Eastern Sierra Avalanche Advisory - 2/27/17 | Eastern Sierra Avalanche Center

THIS AVALANCHE ADVISORY EXPIRED ON March 1, 2017 @ 7:00 am
Avalanche Advisory published on February 27, 2017 @ 7:00 am

Issued by Doug Lewis - Eastern Sierra Avalanche Center

Bottom Line

The primary avalanche concern for Monday thru Tuesday will be newly formed Wind Slabs primarily on NW-N-NE-E-SE-S aspects in the middle and upper elevations. However, moderate/gusty winds and localized channeling in the mid elevations have formed Wind Slabs well below ridgelines and on potentially unusual aspects, especially the southern portions of the forecast region.

Monday - Natural avalanches possible, human triggered avalanches are likely in mid to upper elevations primarily on NW-N-NE-E-SE-S aspects. Dangerous avalanche conditions exist below ridgelines, near and around terrain features that promote drifting, crossloaded gullies and shallow depressions. Careful snowpack evaluation, cautious route-finding and conservative decision-making is essential. Caution on exposed slopes 35 degrees and steeper.

Tuesday – Winds will decrease, which will reduce snow transport onto leeward slopes and allow newly formed Wind Slabs to begin bonding to the underlying snow, reducing the potential for natural releases. Mid to upper elevations, NW-N-NE-E-SE-S aspects - natural avalanches will be unlikely, human- triggered avalanches will remain possible below ridgelines, sidewalls of gullies, crossloaded shallow depressions, or near terrain features that promote crossloading or drifting. Evaluate snow and terrain carefully; identify possible wind loaded areas. Carefully evaluate the snowpack and terrain before getting on steep mid to upper elevation slopes.

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Above Treeline

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Near Treeline

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Below Treeline

Avalanche Problem 1: Wind Slab

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Moderate SW to W winds Sunday at the upper mid to upper elevations produced Wind Slabs primarily on NW-N-NE-E-SE-S aspects. However, due to localized channeling and the gusty nature of the winds, Wind Slabs formed well below ridgelines and on unusual aspects in the mid elevations, especially the southern portions of the forecast region. A weak cold front moved into the region Sunday night and is forecasted to linger through Monday with 2 to 7 inches of new low-density snow possible in the mid to upper elevations possible.

Monday - Forecasted moderate Southwest to Westerly winds (20 to 30 mph, gusts up to 65 mph decreasing to 45 mph in the afternoon) Monday will easily transport the new snow to leeward aspects, forming potentially very sensitive Wind Slabs throughout the forecast area primarily on NW-N-NE-E-SE-S aspects in the mid to upper elevations. Natural avalanches possible, human triggered avalanches are likely in mid to upper elevations primarily on NW-N-NE-E-SE-S aspects. Dangerous avalanche conditions exist below ridgelines, near and around terrain features that promote drifting, crossloaded gullies and shallow depressions. Careful snowpack evaluation, cautious route-finding and conservative decision-making is essential. Caution on exposed slopes 35 degrees and steeper. However, localized channeling and the gusty nature of the winds, Wind Slabs may be encountered well below ridgelines and on unusual aspects in the mid elevations, especially the southern portions of the forecast region.

Tuesday - Winds are forecasted to diminish by Monday night limiting the formation of new Wind Slabs in the mid to upper elevations, which will allow recently formed Wind Slabs to begin bonding to the underlying snow and reducing the potential for natural avalanches but triggered releases will remain possible in the mid to upper elevations on NW-N-NE-E-SE-S aspects. Natural avalanches will be unlikely, human- triggered avalanches will remain possible below ridgelines, sidewalls of gullies, crossloaded shallow depressions, or near terrain features that promote crossloading or drifting. Evaluate snow and terrain carefully; identify possible wind loaded areas. Carefully evaluate the snowpack and terrain before getting on steep mid to upper elevation slopes. Be wary of firm denser snow, hollow sounding slabs, and recently formed drifts on slopes 35 degrees and steeper. Do your own localized stability assessments, and avoid steep slopes, especially convex areas, where suspect wind-slabs may exist. Even a small avalanche can carry a rider dangerous terrain.

Avalanche Problem 2: Loose Wet

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Tuesday - Isolated small Loose Wet release are possible below ~ 8000’ on solar aspects (SE-S-SW) as temperatures begin to moderate, especially where strong warming (sheltered sunny locations) can soften and weaken the surface snow. Small natural releases possible, triggered releases likely. Generally not a hazard to riders unless unusually large, associated with a terrain trap resulting in deep burial, or where a rider is carried into hazardous terrain (cliffs).

advisory discussion

The most recent Atmospheric River moved out of the region Wednesday (2/22) with over two feet of new snow in favored areas (Mammoth and June Lakes). As the week progressed, Storm Slab instability declined as the snowpack settled and strengthened. The snowpack continues to settle in sheltered locations, with snow sensors showing continued settlement over the past 72 hours.

