Eastern Sierra Avalanche Advisory - 4/3/17

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THIS AVALANCHE ADVISORY EXPIRED ON April 5, 2017 @ 6:18 am
Avalanche Advisory published on April 3, 2017 @ 6:18 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Spring conditions will continue to dominate the early part of the week with sunny skies, warm temperatures, and light to moderate Northwest winds, with solar aspects warming throughout the day (easterly > southerly > westerly aspects).  In the mid to upper elevations, natural and human triggered Loose Wet avalanches will become increasingly possible as temperatures climb through the day, especially in and around rock bands, outcroppings, and large rock faces where the snow can heat-up dramatically. Loose Wet slides move slower than dry slab avalanches but the snow is heavy and dense, if entrained, it can be difficult to extract yourself and are capable of carrying a rider into hazardous terrain or entraining enough snow to bury a person, especially if 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 and give cornices extra berth while traveling along ridgelines. Potential slide-for life conditions may exist. Crampons, ice axe, or whippets and good self-arrest skills maybe required for safe travel. Evaluate snow, terrain, and runouts carefully.

 

How to read the advisory

Bottom Line

Spring conditions will continue to dominate the early part of the week with sunny skies, warm temperatures, and light to moderate Northwest winds, with solar aspects warming throughout the day (easterly > southerly > westerly aspects).  In the mid to upper elevations, natural and human triggered Loose Wet avalanches will become increasingly possible as temperatures climb through the day, especially in and around rock bands, outcroppings, and large rock faces where the snow can heat-up dramatically. Loose Wet slides move slower than dry slab avalanches but the snow is heavy and dense, if entrained, it can be difficult to extract yourself and are capable of carrying a rider into hazardous terrain or entraining enough snow to bury a person, especially if 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 and give cornices extra berth while traveling along ridgelines. Potential slide-for life conditions may exist. Crampons, ice axe, or whippets and good self-arrest skills maybe required for safe travel. Evaluate snow, terrain, and runouts carefully.

 

Avalanche Problem 1: Loose Wet
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  • Size ?
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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 small natural and human triggered Loose Wet avalanches. A very mild Sunday night with temperatures barely falling into the low 30s will allow the surface snow to thaw quickly Monday. Monday night, temperatures are forecasted to fall into the teens and 20’s, which will help setup the snow for a hard freeze and slow the daily thaw Tuesday, which will help maintain good riding conditions later into the day. Time of day is critical for wet point releases. Easterly aspects soften first in the morning, followed by south, then west in the afternoon. Localized thawing around cliffs, trees, rock features and faces can elevate the hazard of loose wet avalanches.  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 one 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 1”-4” of new wind-blown snow was deposited on Thursday / Friday (3/30/2017 – 3/31/2017). Thou unlikely, small isolated sensitive Wind Slabs prone to human triggering are possible in the upper elevations. Caution in steep or exposed areas where a small wind slab release can have high consequences.  

advisory discussion

Spring is in full swing with the snowpack mostly consolidated and characterized by Melt-Freeze crusts separated by rounds or decomposing grains closer to the snow surface with a few minor exceptions in the upper elevations where some dry cold snow conditions linger, primarily on Northwest – North – Northeast aspects.  Springtime’s warm days and cool nights will continue into mid-week. As a result, avalanche concerns are centered on Loose Wet avalanches and the occasional isolated small sensitive Wind Slabs in the upper elevations. A weak storm moved through the region last Thursday/Friday (3/30/2017 – 3/31/2017) depositing 1” - 4” of new snow throughout the forecast area accompanied by strong gusty Southwest winds, which veered to the North as the shortwave exited to the east. The strong winds associated with this system created isolated small Wind Slabs on all aspects in the mid to upper elevations. As of Sunday, the Wind Slabs have strengthened and stabilized enough that only very isolated small triggered Wind Slab activity Monday and Tuesday in the upper elevations is anticipated. The bigger concern is for Loose Wet activity on solar aspects as the snow thaws under the intense April sun. Low temperatures Sunday night were exceptionally mild (struggled to fall below 32 degrees in many locations) with highs forecasted to climb into the mid 30s to mid and upper 40s Monday (4/3/2017). With such a weak freeze, expect the snow surface to thaw fairly quickly Monday on solar aspects, especially the mid to lower elevations. Monday night, temperatures are forecasted to fall into the teens and 20’s, which will help setup the snow for a hard freeze and slow the daily thaw Tuesday, which will help maintain better riding conditions later into the day. Localized thawing around cliffs, trees, rock features and faces can elevate the hazard of loose wet avalanches.

