Eastern Sierra Avalanche Advisory - 2/19/17

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THIS AVALANCHE ADVISORY EXPIRED ON February 20, 2017 @ 7:01 am
Avalanche Advisory published on February 19, 2017 @ 7:01 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Wind Slab -With 12” to 30” of new transportable snow, the primary avalanche concern for the forecast period will be wind slabs at middle and upper elevations as moderate Westerly to Southwesterly pre-frontal winds increase over the region ahead of the next storm system due to move in Sunday night. Isolated pockets of Wind Slab will form in favored locations (gullies, shallow depressions, rock outcroppings, etc.) Evaluate snow and terrain carefully and identify windloaded features such as below ridgelines and along the sidewalls of gullies.

Storm Slab – the storm slab threat is improving as the new snow settles and strengthens. Natural avalanches possible, rider triggered releases likely in the mid to upper elevations. Lower elevations - natural avalanches unlikely, triggered release possible, especially in complex or steep terrain. Sloughing possible in steeper terrain.

Loose Wet - ahead of the next system warm temperatures forecasted for Sunday will maintain the threat of loose wet avalanches on solar aspects at middle and lower elevations. Above ~ 11,000 feet, natural avalanches are unlikely, but human triggered avalanches possible. Below ~ 11,000 feet, natural possible and human triggered avalanches are likely, especially around rock outcrops, cliffs, and rock faces where strong localize warming can occur.  

How to read the advisory

Bottom Line

Wind Slab -With 12” to 30” of new transportable snow, the primary avalanche concern for the forecast period will be wind slabs at middle and upper elevations as moderate Westerly to Southwesterly pre-frontal winds increase over the region ahead of the next storm system due to move in Sunday night. Isolated pockets of Wind Slab will form in favored locations (gullies, shallow depressions, rock outcroppings, etc.) Evaluate snow and terrain carefully and identify windloaded features such as below ridgelines and along the sidewalls of gullies.

Storm Slab – the storm slab threat is improving as the new snow settles and strengthens. Natural avalanches possible, rider triggered releases likely in the mid to upper elevations. Lower elevations - natural avalanches unlikely, triggered release possible, especially in complex or steep terrain. Sloughing possible in steeper terrain.

Loose Wet - ahead of the next system warm temperatures forecasted for Sunday will maintain the threat of loose wet avalanches on solar aspects at middle and lower elevations. Above ~ 11,000 feet, natural avalanches are unlikely, but human triggered avalanches possible. Below ~ 11,000 feet, natural possible and human triggered avalanches are likely, especially around rock outcrops, cliffs, and rock faces where strong localize warming can occur.  

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

Moderate West to Southwesterly pre-frontal winds and 1 to nearly 3 feet of new transportable snow have likely formed sensitive wind slabs on NW-N-NE-E-SE-S aspects. Winds are forecasted to increase Sunday afternoon with Wind Slabs becoming more widespread on Leeward aspects, especially in and around exposed terrain features in the mid to upper elevations. Heightened avalanche conditions exist along ridgelines, crossloaded gullies, and depressions, in and around terrain features that promote drifting and loading in the upper and middle elevations. Be wary of hollow, drum-like sounding snow or cracks shooting out from your feet. Natural avalanches possible, human triggered avalanches likely. 

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

1 to nearly 3 feet of new snow fell Thursday into Friday with light winds. The new snow bonded relatively well to the old snow interface. However, a slight density change within the new snow produced a weak Storm Slab that was quite reactive during the storm cycle with extensive avalanching observed. The new snow shows signs of strengthening and bonding with significant settlement since the storm moved out of the area Friday. Natural avalanches unlikely, human triggered avalanches possible on slopes of 35 degrees and steeper. Sloughing possible in steeper terrain. Solar aspects below treeline have stabilized more quickly due to strong solar effect. The potential for Storm Slab releases will decline through the day but increase tonight as the next storm system moves into the the region.

Avalanche Problem 3: Loose Wet
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small
  • Trend ?
    Increasing Danger

Warm temperatures forecasted for today (Sunday) will continue to drive Loose Wet releases, especially around rock outcrops and below cliff bands. Time of day is critical with east aspects receiving the first radiation of the day, then south, then west. Be on the lookout for moist surface snow, rollerballs (pinwheels) rolling down around you, and in steep rocky terrain.

advisory discussion

A weak short wave moved through the region Thursday 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 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 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. 

