With between 4 and 10 inches of snow since Tuesday morning and moderate to strong winds, expect sensitive wind slabs above about 8500’ on leeward slopes as winds continue through Thursday. For Wednesday, southwest winds will favor wind slab formation on northwest through southeast aspects. Thursday, as the winds shift to the northwest, new slabs may develop on southerly and easterly facing slopes as well. As wind continues to load slopes, natural avalanches may be possible, and human triggered avalanches will be likely. Expect to trigger small wind slabs in many areas and large avalanches on steep terrain or the sidewalls of gullies, near convexities, and below ridge lines. Careful snow pack evaluation, cautious route-finding and conservative decision-making will be essential.
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The fast moving spring storm that will push out of the area Wednesday night or Thursday morning has dropped between 4 and 10 inches of wet, heavy snow (as 5:00 AM on Wednesday morning). Snow levels stayed above 8000’ for most of Tuesday, and could fall to as low as 6000’ as the storm tapers off. All of these spring showers have been accompanied by moderate to strong southwest winds that transported much of that new snow onto leeward slopes and packed it into dense wind slabs. These winds will continue Wednesday, and wind slabs will remain sensitive on aspects from northwest through southeast. On Thursday winds will shift out of the northwest, potentially creating new wind slabs on the other side of the compass. Note blowing snow and cornice formation as you travel in the backcountry. Expect to see wind loading just below ridges, on cross loaded slopes, and on the leeward sides of convexities. Dense, hollow sounding snow or cracks shooting out from under your feet are a sign that you may already have ventured onto unstable slabs.
Spring in the Sierra can mean warm sunny days one week and stormy weather the next. The whiplash can be painful.
After a period of above average temperatures and the melt-freeze cycle dominating our backcountry pursuits, a series of fast moving storms are beginning to affect the region once again. The last time we experienced any notable precipitation was March 5th when we received around 7” of cold winter snow and a typically strong southwest flow.
Since then, temperatures have climbed to the 50’s near 10,000’ and even into the 60’s below about 9,000’. Coupled with good re-freezing on most nights at most locations, the snow pack has gained strength and settled (and melted too). During the warm-up, solar aspects saw a significant loss of snow cover with some slopes becoming completely bare below ~8000’. Loose Wet avalanches were observed in the Low to Mid elevations, slowly creeping into the upper elevations as temperatures continued to rise into unseasonably warm territory this last week.
The bond between the melt-freeze crust from last week’s high pressure and the moist new snow varies by aspect and elevation. At lower and middle elevations, the wet storm snow seems to have bonded relatively well to the wet surface underneath. At higher elevations where the snow surface never quite warmed through, and where winds have been depositing heavy, moist slabs since Tuesday morning, we can expect this bond to be more tenuous. On leeward slopes of about 35 degrees or steeper, and around features that promote drifting and loading, watch for round, pillow shaped wind drifts, cornice formation, and blowing snow. Use these clues to guide your terrain choices. Be cautious and make your decisions conservatively. Drum-like, hollow sounding snow indicates that you’re already standing on a wind loaded slope. Cracks shooting out from your feet should set off alarm bells in your head.
Precipitation Totals (as of 5:00 AM, 3/22/17)
Location Snow Water
VA Lakes (9445’) 9” 0.8”
Tioga Pass (9798’) 8” --
June (9148’) 8” --
Mammoth Sesame (9014’) 6” 1.34”
Rock Creek (9600’) 10” --
Saw Mill (10200’) 6” --
Big Pine Creek (10000’) 4” --
A moderately strong consolidated band of convection will move east through central California this morning. The band is north-south oriented along a weak, mid-level cold front. Some moderate convective snow showers are possible this morning and likely through this afternoon, especially right along the Sierra crest. Snow levels will be dragged down under convective precipitation possibly as low as 6000 feet or so. But, temperatures should again be warm enough that snow should melt off fairly quickly below 7000 feet. Isolated thunder showers are not out of the question.
Thursday we should see a break in the weather as ridging temporarily builds in and drier air works into the region.
On Friday another round of moisture pushes into the region driven by a sharp trough and northeasterly oriented jet.