Recent storms in central and southern Idaho boosted snowpack that had fallen well behind long-term averages after a warm, dry November.
Storms Dec. 11-12 affected snow-water equivalents in the state’s central mountains, while a weather system that arrived late Dec. 13 and continued the next day targeted areas south of the Interstate 84-86 corridor.
The first storms “added a considerable amount to our central mountains,” said Tim Axford, warning coordination meteorologist with the National Weather Service in Pocatello. “Even though they didn’t get us back up to where we typically are at this time of year, they did increase snow totals substantially across the Sawtooths and central mountains,” including around Sun Valley and Stanley.
After a mostly dry Dec. 13, storms that arrived that night and lasted into the next day added significant snowpack to the south and east, he said. This area also previously lagged the 30-year average for the time of year.
Eastern Idaho snowpack is important because it supplies water the upper reaches of a Snake River that supports irrigation, power, recreation and other uses across a large area. The region’s high-altitude melt can continue into late summer.
“At the headwaters of the Snake above Palisades Reservoir, we are sitting right at 100% of normal” as of Dec. 16, Axford said. “Anywhere south of that, we are at or above our seasonal average so far.”
Reservoirs in the Upper Snake River system are at 68% of capacity, around year-earlier levels, he said.
Snowpack as of mid-December in Idaho’s central mountains and northern region is running around one-third to one-fourth below long-term averages following the more southerly storm tracks of recent weeks, Axford said. Snowpack thus is well above normal in Utah, Colorado and parts of Wyoming, and behind long-term averages from central Idaho through the Northwest.
Snowpack in many basins dropped from 150 to 200% of normal at the end of October to well below normal a month later, Axford said.
The dry November, which put snowpack totals farther behind season-to-date averages as time progressed, largely was a result of persistent high pressure over the Western U.S. “That cut us off from any moisture coming in from the Pacific,” he said.
Troy Lindquist, NWS senior service hydrologist in Boise, said most forecast models for December through February call for precipitation to be above normal in northern and eastern Idaho, and near normal in the southwest region.
Dec. 16 percentages of average for southeastern Idaho snowpack included 101 in the Snake Basin above Palisades, 105 for Willow-Blackfoot-Portneuf, and 129 for Bear River, the USDA Natural Resources Conservation Service reported. Percentages in the central mountains and basins to the northeast ranged from the low 60s to high 80s.