Eastern Sierra Avalanche Advisory - 4/12/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 14, 2017 @ 6:38 am
Avalanche Advisory published on April 12, 2017 @ 6:38 am
Issued by Clancy Nelson - Eastern Sierra Avalanche Center
bottom line

For Wednesday: As temperatures climb into the upper 40s and low 50s Wednesday, isolated loose wet avalanches on sun-heated aspects at middle and lower elevations will be possible. Increasing southwest winds will keep wind slab avalanches a possibility on isolated, leeward alpine slopes. Heightened avalanche conditions will exist on specific terrain features. Evaluate the snowpack and terrain carefully to identify features of concern.

For Thursday: A moderate spring cold front will blow through our area on strong southwest winds. There could be up to 9” of new snow. Because of uncertainty over precipitation amounts and snow levels, human triggered wind slab avalanches may be likely on Thursday at middle and upper elevations where enough snow accumulates. Natural avalanches will be possible, but human triggered avalanches will be likely in these areas. Watch for blowing snow and avoid the leeward side of features that promote wind drifting.

How to read the advisory

Bottom Line

For Wednesday: As temperatures climb into the upper 40s and low 50s Wednesday, isolated loose wet avalanches on sun-heated aspects at middle and lower elevations will be possible. Increasing southwest winds will keep wind slab avalanches a possibility on isolated, leeward alpine slopes. Heightened avalanche conditions will exist on specific terrain features. Evaluate the snowpack and terrain carefully to identify features of concern.

For Thursday: A moderate spring cold front will blow through our area on strong southwest winds. There could be up to 9” of new snow. Because of uncertainty over precipitation amounts and snow levels, human triggered wind slab avalanches may be likely on Thursday at middle and upper elevations where enough snow accumulates. Natural avalanches will be possible, but human triggered avalanches will be likely in these areas. Watch for blowing snow and avoid the leeward side of features that promote wind drifting.

Avalanche Problem 1: Loose Wet
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Highs are forecast to be in the upper 40s to mid 50s on Wednesday with partly cloudy skies. It will also be windy and southwest winds will help keep the snow surface colder, particularly as winds increase in the afternoon. But as the snow surface warms quickly to its melting point in the middle of the day, especially around some lower elevation rock outcrops, below cliff bands, and in open bowls, isolated loose wet avalanches will be a possibility. 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 carry you into undesirable terrain.

In areas where a thin melt-freeze crust formed Sunday into Tuesday, hard and fast conditions may be found in the mornings. On steep exposed slopes, early in the day or as strong winds prevent surface melting, a slip and fall may have higher consequences.

Avalanche Problem 2: Wind Slab
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The wind slabs formed during last weekend’s storm have generally been bonding to the underlying snow and though possible on isolated alpine slopes on Wednesday, triggering these older wind slabs at lower or middle elevations will be unlikely.

But another fast moving spring storm is expected to push though our area Wednesday night into Thursday morning. Up to 9” of new snow are possible by Thursday afternoon, though there is a good amount of uncertainty about actual amounts and snow levels. What is certain is that winds will increase at all elevations throughout Wednesday as the cold front approaches. Snow levels may fall to around 6,000 ft. by mid Thursday, and snow showers are likely to continue throughout the day. By Thursday afternoon, new wind slabs will be found on leeward terrain features at upper and middle elevations. Expect to see wind loading just below ridges, on cross loaded slopes, and on the down wind sides of convexities in areas where the most snow accumulates, and particularly in the northern half of the forecast zone. Be wary of hollow, drum-like sounding snow or cracks shooting out from your feet.

advisory discussion

An unusually late season Atmospheric River moved into the region Thursday afternoon and dropped 1.2” to 6” of water along the eastern Sierra by Saturday evening. Starting warm, with heavy wet snow above about 7500’, things cooled and lighter density snow fell as the system exited the region. Storm totals ranged from 19” at Tioga Pass to 37” at Mammoth Mountain. Snowfall was heaviest from June Mountain south. The system came in with moderate to strong Southwesterly flow (typical of these Atmospheric River storms) with winds gusting over 100mph at ridge tops. Wind Slabs that formed between Thursday and Sunday were reactive in some areas through Monday when natural avalanches were observed in Lundy canyon. In the Mammoth area, wind deposits have become more stubborn to trigger. Though with little data from elsewhere in the forecast zone, I wouldn’t be surprised if some isolated high elevation slopes remained sensitive through Wednesday.

Temperatures will again be spring like on Wednesday, with highs in the upper 40s to mid 50s. As temperatures climb, so will the risk of loose wet avalanches at lower elevations. Moderate to strong southwest winds cooling the snow surface may keep the danger relatively low. But 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 can be a good indicator that loose wet avalanches are becoming increasingly possible.

The active spring weather pattern continues, so be ready for another ride on the roller coaster. On Thursday, another moderate spring storm will push through our area bringing snow showers, and most importantly wind. Winds will be strong and sustained starting Wednesday and will increase going into early Thursday as the main cold front moves through. As is common with these kinds of spring storms, precipitation totals and snow levels can be difficult to predict. Though precipitation amounts are expected to be around 7”, remember that wind can deposit new snow onto leeward slopes 3 to 5 times faster than it can fall from the sky. So even smaller amounts of snow can create large wind slabs in just a short amount of time. It’s this kind of rapid loading that creates wind slabs sensitive enough for you to trigger. In areas where the most snow accumulates, just under ridge lines, convexities, and the side walls of gullies, you will most likely find hollow-sounding wind slabs. Areas north of Mammoth will see greater snow fall amounts. Blowing snow and cornice formation will point to where the avalanche danger is increasing. Use these observations to make conservative terrain choices that keep you out of harms way.

weather

Winds increase today as a moderate Spring storm moves into the region. Snow and valley rain begin will spread down the eastern Sierra overnight. Gusty winds will peak tonight as a cold front moves through. Backcountry travel may be hazardous at times. Expect breezy conditions Thursday with some lingering showers. Friday and Saturday will be more pleasant with another series of waves moving into the Sierra on Easter continuing into next week.

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: Partly cloudy. Mostly cloudy. Slight chance of rain in the evening. Chance of snow through the night. Mostly cloudy. Chance of snow showers in the morning, then snow showers likely in the afternoon.
Temperatures: 43 to 53 deg. F. 24 to 29 deg. F. 30 to 40 deg. F.
Wind direction: SW SW SW
Wind speed: 15 to 25 mph. Gusts up to 45 mph increasing to 60 mph in the afternoon. 20 to 35 mph. Gusts up to 65 mph increasing to 75 mph after midnight. 15 to 25 mph. Gusts up to 60 mph decreasing to 40 mph in the afternoon.
Expected snowfall: 0 in. 80% probability...up to 3 in. 20% probability...3 to 5 in. 80% probability...up to 4 in. 20% probability...0 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: Partly cloudy. Mostly cloudy. Chance of snow. Mostly cloudy. Snow showers likely through the day.
Temperatures: 37 to 47 deg. F. 17 to 23 deg. F. 23 to 33 deg. F.
Wind direction: SW SW SW
Wind speed: 20 to 35 mph. Gusts up to 60 mph increasing to 75 mph in the afternoon. 30 to 50 mph with gusts to 90 mph increasing to 45 to 60 mph with gusts to 105 mph after midnight. 30 to 45 mph with gusts to 80 mph decreasing to 20 to 30 mph with gusts to 60 mph in the afternoon.
Expected snowfall: 0 in. 80% probability...up to 3 in. 20% probability...3 to 5 in. 80% probability...up to 4 in. 20% probability...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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