Eastern Sierra Snowpack Summary - 2016-02-18 07:25

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THIS ADVISORY EXPIRED ON February 20, 2016 @ 7:25 am
Avalanche Advisory published on February 18, 2016 @ 7:25 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Sensitive wind slabs continue to form throughout Thursday with strong South veering to Southwest winds (90 to 95 mph, gusts to 120 mph, decreasing after midnight). Natural avalanches are likely and human triggered avalanches very likely in wind loaded terrain >30 degrees at mid to upper elevations initially on W-NW-N-NE-E-SE aspects below ridgelines and cross-loaded slopes. These wind slabs will be fairly obvious, denser, hollow sounding snow. While exposed slopes will be of most obvious concern, high winds can also deposit snow into deeper soft wind slabs that could be sensitive to human triggering in open forested terrain and may be only slightly denser than un-wind affected areas. Shooting cracks are a warning sign that these softer wind deposits exist and may be unstable. These avalanches may range from small slabs to larger releases capable of carrying a person into undesirable terrain, bury, injure, or kill a person.

Avalanche Character 2: Storm Slab
Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

Up to 2 feet of new snow forecasted to fall through this recent cycle. As the new snow settles, it will become more slab-like while still forming bonds to the underlying snow. This is potentially a weak combination and will need a couple of days to stabilize. In sheltered areas, natural avalanches are unlikely but triggered releases are possible on slopes of 35 degrees and steeper. Mid to upper elevations are the highest concern, where snowfall amounts are greatest. Be on the lookout for red flag indicators of instability including: whumphing, shooting cracks, and recent avalanche activity. The depth of the new snow is sufficient that even a small release may be capable of burying a skier. Caution is strongly advised while traveling in the backcountry and regularly reassess snow conditions and stability as you move into to higher or steeper terrain and new areas. 

Snowpack Discussion

Primary avalanche concern for the forecast period are fresh windslabs in exposed terrain throughout the mid to upper elevations on NW-SE aspects and storm slabs on steeper terrain due to possible density changes in the new snow, or on more southerly aspects poor bonding to the underlying melt-freeze crust.

The February thaw has come to an end with a fast moving cold front passing through the region depositing 12” to 24” of new snow in the mid to upper elevations, accompanied by strong South/Southwest winds (90 to 95 mph, gusts to 120 mph, decreasing after midnight). This has formed widespread windslabs on all exposed mid to upper elevation leeward slopes (W-NW-N-NE-E-SE), possibly extending down to exposed lower elevation slopes where snow is still present. The strong winds combined with significant amounts of transportable snow have created large sensitive windslabs in favored areas (below ridgelines and cross-loaded slopes and gullies). These windslabs may extend further down slope than anticipated as a result of the strong winds. Natural avalanches are likely and triggered releases are most likely on slopes of 30 degrees and higher. Extreme caution is strongly advised if traveling in complex or exposed terrain. An additional concern is the potential for storm slab weaknesses within the new snow as well as poor bonding to the underlying melt-freeze crust on southerly aspects. Until further observations and information becomes available, travelers must regularly assess the stability of the slopes encountered. Friday will bring some warming and help to promote strengthening and bonding but also may result in localized rapid warming of southerly slopes, especially on large steep southerly faces with complex terrain and multiple aspects.        

 

recent observations

Sherwin Forest (2/17/16) - East facing slopes to ridgetop had frozen melt/freeze crust (0.25”-2"), new snow up to 2" in wind deposited areas. Northerly aspects above ~8500', faceted loose snow and freshly transported snow at mid to upper elevations. Below ridgeline, east-facing slope with new wind deposited snow sloughed easily from the melt-freeze crust.    

weather

...Winter Storm Warning in effect until noon PST Thursday...

 * The heaviest snow (possibly 2 to 3 inches per hour) should tapper off after early to mid-morning.

 * Snow accumulations: 12 to 24 inches above 7500 feet with 6 to 12 inches between 6500 and 7500 feet...2 to 6 inches east of highway 395 in mono county.

 * Winds (Valley Elevations): Southwest 15 to 25 mph with gusts to 40 mph then, decreasing late Thursday AM with Mono County experiencing the strongest winds. Higher elevations will see much stronger winds (Southwest 40’s to 50’s mph, gusts to 85 mph).

 * Snow levels: valley floors

 * Impacts: The combination of wind and heavy snow may create periods of near zero visibility, especially during the early AM.

Thurs-Friday - A burst of very strong winds early Wednesday signaled a brief return to winter for Thursday. Snow levels started out above 7,000’ but fell to the valley floors overnight. The system is forecast to move quickly out of the region today (Thursday) with lingering showers decreasing thru the afternoon and evening. A weak high-pressure ridge builds briefly for Friday with some warming.

Sat-Sunday - The models continue to bring a weak system into the northwest part of the state early Saturday with precip staying well to the north. Will likely see some cloud cover from Mammoth Lakes and north, which will keep the temperatures down a bit. South of Mammoth Lakes, temperatures will rebound more quickly under clearing skies. Winds will pick-up briefly throughout the area Saturday. Mild and dry conditions prevail in the extended forecast with high pressure in control of the large-scale pattern.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Thursday Thursday Night Friday
Weather: CLOUDY. SNOW IN THE MORNING...THEN CHANCE OF SNOW IN THE AFTERNOON. MOSTLY CLOUDY. PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY.
Temperatures: 32 TO 38 deg. F. 17 TO 23 deg. F. 37 TO 45 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 35 TO 45 MPH WITH GUSTS TO 70 MPH 30 TO 35 MPH WITH GUSTS TO 50 MPH BECOMING SOUTH 20 TO 25 MPH WITH GUSTS TO 40 MPH AFTER MIDNIGHT. 35 TO 45 MPH. GUSTS UP TO 60 MPH INCREASING TO 70 MPH IN THE AFTERNOON.
Expected snowfall: 1 TO 3 in. 0 in. 0 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: CLOUDY. SNOW IN THE MORNING...THEN CHANCE OF SNOW IN THE AFTERNOON. MOSTLY CLOUDY PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY.
Temperatures: 25 TO 32 deg. F. 11 TO 18 deg. F. 34 TO 40 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 45 TO 55 MPH WITH GUSTS TO 85 MPH. 25 TO 35 MPH WITH GUSTS TO 50 MPH. 45 TO 55 MPH WITH GUSTS TO 80 MPH.
Expected snowfall: 2 TO 4 in. 0 in. 0 in.
Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. 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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