While tornado season is generally considered to be in April and May, fall is the second most common time of year for tornadoes. With 30 years of forecasting experience, Senior Meteorologist Dave Gorham shares insight into tornadoes, how to forecast them, and how to stay safe.
Tornadic supercell in the American Plains
Severe weather, including tornadoes, is often caused by cold air colliding with warm air, which can happen any time of year. It’s often thought that hot air rises, but actually, hot air only rises when it’s hotter than the air surrounding it. As air rises into a cool atmosphere, it condenses to create clouds—small cumulus clouds on a summer afternoon, or monstrous cumulonimbus clouds along a cold front. The greater the contrast between temperatures, the more rapidly the air rises. When the air rushes upwards fast, it can be strong enough to rip the wings off an airplane. When updrafts are at their peak, cumulonimbus clouds can exceed 55,000 feet high (more than 10 miles)!
As air rises inside the cumulonimbus supercell cloud, it turns counterclockwise. This rotation combined with winds and updrafts/downdrafts creates an environment conducive to tornado formation.
A tornado on the ground is one of nature’s most destructive forces. Violently rotating winds can reach up to 300 mph (480 kph). Supercells are massive and can create tornadoes as wide as two miles (three km) and if the exact conditions persist long enough, a tornado can travel dozens of miles—creating an extensive path of destruction. Most tornadoes are much smaller, with winds of around 110 mph (180 kph), a width of 250 ft (80 m) and travel only a mile or two (2-3 km). These smaller or weaker tornadoes can still be very destructive.
Tornadoes can last from several seconds to more than an hour, though they typically last for about ten minutes. The average distance tornadoes travel is about 3.5 miles (5.6 km).
Forecasting a specific tornado is difficult even an hour before it’s visible on weather radar. Where exactly a tornado forms, when it will touch down and for how long it will remain on the ground are some of the most difficult considerations a weather forecaster will face. This is because the dynamics within a supercell cumulonimbus are complicated and not entirely understood by meteorologists. Conditions that are ideal for tornado formation do not always produce a tornado. However, the atmospheric conditions that allow cumulonimbus supercell development are well understood and allow weather forecasters to identify suspect regions days or even a week in advance of an actual tornado reaching the ground.
That being said, research in this area continues. Some have found that the location and frequency of lightning in the cloud can be used to predict tornado development. This technique is especially useful in remote areas beyond the range of weather radar.
Download this guide on how to use twitter as a lifesaving tool in a storm or emergency
Being weather aware and tuned into local media and weather reports is the best way to prepare for a tornado. If you’re aware of the potential for tornadoes, you increase the margin of safety for you, your family and your employees. In the United States, tornadoes have occurred in every state and month of the year. Though most common during the hottest time of day (late afternoon), tornadoes can form at any time of day or night. Your awareness toolkit should include social media, including Facebook and Twitter—where your network and local authorities are posting critical storm information. Be sure your smart phone has weather apps that include radar and information from the National Weather Service.
Seeking proper shelter is the most important tip. However, in order to do this, you must identify what the nearest proper shelter is.
For added protection, get under something sturdy such as a heavy table or workbench. Cover your body with a blanket, sleeping bag or mattress. Protect your head with anything available.
Myths related to tornadoes and tornado safety originated long ago – well before we understood the meteorology and science of tornado formation and movement. Here are some that continue to be believed:
Cars and homes were heavily damaged by a tornado that swept through Maryland Heights in the suburbs of St. Louis on Good Friday, April 22, 2011
Waterspouts
A waterspout is a tornado over water and are common along the southeast US coast. They can happen over seas, bays and lakes worldwide. Although waterspouts are always tornadoes by definition; they don't officially count in tornado records unless they hit land. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn boats, damage larger ships, do significant damage when hitting land, and kill people. The National Weather Service will often issue special marine warnings when waterspouts are likely or have been sighted over coastal waters, or tornado warnings when waterspouts can move onshore.
Tornado warning: A tornado warning is issued when a tornado has been sighted or has been indicated on radar in your area. When this happens, anyone in the area must immediately take shelter in a safe, sturdy structure. Ideally, this shelter area has a battery-operated radio so that you can stay informed of the up-to-the-minute conditions from the media and your local weather office.
Tornado Watch: A tornado watch is issued to alert people to the possibility of a tornado forming in their area. There has been no tornado sighted, but conditions are favorable for tornado development. Stay alert, listen to local news and weather reports, and make sure your family and co-workers are aware of the threat and ready to quickly seek shelter.
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