 

Storm Totals and Hi Temperatures (2/17 -2/18)
Location                                  Storm Snow    Storm Water    Temps
VA Lakes (9445’)                    N/A                0.5-0.7”         34.2
Tioga Pass (9798’)                  12”                 N/A               N/A
Ellery Lake (9645’)                  N/A                0.6”               34
June (9148’)                           20”                1.9”               36.3
Gem Pass (10750’)                  N/A                1.2”              N/A
Mammoth Sesame St (9014’)   25”                 2.1”              32.7
Mammoth Pass (9,500’)           N/A                N/A               36
Rock Creek (9600’)                 7”                   N/A               33
Saw Mill-Big Pine (10200’)       10”                 N/A               31
Big Pine Creek (10000’)          16”                 N/A               N/A

weather

...Winter Storm Warning In Effect From 10 Pm Sunday To 4 Am PST Tuesday...

The National Weather Service in Reno has issued a Winter Storm Warning above 7000 feet for heavy snow, which is in effect from 10 PM Sunday to 4 AM PST Tuesday. 
 
 * Timing: Heavy snow will push into the region late Sunday night through Tuesday morning. 
 
 * Snow Accumulations: 1 to 3 feet above 7000 feet. Up to 4 feet along the Sierra Crest. Generally less than 6 inches of snow expected below 7000 feet.
 
 * Winds: South to Southwest winds 25 to 35 mph with gusts to 55 mph. Sierra ridges gusting to over 130 mph.
 
•    Snow Levels: 7000 to 7500 feet.

Sunday thru Monday - A weak shortwave passing through the Pacific Northwest will bring bands of light snow near the crest during the day, with limited moisture pushing beyond the crest with accumulations generally 1 inch or less. Sunday night through Monday night, a strong atmospheric river storm remains on track to make a direct hit to the Sierra bringing extreme amounts of rain and higher elevation snow, similar to some of the strongest AR systems already experienced so far this season. The progression of this system is a little faster, with the leading edge of the more intense precip arriving by early Monday morning. Model guidance remains consistent with very large amounts of precip for all of eastern CA. By Monday evening, most guidance sources lift the heaviest precip rates northward into northeast CA. Around 3-6 inches of water with locally higher amounts of precip forecasted for the Sierra. Snow levels will rise to 7500-8000 feet in Mono County Monday morning through Monday evening with increased flooding concerns in the Sierra below 7000 feet. Snow levels could get pushed down by as much as 500-1000 feet lower during periods of heaviest precip. Elevations above 7500 -8000 feet can expect 2-5 feet of snowfall with highest amounts likely along the Sierra crest. Snow amounts between 6500-7500 feet will vary widely, ranging anywhere from about 6 inches to at least 2 feet depending on how much rain mixes in during the peak precip rates, while most areas below 6500 feet receive limited snowfall amounts. As the stronger winds aloft push in by Monday, expect to see strong winds gusting over 140 mph during the main storm on Monday. 
 
Tuesday thru Wednesday - models are a little slower with lowering snow levels Tuesday. QPF numbers are a little higher for early Tuesday for Mono County where additional snow is expected at the higher elevations. Models have trended a little drier Tuesday night into Wednesday night. The main upper trough begins to slide over the region Wednesday. With the slight decrease in expected QPF for Wednesday the amount of snow for Mono County for Wednesday and Wednesday night was lowered 
 Slightly, generally less than a foot around Mammoth Lakes, more at the higher elevations.
 

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: Partly cloudy. Slight chance of snow. Cloudy. Chance of snow in the evening, then snow after midnight. Cloudy. Snow and rain.
Temperatures: 35 to 41 deg. F. 28 to 33 deg. F. 35 to 41 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 15 to 20 mph. Gusts up to 30 mph increasing to 40 mph in the afternoon. 15 to 25 mph. Gusts up to 45 mph increasing to 60 mph after midnight. 20 to 35 mph with gusts to 85 mph.
Expected snowfall: Up to 1 in. 2 to 6 in. 8 to 16 in.
Over 10,000 ft.
  Sunday Sunday Night Monday
Weather: Partly cloudy. Slight chance of snow. Cloudy. Slight chance of snow in the evening, then snow after midnight. Cloudy. Snow.
Temperatures: 29 to 35 deg. F. 23 to 29 deg. F. 29 to 35 deg. F.
Wind direction: West Southwest Southwest
Wind speed: 20 to 35 mph. Gusts up to 45 mph increasing to 65 mph in the afternoon. 25 to 35 mph with gusts to 65 mph increasing to 30 to 45 mph with gusts to 80 mph after midnight. 40 to 60 mph with gusts to 105 mph.
Expected snowfall: Up to 1 in. 3 to 8 in. 10 to 20 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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