Predictions are for a winter storm in the Wood River Valley, but warming temperatures could prevent the valley floor from seeing any accumulation.
The National Weather Service announced yesterday that storms forming over the Pacific coast could bring several inches of snow to the mountains around the valley, with almost 30 inches at Galena Summit.
However, forecasters say warming temperatures could prevent any snow from sticking below 7,000 feet elevation.
"I don't mind saying it's a very difficult proposition to nail down snow amounts," said Mike Huston, lead forecaster for the National Weather Service in Pocatello.
A high-pressure ridge that has been dominating the region for about a month shaded the area from receiving precipitation for nearly six weeks, diverting storm systems from Canada and dumping heavy snow far to the north of Blaine County.
Huston said several low-pressure systems have since passed over the top of the ridge, weakening it enough so it has finally broken in time for several storm systems to arrive from the Pacific.
"Basically, what you will see through the week is pulses of stronger precipitation," he said.
The first major wave is expected to begin creeping into the area Wednesday morning, ramping up through Wednesday night and Thursday. Huston said the valley will likely see a break in precipitation on Thursday night and early Friday, but will be hit by a second wave of precipitation during the day on Friday and into Friday night.
"Right now, if you drew a line straight west from Portland and a line straight south from Anchorage, that's where the storm is," Huston said Tuesday afternoon.
The storm is expected to head directly west through Washington and Oregon before hitting Sun Valley.
But snow totals will be deeply affected by warming temperatures, Huston said. The breakdown of the high-pressure ridge shielding the Wood River Valley from precipitation also trapped cold air in the region. With the breakdown of that ridge, Huston said, warmer air will move more freely and cause temperatures to rise.
"It will seem like a warming trend," he said. "Now that the blocking pattern has been erased, we have all this warm, moist air coming off of the Pacific."
Unfortunately for accumulation, that warm, moist air will cause the snow levels to rise to between 6,000 and 7,000 feet by Wednesday afternoon. According to Huston, the valley floor is more likely to see rain or a little bit of snow.
"This isn't looking too great for Ketchum itself," he said, adding that the valley could see up to 2 inches.
However, he said, the area around Ketchum is likely to see "healthy" accumulations. Huston is predicting 8 inches of snow in the Sawtooth Mountains west of Ketchum, with 20 inches in Stanley and 30 inches on Galena Summit.
"Above 7,000 feet, we're going to get clobbered," he said.
The lack of accumulation is not due to the precipitation shadow that existed earlier this month, which was blocking any moisture from Canadian storms.
"That shadowing effect is most pronounced for you in a northwest flow," he said, adding that any snow coming from that direction generally gets dumped in McCall or the panhandle. "We're [now] in a westerly flow here. While Ketchum proper won't do great, the mountains to the west will do just fine."
Any accumulation in the mountains will cause increasing avalanche danger, according to a Sawtooth National Forest Avalanche Center advisory Tuesday.
"The avalanche danger will rise over the next few days as a series of fast-moving storms sweeps across our area," wrote Blase Reardon in the advisory. "The dry weather of the past four weeks has dramatically weakened the existing snowpack."
Reardon said as little as 6 inches of snow on the current weak and faceted snow layer could cause natural avalanches in some areas, the most dangerous of which will be shaded mid-elevation slopes where the snow cover is the deepest.
"The forecasted combination of sustained snowfall, rising temperatures and increasing winds has the potential to create thick, cohesive slabs," Reardon said. "By Thursday morning, dangerous avalanche conditions could be widespread."
As for the weather after Friday, Huston said models disagree as to whether the region will see more snow or will dry out. Conditions are expected to be dry and sunny on Saturday and Sunday, but the early part of next week could result in either another high-pressure ridge or another Pacific storm system.
"We're not really sure how it's going to pan out," Huston said. "The early part of next week is a big question mark."
Katherine Wutz: firstname.lastname@example.org