JACKSON, Wyo. – On July 21, 1987, one of the most extraordinary weather events to ever occur in North America happened in Teton County when a violent high-elevation tornado tore through the Teton Wilderness east of Jackson Lake.
This tornado was rated an F-4 (out of 5) on the Fujita scale with maximum winds estimated between 207 and 260 mph. Tornadoes that are rated as F-4 or F-5 (or EF-4 to EF-5 on the newer Enhanced Fujita scale) are considered to be violent tornadoes.
Dr. Ted Fujita himself, who was well known in the weather community as “Mr. Tornado,” studied this tornado extensively. He led three damage surveys after the storm, which helped to confirm and classify the tornado.
Fortunately, the tornado occurred in such a rural area, even by Wyoming standards, that no injuries or fatalities occurred. At the same time, nobody witnessed the tornado either, but a group of nine campers in the area heard a “roar like a train” and encountered golf ball size hail as the storm passed nearby.
The tornado was both massive and powerful, uprooting an estimated one million trees in the Teton Wilderness Area. The tornado formed along Lava Creek between Gravel Mountain and Mt. Randolph and stayed on the ground for 24.3 miles as it tracked northeast, leaving a damage swath up to 1.5 miles wide at times.
The tornado passed near Enos Lake (where the nine campers heard the roar) and crossed the Continental Divide at over 10,000 feet. The tornado eventually crossed the Yellowstone National Park border into the Thorofare Region, before dissipating shortly after crossing the Yellowstone River.
In the September 1989 Monthly Weather Review, Dr. Fujita stated that the severity of the tree damage surveyed in this tornado exceeded that of more well-known violent tornadoes he had investigated in the 1970s in Birmingham, Ala. and Xenia, Ohio. Fujita noted that trees were stripped of leaves and branches, and that tree trunks were debarked.
According to Dr. Fujita, “The only F4 damage in a forest comparable to this one was photographed from a Cessna by the author in the Appalachians after the F4 Murphy Tornado in North Carolina on 3 April 1974.”
Dr. Fujita planned to conduct additional damage studies on this tornado, but ironically enough, most of the downed trees in the damage path were consumed in the 1988 Yellowstone fires just one year later.
The thunderstorm that produced the tornado actually passed over Jackson Hole before producing the tornado to the north of US-26 and east of US-191. The Jackson Hole Airport recorded a straight-line wind gust of 58 mph and storm knocked over 10-15 trees north of Teton Village.
It’s extremely fortunate that the tornado did not form until the storm had already moved through the more populated and visited areas of the Jackson Hole Valley.
The Teton-Yellowstone Tornado, as it’s known in the meteorology community, still holds several records as far as tornadoes go, including the following:
1) Strongest tornado to ever occur west of the Continental Divide, and the only violent (F4/EF4 or higher) tornado to occur west of the Divide.
2) Strongest tornado to ever occur in the State of Wyoming, and the only violent (F4/EF4 or higher) tornado to ever occur in Wyoming. This is impressive because tornadoes are actually fairly common across Eastern Wyoming.
3) Highest elevation violent tornado (F4/EF4 or higher) to ever occur in the United States. The elevation range of this tornado was between 8,500 and 10,070 feet.
4) The only “officially” confirmed tornado to have ever touched down in Teton County. However, it is likely that other tornadoes have touched down in Teton County before, but have been undocumented 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 happen under the right conditions. However, for an F4 tornado to occur at such a high elevation west of the Continental Divide is extraordinary.
Cooler temperatures with increasing altitude, typically-drier air in the Western U.S. and terrain influences on wind shear are a few of the factors that work against tornadoes in this part of the country.
However, on occasion the ingredients do come together for tornadoes to occur in the mountainous regions of the Western U.S. While mountain tornadoes are more likely in the East compared to the West, recent research has actually found that when conditions are otherwise favorable for tornadoes, mountainous terrain can actually enhance the strength of a rotating updraft associated with a tornado – contrary to popular belief.
The large-scale weather pattern leading up to this event featured a large trough of low pressure centered over Northern California. The trough was positioned in a way that contributed to 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, winds near the surface were out of the southeast, winds at the mid-levels were out of the south/southwest, and winds at the upper levels of the atmosphere were out of the southwest. This “veering” of winds with altitude created an environment conducive for storms to rotate.
A strong jet stream also moved into Western Wyoming during this set up, which helped to enhance the rate of vertical motion in the atmosphere, which in turn, supported a stronger updraft in the tornado-producing thunderstorm.
In addition, the position of the trough allowed for a significant influx of monsoonal moisture to arrive from the south and work its way into Western Wyoming. Moisture levels typically need to be much higher than average for severe thunderstorms and tornadoes to form in our region, and in this case, cloud bases were unusually low as well.
At the surface, winds associated with a small-scale area of high pressure along with a prior cold front that had moved through likely interacted to aid in thunderstorm development and rotation as well.
Could A Tornado Like This Happen Again?
It’s possible, but unlikely, that a tornado of this magnitude would occur in Teton County again in our lifetime. This event took a rare set of meteorological ingredients and remains the only violent tornado to have ever been documented west of the Continental Divide.
Also, there is no evidence that tornadoes across the Western U.S. will become stronger or more frequent in a warming climate.
Teton County remains one of the lesser tornado-prone regions in the country. That being said, it is somewhat surprising that additional weaker tornadoes have not occurred or at least been documented in or near Teton County since this event occurred in 1987.
Tornadoes can occur anywhere in the United States, and it’s likely that additional tornadoes will touch down again in Teton County in the future, even if infrequently.
