WASHINGTON, May 21, 2013 — The severe thunderstorms and tornadoes that recently hit Oklahoma were among the most devastating and destructive on record. Here is actual footage from this historic tornadic outbreak (on May 19, 2013):
Definition: A tornado is a violently rotating column of air extending from the base of a cumulonimbus cloud down to the ground surface below. If the rotating column of air does not reach the earth’s surface, it is called a funnel cloud. If it extends down to contact a body of water, it is called a waterspout. Tornadoes only form under cumulonimbus clouds, the same clouds that spawn thunderstorms.
Locations: The United States of America, which experiences an average of over 1000 tornadoes annually, is by far the region of maximum tornado activity on planet earth. Australia is second with an average of 200 to 300 each year. Every state in the U.S.(including Hawaii and Alaska) has recorded a tornado occurrence. The epicenter for severe thunderstorms and tornadoes is the Texas-Oklahoma panhandle. Oklahoma and Kansas have more tornadoes per square mile than any other region of the U.S., though tornadoes are relatively frequent across “tornado alley,” the area between the Rocky Mountains and the Appalachians.
Florida, which has more thunderstorms than any other state, has frequent tornadoes, but Florida Tornadoes rarely achieve the destructive power of storms in Texas, Oklahoma, Kansas, Missouri, and the other states in tornado alley. Uganda has even more thunderstorms, but rarely any tornadoes.
Why are America’s central states such fertile tornado breeding ground? That’s where cold, dry arctic air flowing down from Canada meets warm, moist air flowing northward from the Gulf of Mexico. The clash of cold, dry air and warm, moist air produces violent weather, and without it, you don’t see the most powerful storms, like the F4 storm that hit Oklahoma. Tornado alley is also relatively flat. Mountains and high structures inhibit tornado formation, which is why you don’t see tornadoes appear in Durango or downtown New York, even during severe thunderstorms.
Seasonal Occurrence: Tornadoes occur most frequently in the U.S. from April through early June. Wintertime tornadoes (Dec – Feb) account for only 10 percent of the occurrences, but (on average) are responsible for over 50 percent of the deaths. This is because they are so rare that when they strike, they usually come without sufficient early warning. Springtime is when waves of cold and warm air are most often going to collide to produce thunderstorms and tornadoes.
Associated Weather: Tornado winds can exceed 200 miles per hour (they’ve been measured at over 300 mph). They generate missile hazards (damage and injury caused by flying debris such as glass, metal, wood, etc.). In addition to the winds on the ground, tornadoes are accompanied by the other effects of severe thunderstorms: wind shear, lightning, hail, torrential rains and flash flooding. These other effects can compound the misery, extend the damage, and complicate rescue efforts.
The energy associated with the upward vertical motion of the winds in a severe thunderstorm is so powerful that the vertical wind velocity exceeds the climb rate of the fastest military aircraft. The total energy released by a large storm is spectacular, on the order of a good-sized nuclear explosion, but it’s released over minutes or hours and across a large area, not in a blinding instant from a single point, so the power and destruction are much, much less.
Dynamics that Produce Tornadoes: The bottom line for tornado formation is cold dry air + warm moist air. But it is just a little more complicated than that. The western part of large high pressure systems, usually situated in the eastern U.S. ― western Atlantic/Caribbean, spin in a clockwise manner. This motion brings large amounts of warm, moist air across the south-central U.S.
The lighter warm, moist air converges with and is lifted by colder, denser air to the west that is pushing eastward. In the illustration above, note that there is a dry line extending almost due south from northeast New Mexico. The temperature difference across the dry line is almost indiscernible, but the moisture (dew point) contrast is large. When advancing dry lines move eastward, they act just like a cold front, and provide mechanical lifting that can spawn severe thunderstorms.
Severe thunderstorms and tornadic activity don’t just happen. They are produced when a variety of meteorological factors come together. And while Doppler radar, computer forecast models and satellite pictures are very helpful in short range forecasts, effective early warning usually finds its roots in good, old fashioned (old school) know-how and homework that applies environmental knowledge, skill and experience to produce a forecast that is worthy to be shared with the customer.
There are other considerations too numerous to discuss here, but the bottom line is that tornado prediction is as much art as science. Professional meteorologists have been chasing storms across tornado alley for decades trying to make predictions more accurate, but if you aren’t paying attention, you’re unlikely to get much warning of a major storm. Your best protection is to pay attention to the weather, listen to emergency weather channels when it starts to look ominous, and to know where to hide when a storm comes your way. Tornadoes can be terrifying, destructive and spectacular, but if you keep your wits about you, they’re also survivable.
Bill Randall is a former military earth science meteorology/oceanography instructor and forecaster. He has served at joint military commands, and a variety of sea/shore environmental support commands/units. He has also given weather-related presentations to local communities since retiring from the U.S. Navy in Jan 2002.
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