Eastern Sierra Snowpack Summary - 2016-03-03 07:37

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THIS ADVISORY EXPIRED ON March 5, 2016 @ 7:37 am
Avalanche Advisory published on March 3, 2016 @ 7:37 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: Loose Wet
Loose Wet avalanches occur when water is running through the snowpack, and release at or below the trigger point. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Melt freeze crusts on E-S-W aspects will be frozen and firm early, and may thaw and soften through the day but forecasted cloud cover and moderate winds will greatly slow or impede this process. Depending on cloud cover, wind, and elevation, Easterly slopes may begin to soften by mid morning, southerly aspects softening by mid-day, and westerly slopes softening by the afternoon. Indicators of overly warm slopes include, punchy or manky snow resulting in sinking past boot-top and significant rollerball activity. These indicators mean the likelihood for natural and human triggered wet point releases is increasing, which may have the potential to lead to larger slope failures. Even small releases can have consequences in potentially hazardous terrain. 

Lower elevation slopes (below 8500’) may soften more quickly on all aspects and the risk of wet loose activity possibly increasing on slopes >35 degrees by the afternoon.  Natural and human triggered wet loose avalanches maybe possible, starting on southeast slopes at mid to upper elevations in the late morning and moving around the compass toward southwesterly by afternoon.

 

 

Avalanche Character 2: 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.

If traveling in higher elevation, isolated small recently formed windslabs that may be tender enough to be triggered by a rider. Though these wont be big enough to result in a burial, they could be enough to knock a skier or boarder off balance, resulting in a slide into terrain with potentially high consequences. These will most likely be found near ridgelines on northerly to easterly facing slopes. 

 

Snowpack Discussion

Snow Sum:  As our spring-like weather comes to a close, our avalanche concerns Thursday thru Friday will shift from Wet-Loose avalanches toward isolated windslabs.

Concern for wet releases exists on southerly/sunny aspects (E-SE-S-SW-W) beginning late morning and into the late afternoon depending on sun exposure with forecasted cloud cover, which will likely slow the thawing of the surface snow. As snow showers develop along the Sierra Crest, moderate Southwest to Westerly winds will likely form isolated small fresh windslabs in upper elevation terrain near ridgetops on easterly to northerly facing slopes.

Melt freeze crusts on E-S-W aspects will be frozen and firm early, and may thaw and soften through the day but forecasted cloud cover and moderate winds will greatly slow or impede this process. Depending on cloud cover, wind, and elevation, Easterly slopes may begin to soften by mid morning, southerly aspects softening by mid-day, and westerly slopes softening by the afternoon. Indicators of overly warm slopes include, punchy or manky snow resulting in sinking past boot-top and significant rollerball activity. These indicators mean the likelihood for natural and human triggered wet point releases is increasing, which may have the potential to lead to larger slope failures. Even small releases can have consequences in potentially hazardous terrain. 

Lower elevation slopes (below 8500’) may soften more quickly on all aspects and the risk of wet loose activity possibly increasing on slopes >35 degrees by the afternoon.  Natural and human triggered wet loose avalanches maybe possible, starting on southeast slopes at mid to upper elevations in the late morning and moving around the compass toward southwesterly by afternoon.

If traveling in higher elevation, isolated small recently formed windslabs that may be tender enough to be triggered by a rider. Though these wont be big enough to result in a burial, they could be enough to knock a skier or boarder off balance, resulting in a slide into terrain with potentially high consequences. These will most likely be found near ridgelines on northerly to easterly facing slopes. 

Firm snow conditions remain a concern for the potential for dangerous slip and falls, which may be encounter on all aspects.  Be especially cautious as you travel in steeper more complex terrain where a fall could result in injury or even death. East to south to west facing slopes are firm and frozen prior to thawing, northerly facing slopes may have very firm wind-board that can make it difficult to arrest a fall. If traveling in exposed terrain, make sure you have the proper equipment (ice axe, self-arrest grips, crampons, etc.), know how to use them effectively, use extreme caution, and have a plan should something go wrong.