Lower elevations – the snowpack is disappearing quickly below ~7500 feet and is quickly being confined to sheltered northerly aspects, 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 skies clear and temperatures rebound. The snowpack fluctuates between weak Melt-Freeze crust at the surface in the AM to fully isothermal by the afternoon.    

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 whippets maybe required for safe travel, especially prior to the snow softening. Creeks continue to open up with snow bridges beginning to sag and fail.

 

 

 

recent observations

4/2/2017 - Wet Avalanche Debris Pinner Couloir/Mendenhall Couloir

4/2/2017- Mt. Morgan North 

4/2/2017 - Mount Mallory/Meysan Valley

4/1/2017 - Negative 1, June Lake

4/1/2017 - Large Loose Wet Slide - 14,000' Split Mtn

4/1/2017 - Tinnemaha

4/1/2017 - White Wing from June Mtn.

 

Monday 0400 Observations          Low    High (Sunday)

Virginia Lakes (Elev. 9445’)          33      48

Ellery Lake (Elev. 9645’)                32      52

Agnew Pass (Elev. 9355’)             32      56

June Mountain (Elev. 9148’)           31      49

Mammoth Pass (Elev. 9500’)        33      59

Rock Creek Lakes (Elev. 9600’)    32      50

Sawmill (Elev.10200’)                     31      50

South Lake Cabin (Elev. 9580’)    33      59

weather

Monday – A weak upper level disturbance is making its way into far northwest NV and is center is projected to track into west central NV through this morning (Monday) bring breezy northwest to north winds and 5-10 degrees of cooling but the potential for any precipitation is low. Locally stronger gusts up to 40 mph are expected for southern portions of Mono and Mineral counties this afternoon and early evening.

Tuesday thru Wednesday – High pressure builds back into the region with a warming and drying trend for mid-week. This ridge is not very amplified and areas of cirrus will spread across most of the region both days. Temperatures will bounce back Tuesday with highs mainly in the 60s for lower elevations, and 50s near the Sierra. With some improved mixing Wednesday as southwest flow aloft returns, further warming is anticipated with highs pushing into the 70s for lower elevations, and lower 60s near the Sierra.

Thursday and beyond - A weak system Thursday will bring a few showers and breezy afternoon winds. This sets the stage for what is shaping up to be a potentially major storm next weekend. Confidence continues to increase in what looks to be a moderate atmospheric river (AR) event the weekend of April 7-9.

 

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: Partly cloudy then becoming sunny. Clear. Partly cloudy then becoming mostly cloudy.
Temperatures: 40 to 45 deg. F. 18 to 26 deg. F. 43 to 49 deg. F.
Wind direction: Northwest Northwest
Wind speed: 15 to 20 mph with gusts to 35 mph. 10 to 15 mph with gusts to 30 mph in the evening becoming light. Light winds. Gusts up to 25 mph in the morning.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: Partly cloudy. Clear. Partly cloudy then becoming mostly cloudy.
Temperatures: 34 to 42 deg. F. 16 to 21 deg. F. 38 to 44 deg. F.
Wind direction: Northwest Northwest Southwest
Wind speed: 20 to 30 mph. Gusts up to 55 mph. 20 to 30 mph with gusts to 50 mph. 15 to 20 mph. Gusts up to 35 mph decreasing to 25 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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