The varied topography of the Western United States—a patchwork of valleys and mountains, basins and plateaus—results in minutely localized weather.
Accordingly, snowfall forecasts for the mountain West often suffer from a lack of precision, with predictions provided as broad ranges of inch depths for a given day or storm cycle.
The crux of the problem lies in the snow-to-liquid ratio (SLR), which varies widely in the West“If you don’t have a good snow-to-liquid ratio, your snowfall forecasts are not going to be as good,” said Peter Veals, a research assistant professor of atmospheric sciences. New research by Veals and a group of University of Utah scientists aims to improve methods of forecasting by applying machine learning to manually collected snowfall data from 14 mountain sites by snow-safety professionals employed by ski areas and transportation departments.
It’s all about snow density
The single most important predictor of SLR is the snow-water equivalent, or SWE, according to Veals.
“It’s because the more SWE you have, the more the storm’s snow weighs and it densifies itself. It compacts under its own weight,” said Veals, the study’s first author. Other factors, including elevation, temperature and wind speed, also play a crucial role in determining what the SLR will be in each storm.
The U research team, led by atmospheric sciences professor Jim Steenburgh, has another study coming out soon applying this method across the continental United States, using snowfall data gathered at 900 locations.
The main reason why snowfall forecasting in the West is so much harder is that the amount of snow piling on the ground depends not just on how much water the snow contains, but how dense or powdery the snow is, that is, its snow-to-liquid ratio. SLRs can be as low as 2-to-1, or two inches of snow equals an inch of liquid water, typical of slush. Or up to 100-to-1 seen in ultralight powder.
The commonly cited 10-to-1 “rule of thumb” for SLR was a product of Eastern weather forecasting.
“Somebody cooked that up in some place a long time ago. It’s easy to use. It’s just multiplying by 10,” Steenburgh said. And it has little relevance to the West.
To build a forecast model tailored to the West, Steenburgh and Veals’ team acquired manually gathered snowfall data from 14 mountain sites across the Western states, spots where avalanches are a winter hazard that must be managed with care.
Three sites were in Utah: Alta in Little Cottonwood Canyon; the Spruces campground in Big Cottonwood Canyon; and Aspen Grove in Provo Canyon. The others were in the Cascade and Sierra Nevada ranges, Colorado, Idaho, Wyoming and Montana.