Jim Steenburgh displays a device for measuring snowfall. Credit: Brian Maffly ^^ Banner photo above: Little Cottonwood Canyon. Credit: UDOT.
Major snowstorms in Utah’s Wasatch Mountains are both a blessing and a curse. They deliver much-needed moisture that supplies water to the state’s biggest metropolitan area and fluffy light snow to support the world’s finest powder skiing.
But heavy snowfall also wreaks havoc on canyon roads and creates extreme avalanche hazards that can sometimes shut down busy winter recreation sites. Alta at the head of Little Cottonwood Canyon, for instance, can be reached by vehicle only via a winding road that rises 3,000 feet in 8 miles, crossing about 50 avalanche paths.
University of Utah atmospheric scientists have set out to better understand extreme snowfall, defined as events in the top 5% in terms of snow accumulations, by analyzing hundreds of events over a 23-year period at Alta, the famed ski destination in the central Wasatch outside Salt Lake City. The resulting study, published this week in Monthly Weather Review, illustrates the remarkable diversity of storm characteristics producing orographic snowfall extremes in the ranges of the Intermountain West.
The orographic effect occurs when air is forced to flow up and over mountains, which cools the air and condenses its water vapor.
Some of the new findings surprised researchers. For example, they looked for an association between heavy snow and a weather factor called “integrated vapor transport,” or IVT, but found a complicated relationship.
“IVT is essentially a measure of the amount of water vapor that is being transported horizontally through the atmosphere, said lead author Michael Wasserstein, a graduate student in atmospheric sciences. “In certain regions high IVT can produce extremely heavy precipitation. That can be the case for the Wasatch, but not always.”
In the West Coast’s Sierra Nevada and Cascade Range, by contrast, there is a stronger relationship between high-IVT storms blowing in from the Pacific and extreme precipitation and snowfall.
Spanning the years 2000 to 2022, the study, which was funded by the National Science Foundation, analyzed a total of 2,707 snow events, each covering a 12-hour period. The average amount of snow deposited during each event was 11.2 centimeters (4.4 inches), while the median amount was just 7.6 (3 inches). Alta ski patrollers did much of the data collection at the monitoring station located near the ski area’s Wildcat Lift.
The researchers homed in on “extreme” events above the 95th percentile, or 138 storms in which 30.5 centimeters (12 inches) or more snow fell. “Those would be snowfall rates of about an average of an inch an hour,” said Jim Steenburgh, the study’s senior author. The biggest 12-hour accumulation was 65 centimeters (26 inches), recorded on March 30, 2005. They also examined “extreme” water-equivalent snowfall events above the 95th percentile, or 116 storms with at least 27.9 mm (1.11 inches) of water equivalent precipitation. The water equivalent of precipitation measures the amount of water in the snowfall and is important for water resources and avalanches.