The violent winds resulted from a “downslope wind event,” when cold, dense air from the northeast spilled overtop the Wasatch Mountains and sloshed downhill. That meant the air dried out, causing it to accelerate downward even faster and reach destructive speeds by the time it impacted communities at lower elevations.
The damaging episode rivals a severe windstorm that raked Utah in 2011, which incurred a $75 million price tag in Davis County, north of Salt Lake City. Winds gusted up to 146 mph during that event in Fruit Heights, just 15 miles or so north of Utah’s capital.
Wind gusts on Tuesday were a bit tamer, but not by much. A gust of 99 mph was measured at the intersection of US-89 and Park Lane in Farmington, near Fruit Heights, at an elevation of 4,288 feet. Just to the north, Hill Air Force Base gusted to 84 mph, while 75 mph wind gusts swept across Interstate 15 in Box Elder County.
Salt Lake City International Airport clocked gusts of 77 mph.
The Salt Lake Tribune reported that at least one person was killed by injuries resulting from the strong winds.
The strong winds prompted travel restrictions in Davis, Weber and Box Elder counties, where many large vehicles were overturned in the gusts.
“Just because it’s not as windy where you are right now doesn’t mean you won’t experience hurricane gusts down the road,” the Utah Highway Patrol tweeted Tuesday. “High profile vehicles PLEASE stay off the interstates and highways in Northern Utah!”
Toppled trucks at one point even forced “numerous closures” on Interstate 15, while tree and power line debris impeded traffic.
Winds to near 90 mph even occurred at the University of Utah, forcing officials to cancel classes. Weber State and Westminster College suspended classes through noon Tuesday.
The Salt Lake City School District canceled classes, reporting power outages at more than half of its schools.
“Due to high winds and dangerous conditions around the building, we are closing the Capitol building,” Gov. Gary Herbert (R) tweeted Tuesday.
Due to the power outages in Salt Lake City and dip in temperatures into the 40s one day after it was in the 90s, the city opened four warming centers for people without heat Tuesday night.
Downslope wind events in Utah are different from the windstorms that residents elsewhere may be used to. Instead of a constant sustained wind with higher gusts, waves of strong wind surge down the mountains every couple of minutes, interspersed by periods of calm. They are also comparatively localized, making it easier for drivers to underestimate how strong the winds will be up ahead.
“If you’re in Centerville or Farmington, what ends up happening is that you have this roaring wind that comes down from the Wasatch,” explained Christine Cruse, a meteorologist at the National Weather Service in Salt Lake City. “The winds were actually breaking at my house. You’ll hear this roar coming, and then it hits where your location is, and it will decrease for a bit, and then the next one will come it.”
Tuesday’s event featured strong gusts about a minute or two apart, with the roar preceding severe gusts by a couple of seconds.
Cruse said her office had begun ramping up their forecasts on Sunday, when it became apparent that conditions were ripe for a downslope wind event.
“We started talking about wind gusts in excess of 75 mph in the core area that gets the strongest winds,” she explained. “When we started looking at it on Monday, we increased that to 85 mph-plus based on the high-resolution guidance. We knew it could rival and or exceed 2011 in terms of wind speeds.”
While the maximum wind gusts observed fell acutely short of what occurred in 2011, the winds were more widespread.
“The wind event in 2011 did have a higher peak speed; in Centerville they recorded 102 mph, but the areal extent was more limited than what we saw” Tuesday, Cruse said. “Areal extent [this time] was much larger.”
The Salt Lake City office even spotted “rotor clouds” from the location; these form as a result of the same density perturbations that give rise to the strong winds. The circulation that gives rise to those clouds is also what helps propel pockets of air downhill.
The wind was driven by a large difference in pressure between an unusually intense zone of high pressure dropping into the Rockies out of Canada and a broad area of low pressure along the West Coast. This pressure difference drove strong winds not only through Utah but also across the Pacific Northwest and California, fanning the devastating wildfire outbreak.
On the eastern side of the intense high-pressure cell, winds coming in from the north triggered a dramatic temperature drop from Utah through the eastern Rockies and Front Range, setting the stage for a rare late summer snowstorm.
“We had the temperature fall precipitously,” Cruse said. “Our high [Tuesday] was 55, and that was a midnight high... most of the day was in the 40s. The day before was 93. We had some flurries reported in the benches, too."
The benches are, in essence, raised plateaus in the foothills of the Wasatch Mountains.
Flurries were reported in the Salt Lake City metro area, while higher elevations saw considerably more. Lyman, Wyo., reported 13 inches of snow, while Rapid City, S.D., saw 0.6 inches — two days after highs maxed out above 100 degrees.
“This 2-day gap between 100°F and measurable snow breaks the U.S. record for any station,” tweeted Brian Brettschneider, a climatologist based in Alaska.
In Denver, a trace of snow was observed. Nearby Boulder wound up with 4.6 inches of “summer." Both locations spent Labor Day in the 90s.
The snowfall in Boulder was the most for so early in the season, while its freezing low on Wednesday morning was its coldest so early in the season. This came after its hottest weather so early in the season, when it hit 99 degrees on Saturday and Sunday.
More seasonable weather will return by the weekend.
Jason Samenow contributed to this article.