Eastern Sierra Avalanche Advisory - 4/2/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 3, 2017 @ 6:52 am
Avalanche Advisory published on April 2, 2017 @ 6:52 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Spring conditions are on order for today with sunny skies, warming temperatures, and light Northwest winds, with solar aspects warming throughout the day (easterly > southerly > westerly).  Natural and human triggered Loose Wet avalanches will become increasingly possible as the day progressively warms, especially in and around rock bands, outcroppings, and adjacent to large rock faces where the snow can heat-up dramatically.  Loose Wet slides generally move at slow speeds (when compared to dry slab avalanches) but the snow is heavy and dense and if entrained can carry a rider into hazardous terrain or entrain enough snow to bury a person, especially when combined with a terrain trap.  Early starts are recommended to avoid slopes before they become saturated and unsupportive. Caution – Spring Hazards: Avoid riding slopes under cornices (they can fail unexpectedly) and give cornices a wide berth while traveling along ridgelines (can fail much further back than expected). Spring snow can be very firm prior to softening and a slip or fall can result in a slide-for life, even on relatively benign slopes.  Evaluate snow, terrain, and runouts carefully. Crampons, ice axe, or whippet maybe required for safe travel, especially prior to the snow softening. Creeks continue to open up with snow bridges beginning to sag and fail.

 

 

How to read the advisory

Bottom Line

Spring conditions are on order for today with sunny skies, warming temperatures, and light Northwest winds, with solar aspects warming throughout the day (easterly > southerly > westerly).  Natural and human triggered Loose Wet avalanches will become increasingly possible as the day progressively warms, especially in and around rock bands, outcroppings, and adjacent to large rock faces where the snow can heat-up dramatically.  Loose Wet slides generally move at slow speeds (when compared to dry slab avalanches) but the snow is heavy and dense and if entrained can carry a rider into hazardous terrain or entrain enough snow to bury a person, especially when combined with a terrain trap.  Early starts are recommended to avoid slopes before they become saturated and unsupportive. Caution – Spring Hazards: Avoid riding slopes under cornices (they can fail unexpectedly) and give cornices a wide berth while traveling along ridgelines (can fail much further back than expected). Spring snow can be very firm prior to softening and a slip or fall can result in a slide-for life, even on relatively benign slopes.  Evaluate snow, terrain, and runouts carefully. Crampons, ice axe, or whippet maybe required for safe travel, especially prior to the snow softening. Creeks continue to open up with snow bridges beginning to sag and fail.

 

 

Avalanche Problem 1: Loose Wet
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As the sun warms and softens the snow surface, the snow can become saturated during the day, increasing the potential for natural and human triggered small Loose Wet avalanches. A mild Saturday night with temperatures falling only into mid 20s to low 30s will allow the surface snow to thaw more quickly. Time of day is critical for wet point releases. Easterly aspects soften first in the morning, followed by south, then west in the afternoon. Extra heat from cliffs, rock features and faces, trees can result in additional localized thawing.  Northerly aspects will soften as well at lower elevations as the day progresses. Watch for signs of decreasing stability such as large rollerballs or deep boot penetration, which indicates the snow is loosing strength and possibly becoming unstable, steep terrain should be avoided where encountered.  The two to four inches of snow recorded on Thursday continues to produce sticky conditions as it softens, especially below ~ 9000.

*Small isolated wind slabs may also be encountered in the upper elevations where Thursday/Friday’s 1-4” of new wind-blown snow was deposited. Thou unlikely, small isolated sensitive Wind Slabs prone to human triggering are possible. Caution in steep exposed areas where a small wind slab release can result in a bad fall.  

