Fremont County, Wyoming
A tornado churns over open country north of Riverton, Wyoming, on May 6, 2014. (Photo credit: Josh Allen)
Nature dished out yet another out-of-season surprise Tuesday as a tornado churned through a part of the country that doesn't often see twisters, especially this early in the year.
Kelly Allen, the fire weather program manager at the National Weather Service in Riverton, Wyoming, tweeted photographs of a tornado that passed over open country about 20 miles north of that central Wyoming city shortly after 2 p.m. Mountain time. The photos were taken by her husband, Josh Allen.
Radar Tuesday afternoon showed a small hook echo at the southern end of a thunderstorm north of Riverton and west of Shoshoni, Wyoming. This is the storm that produced the tornado.
Allen said the tornado, which came from a rotating supercell thunderstorm, lasted only about 2 minutes and hit an undeveloped area. "Nothing there but sagebrush and pronghorn," she tweeted.
The National Weather Service in Riverton issued a severe thunderstorm warning for the area around the time the tornado was spotted.
NWS meteorologist Trevor LaVoie said that in the absence of any damage to human-built objects, the tornado would likely be rated an EF0 on the Enhanced Fujita Scale, based on a radar signature indicating about 90 miles per hour of "gate to gate shear" – technical jargon describing the difference between winds blowing toward and away from the radar in very close proximity. That is the way Doppler radar visualizes tornadic winds.
Tornadoes in Wyoming
Tornadoes are relatively uncommon in Wyoming. The state has averaged about 10 tornadoes per year since 1950, according to The Weather Channel severe weather expert Dr. Greg Forbes. Given the state's large land area, this puts Wyoming 37th out of the 50 states in terms of annual tornadoes per square mile. Most of those tornadoes occur in the state's eastern sections, closer to the traditional Tornado Alley of the Plains states.
Fremont County, where Tuesday's tornado occurred, had only recorded six previous tornadoes since 1996. Five of the six caused no damage at all, and the other caused $10,000 damage and one minor injury from broken glass in the town of Dubois in 2008. At over 9,000 square miles, Fremont County is the size of Vermont and is the 14th-largest county in the U.S., geographically.
Tuesday's tornado was unusually early for Wyoming, where the peak tornado months are typically June and July. Wyoming had not recorded a tornado this early in the year since 2001, when a twister struck James Town in Sweetwater County on April 20.
The last tornado to hit Wyoming before this was on Sept. 15, 2013. All eight Wyoming tornadoes in 2013 were rated EF0, and only one caused any damage.
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MORE: Video of Riverton, Wyoming Tornado (courtesy Kelly Allen)