most common in spring and early autumn. About 100,000 thunderstorms strike the U.S. each year, and about 10,000 of them become severe. These are five things every trucker should know about severe thunderstorms.
1. Define me
All thunderstorms require moisture, rising unstable air that keeps rising when given a nudge, and a lifting mechanism, such as a frontal boundary, to provide the nudge. If enough instability builds up, a storm can turn severe. The National Weather Service (NWS) classifies a thunderstorm as severe if it produces one or more of the following:
• Hail 1 inch or greater in diameter.
• Wind gusts exceeding 57.5 mph (50 knots).
• A tornado.
NWS meteorologists identify severe thunderstorms based on radar analysis and/or eyewitness reports.
2. Watch vs. warning
The NWS issues a severe thunderstorm watch when weather conditions will be favorable for such storms. A watch can cover parts of a state or several states and is usually posted several hours in advance. It’s a “tap on the shoulder” telling people to watch the skies and to make sure they have a way to receive alerts — NOAA Weather Radio, cell phone apps, etc. — in case warnings are issued.
The NWS issues a severe thunderstorm warning when severe hail and/or winds have been indicated on radar or reported by eyewitnesses. A warning means there is an imminent threat to life and property in the path of the storm. A warning can cover portions of one or several counties. If a warning is issued, find a safe shelter as quickly as possible.
The same watch and warning conventions are used for tornadoes.
3. Tornado tidbits
A tornado is a violently rotating column of air extending from the base of a thunderstorm down to the ground. Tornadoes are capable of destroying sturdy structures, uprooting trees and hurling heavy objects (including semis) through the air. Tornadoes can occur at any time of day or night and at any time of the year. Although tornadoes are most common in the Central Plains and the Southeast, they have been reported in all 50 states.
Truckers should never try to outrun tornadoes when on the road. Tornadoes can move quickly with forward speeds up to 60 mph, and they sometimes change direction. If drivers are heading through areas where the NWS has issued tornado warnings, they should get off the road as quickly as possible and find shelter. If that’s not possible, pull over, then lie face down in a ditch or other low-lying area off the road. Do not seek shelter under an overpass. Additional tornado safety tips can be found here.
4. Who’ll stop the rain?
Although heavy rainfall isn’t an official characteristic of severe thunderstorms, it can sometimes stop truckers in their tracks. Flash flooding is often a dangerous byproduct of severe and nonsevere thunderstorms. Flooding kills more people each year than hurricanes, tornadoes or lightning, and the NWS issues an average of more than 4,000 flash flood warnings across the country annually.
Truckers, as well as all other drivers, should never try to drive through floodwaters. The depth of water across a road can be easily underestimated, and the road could wash out, leaving a driver in a huge sinkhole.
5. Hail no!
Hail size is often compared to familiar objects, like a marble, a quarter (1 inch), or a golf ball. Large hail can do major damage, and even small hail can injure people. The biggest hailstone on record in the U.S. measured 8 inches in diameter — twice the size of a softball — and weighed almost 2 pounds. It fell near Vivian, South Dakota, on July 23, 2010.
Hailstorms cause an average of $15 billion in damage to homes, cars and crops in the U.S. each year. This total has increased in recent decades. The estimate for the 1990s was $1.2 billion per year, and that itself was an increase over prior decades.