advisory discussion

The springtime pattern continues with brief shortwave storms passing through the area with light snowfalls and gusty winds followed by warm-ups and corn snow. As a result, avalanche concerns have revolved around sensitive Wind Slabs with each new snowfall and Loose Wet avalanches as the storm systems move out of the region and temperatures rebound between near and above normal. One to four inches of new snow fell Thursday/Friday accompanied by strong gusty winds (which began out of the SW and then shifted out of the N) creating isolated small Wind Slabs on all aspects where the snow deposited in the mid to upper elevations. As of Saturday, the recently formed Wind Slabs have had time to gain some strength with only very isolated small Wind Slab activity anticipated Sunday in the mid to upper elevations. The bigger concern is for small Loose Wet activity on solar aspects as the snow thaws. Temperatures lows Saturday night were mild and are forecast to climb into the mid 30s and 40s today (Sunday) alllowing the snow to thaw more quickly. Rocky outcrops and trees can trap the heat and reflect in back down onto the snow, further elevating the hazard of loose wet avalanches.

Lower elevations – the snowpack is disappearing quickly below ~7500 feet, primarily confined to sheltered northerly aspects or shaded slopes, terrain features that promote crossloading, or where earlier avalanches spilled into low elevation terrain. The spring storms have deposited limited amounts of snow down low, which quickly melts as the clear skies return. The snowpack vacillates between weak melt/freeze crust at the surface to fully isothermal.    

Caution – Spring Hazards: Avoid riding slopes under cornices (they can fail unexpectedly) and give cornices a wide berth while traveling along ridgelines (can fail much further back than expected). Spring snow can be very firm prior to softening and a slip or fall can result in a slide-for life, even on relatively benign slopes.  Evaluate snow, terrain, and runouts carefully. Crampons, ice axe, or whippet maybe required for safe travel, especially prior to the snow softening. Creeks continue to open up with snow bridges beginning to sag and fail.

weather

Sunday thru Monday - A series of weak shortwaves will move through the area Sunday thru Monday. The first wave moving across the northern Rockies is projected to drag a shallow back door cold front through the region today (Sunday) with increasing clouds, cooler temperatures, and moderate north winds with gusts 20-25 mph. Sunday night, modest dynamics combined with upslope surface flow could produce scattered light rain and snow showers along the eastern Sierra later tonight (Sunday) into Monday morning. Liquid amounts less than 0.10 inch and less than 1 inch of snow near the Sierra. Monday, temperatures will drop another 5-10 degrees from today as cooler north winds prevail behind the departing shortwave with southern Mono county seeing more notable gusts (up to 40 mph) in the afternoon as a tighter pressure gradient is projected.

Tuesday thru Wednesday - a decent warm-up is expected as upper level ridge axis moves over western NV with highs climbing into the 60s across most lower elevations, with 50s near the Sierra. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Sunday Sunday Night Monday
Weather: Sunny Mostly cloudy. Slight chance of showers. Mostly cloudy then becoming partly cloudy. Chance of snow showers in the morning, then slight chance of snow showers in the afternoon.
Temperatures: 38 to 44 deg. F. 25 to 31 deg. F. 32 to 38 deg. F.
Wind direction: Light winds. Light winds. Gusts up to 25 mph after midnight. Northwest
Wind speed: 15 to 20 mph. Gusts up to 30 mph increasing to 45 mph in the afternoon.
Expected snowfall: 0 in. Up to 1 inch. in. Up to 1 inch. in.
Over 10,000 ft.
  Sunday Sunday Night Monday
Weather: Sunny Mostly cloudy. Slight chance of snow showers in the evening, then chance of snow showers after midnight. Mostly cloudy then becoming partly cloudy. Chance of snow showers in the morning, then slight chance of snow showers in the afternoon.
Temperatures: 34 to 40 deg. F. 19 to 25 deg. F. 26 to 32 deg. F.
Wind direction: Northwest Northwest Northwest
Wind speed: 10 to 15 mph with gusts to 30 mph. 10 to 15 mph. Gusts up to 35 mph after midnight. 20 to 30 mph. Gusts up to 45 mph increasing to 55 mph in the afternoon.
Expected snowfall: 0 in. Up to 1 inch in. Up to 1 inch. 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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