JACKSON, Wyo. — A relatively rare supercell thunderstorm moved across Teton County on Sunday afternoon. Wind and hail damage along with frequent lightning and heavy downpours were reported with this storm as it moved across Teton Pass before tracking just north of the Town of Jackson.
The storm initially formed over the Caribou Mountains southwest of Swan Valley in Idaho between 3:00 and 3:30 p.m., before intensifying as it approached the Wyoming border around 4:00 p.m., prompting the National Weather Service in Riverton to issue a Severe Thunderstorm Warning for Teton County.
The storm was fast-moving and took an east/northeast track at about 55 mph, moving across Teton Pass at 4:10 p.m. and Moose and Kelly around 4:30 p.m.
Large hail estimated to be at least one inch in diameter was reported over Teton Pass as the storm moved through, denting the hood of a vehicle per a National Weather Service report.
Wind damage was also reported in the Town of Jackson by a local emergency manager, who forwarded a social media post to the National Weather Service showing a large pine tree that had fallen over a road.
The storm formed in a favorable environment, as cloud cover from earlier in the day had given way to sunny mid-afternoon conditions across Southern Teton County and points south, allowing solar radiation to heat up the lower levels of the atmosphere.
Moisture levels were already much higher than usual across the area, with dewpoint temperatures of around 50 degrees. The low density warm, moist air near the surface was able to rise rapidly into much colder air aloft, helped by a wind convergence zone (winds blowing from opposing directions that force pockets of warm, moist air upward) that was in place south of Jackson.
Wind shear was also very strong on Sunday afternoon, meaning that winds increased rapidly with altitude. This helped to tilt the thunderstorm’s updraft, allowing it to strengthen and sustain itself for longer since it was displaced from the rain-cooled downdraft which can weaken a thunderstorm more quickly.
As the storm continued to strengthen, it began to take on the shape of a “hook” on radar, which indicated that the thunderstorm’s updraft was rotating and the storm was developing into what is known as a supercell.
The storm’s rotation was very evident visually, and a wall cloud even formed beneath the based of the storm, indicating the lower portion of a strong updraft. Rotation was evident but fairly slow, and directional wind shear (veering of wind directions with altitude) was nonexistent, thus there was no threat of a tornado with this storm despite the ominous look.
The ingredients needed for this storm to develop, most notably the significant wind shear and higher than average levels of moisture, were noted by the National Weather Service’s Storm Prediction Center on Sunday morning, which highlighted all of Teton County under a Marginal (level 1 out of 5) risk for severe weather. Thunderstorms are classified as “severe” when they produce hail of one inch or more in diameter and/or wind gusts of 58 mph or higher.
Supercell thunderstorms are rare in Teton County and areas west of the Continental Divide due to significant terrain influences on moisture and wind profiles compared to the Central and Eastern U.S. However, they are not unprecedented in Teton County either and can form when the conditions are just right.
Supercells are more common across Eastern Wyoming, where moisture originating from the Gulf of Mexico often arrives at this time of year and interacts with strong winds aloft associated with upper air disturbances approaching from the west. The State of Wyoming averages 11 tornadoes annually, and they occur most often in the eastern half of the state with June being the peak month.
Alan Smith, Meteorologist