Eastern Sierra Avalanche Advisory - 3/22/17

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THIS AVALANCHE ADVISORY EXPIRED ON March 24, 2017 @ 6:20 am
Avalanche Advisory published on March 22, 2017 @ 6:20 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center
bottom line

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.

How to read the advisory

Bottom Line

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.

Avalanche Problem 1: Wind Slab
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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.

advisory discussion

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.

recent observations

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”            --

3/21- Wet storm snow and winds on the Mammoth Crest

3/21- Wind loading and wet storm snow on the Sherwins

3/20- Spring conditions and increasing winds on Basin Mtn

3/20- Strong winds and sun softening of surface snow in Lundy Canyon

 

weather

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.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Slight chance of snow showers in the morning, then snow showers likely and slight chance of thunderstorms in the afternoon. Mostly cloudy. Chance of snow showers and slight chance of thunderstorms in the evening, then slight chance of snow showers after midnight. Partly cloudy.
Temperatures: 26 to 36 deg. F. 15 to 23 deg. F. 39 to 45 deg. F.
Wind direction: SW NW
Wind speed: 10 to 15 mph with gusts to 45 mph in the morning becoming light. 10 to 15 mph in the evening becoming light. Gusts up to 40 mph. Light.
Expected snowfall: about 2 in. up to 3 in. 0 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: Mostly cloudy. Slight chance of snow showers in the morning, then snow showers likely and slight chance of thunderstorms in the afternoon. Cloudy. Snow showers likely and slight chance of thunderstorms in the evening, then chance of snow showers after midnight. Partly cloudy.
Temperatures: 22 to 32 deg. F. 11 to 17 deg. F. 35 to 41 deg. F.
Wind direction: SW NW NW
Wind speed: 20 to 30 mph with gusts to 55 mph decreasing to 15 to 20 mph with gusts to 40 mph in the afternoon. 15 to 25 mph. Gusts up to 40 mph increasing to 50 mph after midnight. 15 to 20 mph. Gusts up to 45 mph decreasing to 35 mph in the afternoon.
Expected snowfall: about 2 in. 1 to 3 in. 0 in.
Disclaimer
This Avalanche Advisory is designed to generally describe avalanche conditions where local variations always occur. This product only applies to backcountry areas located outside established ski area boundaries. The information in this Snowpack Summary is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

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