Eastern Sierra Avalanche Advisory - 4/14/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 15, 2017 @ 6:37 am
Avalanche Advisory published on April 14, 2017 @ 6:37 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center
bottom line

Strong winds began transporting snow over ridge tops on Wednesday and increased through Thursday when we received up to 9” of new snow which added to the wind slab problem. Look for and avoid round, smooth wind slabs on the leeward side of features that have been promoting drifting. Another concern will be loose wet avalanches where temperatures are the warmest by this afternoon: on solar aspects and near rocky areas. Today, natural avalanches will be possible, and human triggered avalanches will be likely. Careful snowpack evaluation, cautious route-finding, and conservative decision-making will be essential to your safety.

How to read the advisory

Bottom Line

Strong winds began transporting snow over ridge tops on Wednesday and increased through Thursday when we received up to 9” of new snow which added to the wind slab problem. Look for and avoid round, smooth wind slabs on the leeward side of features that have been promoting drifting. Another concern will be loose wet avalanches where temperatures are the warmest by this afternoon: on solar aspects and near rocky areas. Today, natural avalanches will be possible, and human triggered avalanches will be likely. Careful snowpack evaluation, cautious route-finding, and conservative decision-making will be essential to your safety.

Avalanche Problem 1: Wind Slab
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Moderate west winds continued to build wind slabs on high leeward slopes on Wednesday. By Wednesday night, a typical spring storm came through with modest snowfall but stronger that usual winds. Between 2” and 9” of new snow fell by Thursday night, primarily from Crowley Lake north. Areas south of Rock Creek were under a High Winds Advisory that lasted until 12 AM this morning. Winds at the top of Lincoln Mountain in Mammoth gusted to 123 mph yesterday afternoon. Wind slabs in the June Lake area yesterday were touchy enough that they could be triggered by a skier’s weight, even on lower angled terrain. Just because things look calmer today doesn't mean that these sensitive wind slabs have had time to bond to the underlying snow. That could take between hours and even days. Expect to see wind slabs today just below ridges, on cross loaded slopes, and on the down wind sides of convexities. Because of the strength and turbulent nature of the winds yesterday, wind slabs may be found on unusual aspects and further down slope that usual. Hollow, drum-like sounding snow or cracks shooting out from your feet should set off alarm bells in your head. Remember that more avalanche accidents occur on the first sunny day after a storm. Stay alert to freshly formed cornices and wind drifts.

Avalanche Problem 2: Loose Wet
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Temperatures will climb back into the upper 40s at around 10,000’ today as spring returns for the weekend. As the snow surface warms quickly to its melting point by mid day, especially around some rock outcrops, below cliff bands, and in cirques and gullies, loose wet avalanches will be increasingly likely. East aspects will receive the first solar radiation in the morning, then south, then west as the sun moves across the sky by late afternoon. Lower elevations will warm more quickly than higher elevations. Watch out for pinwheels rolling down the slopes around and above you. Even small point releases are heavy and can be hard to escape, and they can be a sign that larger avalanches are possible.

advisory discussion

Wind Slabs that formed during an unusually late season Atmospheric River storm between April 6th and 8th were reactive in some areas through Monday when natural avalanches were observed in Lundy canyon. In the Mammoth area, wind deposits from that storm were healing and were more stubborn to trigger by early in the week. But while sunny skies and warm temperatures prevailed through Wednesday night, moderate west winds still had snow to blow around on alpine ridge tops where wind slabs where most likely continuing to grow.

Meanwhile, at elevations below about 11,000’ in the southern forecast area, and below about 9,500’ in the northern zone, warm temperatures and the intense spring sun warmed up last weekend’s storm snow. Loose wet avalanches ran on many slopes up and down the east side through Wednesday when large roller balls and point releases were observed in the southern Sierra and between Rock Creek and Convict Lake.

Today will feel like spring again after a short burst of wind and light precipitation rushed through the northern area on Thursday. Winds from this quick hitter were intense at times and snowfall amounts were just enough to build large wind slabs on leeward slopes. It’s this kind of rapid loading that creates wind slabs sensitive enough for you to trigger. In areas where the most snow accumulated, north of Crowley Lake and just under ridge lines, convexities, and the side walls of gullies, you will most likely find hollow-sounding wind slabs today. Recently formed cornices and round, pillow shaped wind slabs point to where the danger has increased. Use these observations to make conservative terrain choices that keep you out of harms way. Remember that sunny weather doesn't mean no danger.

As temperatures rebound today, with highs in the upper 40s, loose wet avalanches will be another concern on solar aspects. As the sun heats the snowpack to its melting point it will become less stable. Extra heat from cliffs, rock faces, and trees can result in additional localized thawing. Large roller balls originating from exposed rocks and trees can be a good indicator that loose wet avalanches are becoming increasingly likely. Starting early and being off of rocky, solar heated slopes before they become unsupportable is the best strategy for avoiding the loose wet avalanche problem.

weather

A quieter pattern will be in place for the next few days with drier conditions, lighter winds, and warming temperatures expected for much of the weekend. Temperatures in the Sierra will warm to the mid 40s today and to the lower 50s on Saturday. Modest winds will be primarily from the southwest. Our next shot of precipitation and wind comes with a shortwave trough that will quickly move through the region Sunday afternoon and evening.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Friday Friday Night Saturday
Weather: Sunny then becoming partly cloudy. Clear. Sunny.
Temperatures: 38 to 46 deg. F. 19 to 24 deg. F. 45 to 53 deg. F.
Wind direction: W Light becoming S.
Wind speed: 10 mph. Gusts up to 25 mph in the morning. Light. 10 to 15 mph with gusts to 25 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Friday Friday Night Saturday
Weather: Sunny then becoming partly cloudy. Clear. Sunny.
Temperatures: 33 to 39 deg. F. 15 to 20 deg. F. 40 to 46 deg. F.
Wind direction: SW SW SW
Wind speed: 10 to 20 mph with gusts up to 45 mph in the morning. 10 to 15 mph. Gusts up to 25 mph after midnight. 10 to 15 mph. Gusts up to 35 mph in the afternoon.
Expected snowfall: 0 in. 0 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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