Eastern Sierra Avalanche Advisory - 2/20/17

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THIS AVALANCHE ADVISORY EXPIRED ON February 22, 2017 @ 6:56 am
Avalanche Advisory published on February 20, 2017 @ 6:56 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

The primary avalanche concern for the forecast period will be Wind Slabs and Storm Slabs above ~7000’. 

Storm Slab - One to five feet of new snow is forecasted for the region above ~7000’ over the next 24 hours, which will elevate the risk of Storm Slab avalanches for terrain above ~7000. The rapid loading will produce potentially large destructive avalanches running well into the lower elevations. Travel in or under avalanche terrain not recommended. Natural avalanches likely, human triggered avalanches very likely.

Wind Slab - Strong Southwesterly (Monday) to Southerly (Monday night) winds and plenty of transportable snow will result in sensitive Wind Slabs forming primarily on W-NW-N-NE-E-SE-S aspects, above ~7000’ in favored locations (gullies, shallow depressions, rock outcroppings, below ridgelines, etc.) However, due to the strong gusty winds and localized wind channeling, Wind Slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Travel in or under avalanche terrain not recommended. Natural avalanches likely, human triggered avalanches very likely

How to read the advisory

Bottom Line

The primary avalanche concern for the forecast period will be Wind Slabs and Storm Slabs above ~7000’. 

Storm Slab - One to five feet of new snow is forecasted for the region above ~7000’ over the next 24 hours, which will elevate the risk of Storm Slab avalanches for terrain above ~7000. The rapid loading will produce potentially large destructive avalanches running well into the lower elevations. Travel in or under avalanche terrain not recommended. Natural avalanches likely, human triggered avalanches very likely.

Wind Slab - Strong Southwesterly (Monday) to Southerly (Monday night) winds and plenty of transportable snow will result in sensitive Wind Slabs forming primarily on W-NW-N-NE-E-SE-S aspects, above ~7000’ in favored locations (gullies, shallow depressions, rock outcroppings, below ridgelines, etc.) However, due to the strong gusty winds and localized wind channeling, Wind Slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Travel in or under avalanche terrain not recommended. Natural avalanches likely, human triggered avalanches very likely

Avalanche Problem 1: Storm Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Increasing Danger

Approaching Atmospheric River is forecasted to produce 2 to 5 feet of snow above ~8000 with 1 to 3 feet of snow between ~6500’ to 8000’, which will result in wide spread Storm Slab development above ~7000’. The rapid loading will produce potentially large destructive avalanches running well into the lower elevations. Travel in or under avalanche terrain not recommended. Natural avalanches likely, human triggered avalanches very likely.

 

Avalanche Problem 2: Wind Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Increasing Danger

Strong Southwesterly (Monday) to Southerly (Monday night) winds and plenty of transportable snow will result in sensitive Wind Slabs forming primarily on W-NW-N-NE-E-SE-S aspects, above ~7000. Due to the strong gusty winds or localized wind channeling, Wind Slabs may be encountered on unusual aspects, in normally sheltered areas, as well as further down slope than usual. Wind Slabs will likely be encountered along ridgelines, crossloaded gullies and depressions, in and around terrain features that promote drifting and loading. Be wary of hollow, drum-like sounding snow or cracks shooting out from your feet. Travel in or under avalanche terrain not recommended. Natural avalanches likely, human triggered avalanches very likely. 

advisory discussion

A weak short wave moved through the region Thursday (2/16) depositing about 2” of snow with strong SW winds, which quickly forming localized sensitive Wind Slabs on NW-N-NE-E-SE aspects. Late Thursday into Friday, heavy snow began falling as another Atmospheric River streamed into the region. Freezing level hovered around 7000’ with 12” to 30” of new snow reported with light winds, which limited Wind Slab formation to isolated exposed mid to upper elevation locations. Friday (2/17) as the storm began to move off to the east, widespread tender Storm Slabs and sloughing were observed, especially in the June / Mammoth region were snowfall was the heaviest. This was confined to soft slab instability within the new snow. There were many reports of extensive soft storm slab releases and sloughing in steeper mid to low elevations Friday around the Mammoth and June area, as well as from Ice Climbers in Lee Vining Canyon with avalanches burying the ice climbing approach boot pack Thursday and Friday. Skies cleared Saturday (2/18) with light Southwesterly to Westerly winds, as the day progressed, winds began to picking-up Saturday afternoon forming Wind Slabs on NW-N-NE-E-SE-S aspects primarily in exposed mid to upper elevation terrain along ridgetops, crossloaded gullies and depressions, and near and around terrain features that promote drifting and loading. As the skies cleared Saturday, temperatures began to climb under the mid-February sun with extensive Loose Wet releases observed throughout the region on all solar aspects, especially around rock outcrops and below cliff bands on steep slopes. Some observed Loose Wet releases were quite large and could have easily buried person, especially if a terrain trap was involved. Saturday’s mild conditions has helped to the snowpack to settle and strengthen, decreasing the Storm Slab potential through Sunday (2/19). Another strong Atmospheric River storm system is forecast to move into the region Monday (2/20) with strong winds and heavy snow above ~7000’. The sub-tropical nature of the system will usher in warm temperatures and produce heavy dense snow, which increase the danger of Storm Slabs for all elevations. Additionally, the strong Southwesterly  to Southerly flow aloft associated with the system will form Wind Slabs throughout the region primarily on W-NW-N-NE-E-SE aspects. However, due to the strong winds, Wind Slabs may be encountered in unusual locations and in normally sheltered areas, possibly further down slope than usual.  

