JACKSON, Wyo. – On July 21, 1987, one of the most extraordinary weather events ever to occur in North America occurred in Teton County when a violent high-altitude tornado tore through the Teton Wilderness east of Jackson Lake .
This tornado was rated F-4 (out of 5) on the Fujita scale with estimated maximum winds between 207 and 260 mph. Tornadoes rated F-4 or F-5 (or EF-4 to EF-5 on the New Improved Fujita Scale) are considered violent tornadoes.
Dr. Ted Fujita himself, who was well known in the weather community as “Mr. Tornado”, studied this tornado extensively. He conducted three post-storm damage surveys, which helped confirm and classify the tornado.
Fortunately, the tornado happened in an area so rural, even by Wyoming standards, that no injuries or deaths occurred. At the same time, no one witnessed the tornado either, but a group of nine campers in the area heard a “roar like a train” and encountered golf ball sized hail as the storm was passing nearby.
The tornado was both massive and powerful, uprooting approximately one million trees in the Teton Wilderness. The tornado formed along Lava Creek between Gravel Mountain and Mt. Randolph and remained on the ground for 24.3 miles as it tracked northeast, leaving a swath of damage up to 1. 5 miles wide.
The tornado passed near Enos Lake (where the nine campers heard the roar) and crossed the Continental Divide at more than 10,000 feet. The tornado eventually crossed the Yellowstone National Park border into the Thorofare area, before dissipating shortly after crossing the Yellowstone River.
In the September 1989 Monthly Weather Review, Dr. Fujita said the severity of tree damage studied in this tornado exceeded that of more well-known violent tornadoes he investigated in the 1970s in Birmingham, Ala. and Xenia, Ohio. Fujita noted that trees were stripped of their leaves and branches, and tree trunks were debarked.
According to Dr. Fujita, “The only F4 damage in a forest comparable to this was photographed from a Cessna by the author in Appalachia after the F4 Murphy tornado in North Carolina on April 3, 1974.”
Dr. Fujita planned to conduct additional damage studies from this tornado, but ironically enough, most of the downed trees in the damage path were consumed in the 1988 Yellowstone fires a year later.
The thunderstorm that produced the tornado actually passed over Jackson Hole before producing the tornado north of US-26 and east of US-191. Jackson Hole Airport recorded a straight-line wind gust of 58 mph and the storm toppled 10 to 15 trees north of Teton Village.
It is extremely fortunate that the tornado did not form until the storm had already passed through the most populated and visited areas of the Jackson Hole valley.
The Teton-Yellowstone tornado, as it is known in the weather community, still holds several records for tornadoes, including the following:
1) The strongest tornado ever west of the Continental Divide and the only violent one (F4/EF4 or higher) to occur west of the Continental Divide.
2) The strongest tornado ever to occur in the state of Wyoming and the only violent tornado (F4/EF4 or higher) ever to occur in Wyoming. This is impressive because tornadoes are actually quite common in eastern Wyoming.
3) The highest violent tornado (F4/EF4 or higher) ever in the United States. The altitude range of this tornado was between 8,500 and 10,070 feet.
4) The only “officially” confirmed tornado to ever touch down in Teton County. However, it is likely that other tornadoes have touched down in Teton County before but have not been documented due to the amount of remote terrain and the fact that weather radar coverage (which can pick up rotation in thunderstorms) is poor in Teton County.
How did this event happen?
Tornadoes are relatively rare in the mountains west of the Continental Divide in North America, but they can and do occur under the right conditions. However, for an F4 tornado to occur at such a high altitude west of the Continental Divide is extraordinary.
Cooler temperatures with increasing elevation, generally drier air in the western United States, and terrain influences on wind shear are some of the factors that act against tornadoes in this part of the country.
However, on occasion, the ingredients come together for tornadoes to occur in the mountainous regions of the western United States. While mountain tornadoes are more likely in the east than the west, recent research has actually found that when conditions are otherwise favorable for tornadoes, mountains and terrain can actually increase the strength of tornadoes. a rotating updraft associated with a tornado – contrary to popular belief.
The large-scale weather pattern that preceded this event featured a large trough of low pressure centered over northern California. The trough was positioned to contribute greater amounts of instability and wind shear, both of which are necessary ingredients for tornadoes.
Wind shear is a term used to describe the change in wind speed and direction as altitude increases. In this case, near-surface winds were from the southeast, mid-level winds were from the south/southwest, and upper-level winds were from the southwest. This “deviation” of the winds with altitude created an environment conducive to the rotation of storms.
A strong jet stream also moved across western Wyoming during this pattern, which helped improve the rate of vertical motion in the atmosphere, which in turn supported a stronger updraft in the tornado-producing storm.
Additionally, the position of the trough allowed a large influx of monsoon moisture to arrive from the south and work its way into western Wyoming. Humidity levels usually need to be much higher than average for severe thunderstorms and tornadoes to form in our area, and in this case the cloud bases were also unusually low.
At the surface, winds associated with a small-scale high pressure area as well as an earlier cold front that had moved likely interacted to aid in the development and rotation of the thunderstorms as well.
Could a tornado like this happen again?
It is possible, but unlikely, that a tornado of this magnitude will occur again in Teton County in our lifetime. This event took a rare set of weather ingredients and remains the only violent tornado ever documented west of the Continental Divide.
Additionally, there is no evidence that tornadoes in the western United States will become stronger or more frequent in a warming climate.
Teton County remains one of the least tornado-prone areas in the country. That being said, it is somewhat surprising that other weaker tornadoes have not occurred or at least not been documented in or near Teton County since this event occurred in 1987.
Tornadoes can occur anywhere in the United States, and it is likely that more tornadoes will touch down in Teton County again in the future, even if they are infrequent.