A Pacific storm system has moved into the forecast area overnight. Snowfall began on the west side of the Sierra Crest and deposited several inches of new snow before snowfall accumulation began east of the Sierra Crest this morning. New snow amounts of 6 to 8 inches have been reported in the southern portion of the forecast area with 3 to 5 inches reported in the northern portion. Snowfall rates in excess of 1 inch per hour are expected until mid morning. After mid morning, snowfall rates are expected to decrease to less than 1 inch per hour along the Sierra Crest with lower snowfall rates occurring further to the east. The next round of high intensity snowfall is expected to begin tonight. Moderate to strong ridgetop winds have averaged from the southwest during the last 24 hours with a brief wind shift to the south observed for a few hours very early this morning.
Yesterday, moderate to strong ridgetop winds redistributed new snow causing wind loading and areas of snowpack instability. Along the Mt. Judah ridge (Donner Summit area), small natural avalanches (R1,D1) were observed in the most heavily wind loaded areas on NE-E aspect avalanche start zones. Crown heights ranged from 4 to 8 inches and measured 10 to 20 feet wide. Shooting cracks up to 10 feet long were observed around newly formed cornice features along the ridge. Small pockets of skier triggered sluffs were observed in steep open areas below mid slope. In the upper Deep Creek drainage (North of Squaw Peak), loud whumpfing was reported in an open meadow below treeline. Shooting cracks up to 15 feet long were reported in wind loaded areas just below the ridgeline. Skier triggered sluffs occurred in steep areas. No slab avalanche activity was observed in this area.
Today, a period of heavy new snowfall rates combined with southwest ridgetop winds will continue to wind load N-NE-E aspect avalanche start zones. The best window for natural avalanche activity is expected this morning before snowfall rates decrease as the day progresses. The threat of human triggered avalanches will remain throughout the day. Backcountry travelers should not be surprised today by the occurance of natural or human triggered avalanches that are large enough to bury, injure, or kill a person. Near and above treeline, human triggered avalanches are likely today on N-NE-E aspects in steep wind loaded areas. Natural avalanches are probable to likely in the most heavily wind loaded areas. Below treeline, human triggered avalanches are possible on steep slopes in the most heavily wind affected areas. In areas not affected by wind, natural and human triggered avalanches are unlikely.
The bottom line: Near and above treeline, avalanche danger is CONSIDERABLE with pockets of HIGH danger on heavily wind loaded N-NE-E aspects, 37 degrees and steeper. Below treeline, avalanche danger is LOW with pockets of MODERATE danger in wind affected areas, 37 degrees and steeper.
Please send us your snow, weather, and avalanche observations by clicking the submit observations link on our contact page.
Brandon Schwartz, Avalanche Forecaster
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