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The 2017-2018 winter started with a few early season high elevation snow storms in October, followed by dry weather until mid-November. The first atmospheric river struck in mid-November with high-elevation snow and copious amounts of rain below 9000 feet. It also brought the first close call of the winter when a party triggered a persistent slab avalanche on a layer of facets that had formed on north aspects during the earlier dry spell.
After that storm the winter turned dry until late February/early March. The December, January, February period was one of the drier DJF periods on record. During those three months only about 95 inches of snow fell, with about 20 inches of that falling in the last week of February. All but one of the storms during that time produced less than a foot of snow. After the storm that brought close to 18 inches of snow in late January, one close call happened due to a human-triggered wind slab failure.
During the last week of February things changed and winter came back with a vengeance. March was one of the snowiest Marches on record. Numerous storms produced almost double the amount of snow seen during the rest of this winter. The loading that these storms provided also caused widespread avalanche cycles. Three distinct deep slab avalanche cycles occurred, two of which failed on a buried facet layer -- one cycle at the beginning of March, and a second around the 24th when a large rain event impacted the region. The third deep slab cycle occurred due to avalanches failing on a buried graupel layer that remained active for several days after it was deposited. This was responsible for many remotely triggered avalanches and a close call in which the seventh person to ski a slope triggered an avalanche and went for a ride.
April started with more high-elevation rain and remained stormy through tax day. After mid-April, warmer more spring like weather moved into the area to close out the winter. Overall, precipitation for the season came in close to average, but due to warmer temperatures and higher snow levels, snowfall amounts came in below average.
The Forest Service forecasters were out every day collecting daily avalanche, snowpack, and weather observations. This data is combined with additional data received from the SAC professional observers, local backcountry guides, professional ski patrols, and the general public. The Tahoe National Forest forecasters are solely responsible for the creation of the daily avalanche advisories issued through the SAC website, as well as any avalanche watches and warnings.
Tahoe National Forest/SAC forecasters Brandon Schwartz, Andy Anderson, and Steve Reynaud resumed their positions again this season, and Travis Feist and David Reichel returned as professional field observers for the southern portion of the forecast area. The SAC website continued to allow public and professional users to submit observations, including video, photos, and maps.
Professional Guide services, including Alpenglow Guides, Alpine Skills International, Donner Summit Avalanche Seminars, Mountain Adventures Seminars, and Tahoe Mountain School shared their field observations with the avalanche center. The Central Sierra Snow Laboratory, Kirkwood, Mt. Rose, Squaw Valley, and Sugar Bowl also submitted observations to the avalanche center. This information, combined with observations submitted by the general public, was an important supplement to the data gathered by USFS forecasters and SAC professional observers.
TNF/SAC forecasters and pro observers collected 263 backcountry snowpack and avalanche observations this season. These were supplemented by an additional 63 backcountry snowpack and avalanche observations submitted by local guide services, and 125 observations from the general public.
From December 1st through April 22nd, the Sierra Avalanche Center issued 142 daily avalanche advisories. The number of advisories issued for each danger level was: 46 Low, 65 Moderate, 26 Considerable, 5 High, and 0 Extreme days. Additionally, 9 early season snow condition reports were posted in November. Advisories were available 24 hours a day, 7 seven days per week online and by phone.