 

24 Hour Precipitation Totals and Temperatures (6:00 AM, 2/20/17)
Location                                 Snow    Water    Current Temps
VA Lakes (9445’)                    N/A      0.6”      30.7
Tioga Pass (9798’)                  4”         N/A      N/A
Ellery Lake (9645’)                  N/A      Trace    28
June (9148’)                           2.6”      .3”       28.1
Gem Pass (10750’)                  N/A      .6”       24
Mammoth Sesame St (9014’)   5”         .9”       26.7
Mammoth Pass (9,500’)           N/A       N/A     30
Rock Creek (9600’)                 Trace     N/A     28
Saw Mill-Big Pine (10200’)        1”         N/A     27
Big Pine Creek (10000’)           1”         N/A     N/A

 

weather

...WINTER STORM WARNING REMAINS IN EFFECT UNTIL 4 PM PST
 TUESDAY...
 
 * Timing: Moderate to heavy snow and rain will continue through Tuesday with the heaviest snow expected this morning (Monday) through early afternoon...and again late tonight. 
 
 * Snow Accumulations: Above 8000 feet...18 to 36 inches with locally up to 4 feet along the Sierra Crest. Between 7000 and 8000 feet...8 to 20 inches. Below 7000 feet...up to 8 inches with less than 4 inches below 6500 feet mainly on Tuesday.
 
 * Winds: South to Southwest winds 25 to 35 mph with gusts to 55mph. Sierra ridge gusts up to 120 mph.
 
 * Snow Levels: Between 7500 and 8000 feet through tonight, possibly lowering to near 7000 feet at times during periods of heavy precipitation. Snow levels will then fall to near 7000 feet by Tuesday morning and near 6500 feet by Tuesday afternoon.
 
 * Impacts: Heavy snow will create dangerous conditions, with travel being severely impacted over the Sierra passes. Snowfall rates will exceed 2 inches per hour at times, producing whiteout conditions over the passes and in the backcountry.

Monday thru Tuesday - A powerful atmospheric river storm moving into the region bring strong winds and very heavy precipitation with higher snow levels through Tuesday. Surface temperatures are near or slightly above freezing this morning (Monday). As precip becomes more consolidated, snow levels will become more uniform, averaging near 7500-8000 feet for Alpine-Mono counties. Elevations above ~7500 feet will get very heavy snowfall, around 2-5 feet through Tuesday afternoon. Snow amounts between 6500-7500 feet will be more variable with roughly 1-3 feet likely at that elevation. Tuesday, colder air will bring snow levels steadily downward with snow level falling to many valley locations Tuesday night, although precip will also be diminishing to scattered showers by this time. Gusty Southwesterly winds associated with this system will increase through the day (Monday) into Tuesday with gusts to 120 mph possible at times tonight along the Crest. High winds and heavy snowfall in the high elevations will create whiteout conditions.

Wednesday thru Thursday - Operational models are in good agreement early in the forecast period...sliding the main upper level trough over the region late Wednesday then east of the area on Thursday. This will help usher much colder temperatures into the region with snow levels down to even the lowest valley floors. Precipitation becomes mainly showery by late Wednesday with less coverage than previous days and far less QPF as the main moisture tap has shifted far to the south and east. Still a few inches of snow are possible in the Sierra Wednesday. Late Thursday a weak flat ridge tries to develop with drying over the region with colder temperatures, lows in the teens and 20s with some single digits in the colder valleys. Highs Thursday will struggle to reach 30.
 

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: Cloudy. Heavy Snow. Cloudy. Snow. Cloudy. Snow.
Temperatures: 36 to 42 deg. F. 27 to 33 deg. F. 30 to 38 deg. F.
Wind direction: Southwest South Southwest
Wind speed: 20 to 30 mph with gusts to 75 mph increasing to 30 to 45 mph with gusts to 85 mph in the afternoon. 30 to 50 mph with gusts to 85 mph. 30 to 45 mph with gusts to 80 mph.
Expected snowfall: 10 to 18 in. 8 to 14 in. 4 to 8 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: Cloudy. Heavy Snow. Cloudy. Snow. Cloudy. Snow.
Temperatures: 29 to 35 deg. F. 20 to 26. deg. F. 23 to 31 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 50 to 70 mph with gusts to 120 mph. 50 to 70 mph. Gusts up to 110 mph increasing to 120 mph after midnight. 45 to 65 mph with gusts to 120 mph decreasing to 40 to 55 mph with gusts to 100 mph in the afternoon.
Expected snowfall: 12 to 20 in. 10 to 16 in. 4 to 8 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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