 

recent observations

Wine Glass Chute  (2/29/16) - Weather was warm with a light breeze. The wind board snow never softened up. Crampons were mandatory for most of the chute. Variable conditions going up the wine glass chute. We started up it around 11am and topped out around 2:40pm. Saw many old slides from all different aspects. We saw a couple old slides right at the base, a couple mid way up, and a couple more in the upper section. No signs of recent avalanche activity. Snow was mostly hard wind board with the upper section holding some soft snow in the lower tide areas.

Virginia Lakes, NE face of South Peak (2/28/16).  Light cloud cover dominated most of the morning with moments of full sun exposure.  At 10 am on open NE facing slope at ~10k ft. found a supportable snowpack with soft surface and approx. 10 cm of ski penetration.  As we traveled from 10,500 to 11k+ experienced very slick windboard and plenty of breakable crust.  At 11am on NE facing slope at ~11k ft. a hand pit near the shallower portion of the snowpack revealed ~3cm of wind crust on top of ~80 cm of loose faceted snow down to the rocks.    

weather

Thurs.-Friday - Weather patterns have begun to change with a weak disturbance will skirt across the northern portion of the region with very light precipitation (above 7,500’) and breezy conditions Thursday though winds should taper off through the day as the jet exits to the east. Friday night into will bring the first part of a strong storm into the Sierra with precipitation mainly impacting the Sierra.

Sat.-Sun: The second part of the storm will be preceded by strong gusty winds on Saturday afternoon with valley wind gusts between 50 to 55 MPH and ridge winds of over 100 MPH. Wind prone locations will likely see wind gusts between 60 to 65 MPH Saturday afternoon and evening. As the cold front drops southward across the Sierra and western Nevada late Saturday night into early Sunday morning, the winds will peak with frontal passage and then drop off sharply as precipitation spills over into western Nevada. With a good atmospheric river and strong forcing in place, precipitation will be intense for a 6 to 12 hour period Saturday night into Sunday morning. Snow Water Equivalent (SWE) amounts Saturday night and Sunday could reach 2 to 3 inches along the Sierra Crest. Snow levels will start at 7000-8500 feet Saturday evening throughout Mono County, and then drop dramatically to below 5000 feet Sunday morning (~ 4AM) behind the strong cold front. Significant travel impacts are anticipated for the Sierra and northeast California with moderate to heavy snowfall rates into Sunday morning with snow showers continuing in the Sierra in the post-frontal unstable flow with a few additional inches possible, possible more.

 

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: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS IN THE MORNING. SLIGHT CHANCE OF SHOWERS IN THE AFTERNOON. MOSTLY CLOUDY. CHANCE OF SNOW SHOWERS IN THE EVENING...THEN SLIGHT CHANCE OF SHOWERS AFTER MIDNIGHT. MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS IN THE MORNING...THEN CHANCE OF SNOW SHOWERS IN THE AFTERNOON.
Temperatures: 46 TO 54 deg. F. 23 TO 30 deg. F. 46 TO 54 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 20 TO 30 MPH. GUSTS UP TO 40 MPH 10 TO 15 MPH WITH GUSTS TO 25 MPH. 20 TO 25 MPH WITH GUSTS TO 40 MPH.
Expected snowfall: 0 in. 1 in. 1 in.
Over 10,000 ft.
  Thursday Thursday Night Friday
Weather: PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY. SLIGHT CHANCE OF SNOW SHOWERS. MOSTLY CLOUDY THEN BECOMING PARTLY CLOUDY. CHANCE OF SNOW SHOWERS. MOSTLY CLOUDY. CHANCE OF SNOW SHOWERS.
Temperatures: 40 TO 48 deg. F. 19 TO 26 deg. F. 38 TO 46 deg. F.
Wind direction: SOUTHWEST SOUTHWEST WEST
Wind speed: 40 TO 45 MPH WITH GUSTS TO 70 MPH DECREASING TO 30 TO 35 MPH WITH GUSTS TO 55 MPH IN THE AFTERNOON. 35 TO 40 MPH WITH GUSTS TO 60 MPH DECREASING TO 25 TO 30 MPH WITH GUSTS TO 45 MPH AFTER MIDNIGHT. 35 TO 40 MPH WITH GUSTS TO 60 MPH BECOMING SOUTHWEST 25 TO 30 MPH WITH GUSTS TO 45 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 1 in. 1 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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