Risk Factors Associated with Weather
From a disaster standpoint, our main concerns are severe weather events. These would be tornadoes, flood and flash-flood inducing precipitation, severe thunderstorms, severe hail storms, and severe winter weather. We're sure that many valley residents have experienced at least one, if not most, of these events. Our risk of thunderstorms are higher in the summer, while severe winter weather occurs late and early in the year.
Dispelling Myths About Tornadoes
An event that may have a few people scratching their heads, however, are tornadoes. There is a myth that tornadoes can't occur in Teton County due to the mountains, elevation, and other factors. Let's dispel those falsehoods right now.
Tornadoes Can't Form at High Elevations - False
The strongest tornado in Wyoming history occurred in Teton County on July 21, 1987 (PDF) on the Teton Wilderness / Yellowstone National Park boundary near Enos Lake. It was estimated to be an F4 on the Fujita scale (where an F5 is the strongest possible), and was studied by Dr. Fujita himself. Unfortunately, this tornado was not witnessed by anyone since it occurred in the back country. Undaunted by this setback, Dr. Fujita used forensic meteorology to determine the strength, area, and direction of this massive tornado.
The trail that you see on the right blocked by trees is the Enos Lake trail. Thankfully no one was injured in this event, but the Forest Service and Park Service estimated damage to forest resources to be in the range of $2.5 million.
Another high altitude tornado event occurred in Sequoia National Park on July 7, 2004. The base of the tornado at ground level is estimated to be at least 12,000 feet, making this the highest elevation tornado ever observed in the US. Read more about this tornado.
Tornadoes Can't Cross Canyons/Rivers/Mountains - False
The Teton / Yellowstone tornado of 1987 ranged in elevation from 8500 feet to 10,000 feet as it went up and down mountains and even crossed the continental divide. An F2 tornado in Salt Lake City on August 11, 1999 descended one side of a canyon and came back up the other side halfway along its path. And the deadliest tornado in US history (Tri-state tornado of March 18, 1925, F5, 695 killed) crossed the Mississippi river without so much as slowing down.
We Just Don't Get Tornadoes Out Here - False
Although the Teton / Yellowstone tornado is the only confirmed tornado to hit Teton County, it was a massive one and only proves that they can happen here. On September 5th, 2007, tornadoes were seen outside of Soda Springs, Idaho. That same storm system produced what is known as a radar-indicated tornado 20 minutes southwest of Hoback Junction.
A radar-indicated tornado means that there is a signature on the radar that can be, but isn't always, a tornado. These radar-indicated tornadoes have to be confirmed by weather spotter reports to actually prove that they are a tornado in the field. There weren't any visual confirmations of this radar-indicated tornado, so it isn't classified as an official tornado.
The fact is we do get tornadoes out here, but due to our low population density, there usually isn't anyone around to see them. If you plot out the tornado reports across the state of Wyoming, interestingly enough a very high percentage of them are seen along Interstate 80 and Interstate 25. This isn't because tornadoes like the interstate, it's because that is where people are to call in the reports of tornadoes