Moderate SW to W winds throughout most of the week redistributed the new snow onto leeward aspects (NW-N-NE-E-SE-S), primarily in the upper mid to upper elevations. Saturday, winds veered to the North resulting in significant snow transport (snow banners) over the high peaks from Whitney to Bishop with reports of natural Wind Slab avalanche activity. Sufficient time has elapsed that the Wind Slabs from earlier in the week have strengthened and bonded to the underlying snow. However, Sunday moderate/gusty SW winds developed mid-day forming new sensitive Wind Slabs on NW-N-NE-E-SE aspects in the mid to upper elevations, which have not had sufficient time to strengthen and bond to the underlying snow.

A weak storm system is forecasted to move thru the region Monday with up to 7” new low-density snow possible and moderate Southwesterly winds of 20-30 mph winds over the higher elevations. Forecasted moderate winds will easily redistribute the low-density snow onto leeward slopes, forming new sensitive Wind Slabs on NW-N-NE-E-SE-S aspects in the mid to upper elevations thru Monday. As a result, natural avalanches possible, human triggered avalanches are likely in mid to upper elevations primarily on NW-N-NE-E-SE-S aspects. Winds are forecasted to diminish by Monday night, which will allow Monday’s freshly formed Wind Slabs to begin strengthening and bonding to the underlying snowpack, reducing the Wind Slab threat by Tuesday. Natural avalanches will be unlikely, human- triggered avalanches will remain possible.

24 Hour Precipitation Totals and Temperatures (5:00 AM, 2/27/17)
Location Snow Water Current Temps
VA Lakes (9445’) N/A 0” 16.9
Tioga Pass (9798’) 2” N/A N/A
Ellery Lake (9645’) N/A 0 13
June (9148’) 1” N/A 14.9
Gem Pass (10750’) N/A .1” N/A
Mammoth Sesame St (9014’) 0 0 15.1
Mammoth Pass (9,500’) N/A N/A 18
Rock Creek (9600’) 0 N/A 14
Saw Mill-Big Pine (10200’) 0 N/A 11
Big Pine Creek (10000’) 0 N/A N/A

recent observations

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(2/26/17) Mammoth Rock, Mammoth Lakes

(2/26/17) Tungsten Bowl, Pine Creek

(2/26/17) Birch Mtn, Big Pine Creek

(2/26/17) Sherwins, Mammoth Lakes

(2/26/17) Panorama Dome

(2/26/17) Bishop Bowl, Bishop Creek

(2/25/17) Solitude Basin, Mammoth Lakes, Avalanche Observation

*Recent report of a large avalanche debris field which occurred toward the end of the last major storm cycle in Lee Vining Canyon filled the approach narrows to the ice climbing area. Snowlines this year are much lower than previous years resulting in anchors being covered and plenty of snow down low in the track for entrainment and lubrication are resulting in slides running into areas that don’t frequently see avalanche activity.

Weather and CURRENT CONDITIONS

weather summary

Monday - snow showers winding down from north to south, with the best potential shifting to areas south into Mono County with an additional accumulations up to 3 inches, locally higher amounts where heavier band persists. By late morning, snow showers will diminish with little or no additional accumulations expected. Gusty winds, especially on ridges, this morning will gradually diminish through the day. Temperatures will remain below average, mid 30s to lower 40s, Lows in the teens to lower 20s.

Tuesday thru Thursday –dry conditions with light winds prevailing Tuesday through midweek as ridge of high pressure builds along the CA coast. Temperatures will remain below average with highs mainly in the mid 30s to lower 40s, then warming into the 40s-lower 50s by Wednesday. Lows mainly in the teens to lower 20s will prevail for the next few nights while colder valleys near the Sierra could see temperatures near or below zero. Seasonable temperatures and dry conditions thru Thrusday.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS

For 8,000 ft. to 10,000 ft.

	Monday	Monday Night	Tuesday
Weather:	Mostly cloudy. Snow likely in the morning, then scattered snow showers in the afternoon.	Mostly cloudy then becoming partly cloudy. Slight chance of snow showers.	Sunny.
Temperatures:	21 to 27 deg. F.	1 to 11 deg. F.	28 to 33 deg. F.
Mid Slope Winds:	West	Light winds.	Light winds.
Expected snowfall:	up to 3 in.	up to 1 in.	0 in.

Over 10,000 ft.

	Monday	Monday Night	Tuesday
Weather:	Mostly cloudy. Snow likely in the morning, then scattered snow showers in the afternoon.	Mostly cloudy then becoming partly cloudy. Chance of snow showers in the evening, then slight chance of snow showers after midnight.	Sunny.
Temperatures:	13 to 19 deg. F.	0 to 6 deg. F.	24 to 29 deg. F.
Ridge Top Winds:	Southwest	West	West
Expected snowfall:	up to 3 in.	up to 1 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.