Eastern Sierra Avalanche Advisory - 3/13/17

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THIS AVALANCHE ADVISORY EXPIRED ON March 15, 2017 @ 6:19 am
Avalanche Advisory published on March 13, 2017 @ 6:19 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Monday thru Tuesday - the main avalanche concern is Loose Wet avalanches. Unseasonably warm temperatures this past week have produced spring-like conditions on most aspects and elevations with a few pockets on North to Northeast aspects spared in the mid to upper elevations. High pressure will continue to dominant during the forecast period. Natural loose-wet releases are becoming unlikely on solar aspects (E-SE-S-SW-W mid to upper elevations, lower elevations the aspect range includes NW/NE as well) but triggered releases are possible in steeper terrain. Timing accents and descents is critical to avoid Loose Wet avalanches. Small point releases are indicators that surface snow is loosing strength and becoming unstable, larger avalanches may become possible with further thawing. Caution - small slides can easily entrain a rider and carry them into dangerous terrain and in rare instances could entrain enough snow to potentially bury a person, especially where a terrain trap is involved. Slide for life conditions may be encountered on steep solar aspects prior to thawing.  

 

How to read the advisory

Bottom Line

Monday thru Tuesday - the main avalanche concern is Loose Wet avalanches. Unseasonably warm temperatures this past week have produced spring-like conditions on most aspects and elevations with a few pockets on North to Northeast aspects spared in the mid to upper elevations. High pressure will continue to dominant during the forecast period. Natural loose-wet releases are becoming unlikely on solar aspects (E-SE-S-SW-W mid to upper elevations, lower elevations the aspect range includes NW/NE as well) but triggered releases are possible in steeper terrain. Timing accents and descents is critical to avoid Loose Wet avalanches. Small point releases are indicators that surface snow is loosing strength and becoming unstable, larger avalanches may become possible with further thawing. Caution - small slides can easily entrain a rider and carry them into dangerous terrain and in rare instances could entrain enough snow to potentially bury a person, especially where a terrain trap is involved. Slide for life conditions may be encountered on steep solar aspects prior to thawing.  

 

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

Warm temperatures and sunny skies have generating small isolated Loose Wet releases on solar aspects (E-SE-S-SW-W) in the mid to upper elevations as the surface snow thaws. In the lower elevations the aspect range increases to include NE & NW aspects too. Natural Loose Wet releases are unlikely on solar aspects but triggered releases are possible in steeper terrain, especially in and around rock bands, cliffs, and rock faces, as the day progresses. In the lower elevations where the snowpack is shallower, possible buried weak layer that can be triggered by larger point releases overloading this layer and propagating into a larger slab failure. Timing is critical to avoid Wet Loose avalanches with easterly aspects thawing first, followed by southerly, then westerly aspects in the late afternoon. Loose Wet releases are indicators that surface snow is loosing strength and becoming unstable, larger avalanches may be possible with further thawing. Caution - small slides can easily entraining a rider and carry them into dangerous terrain and in rare instances could entrain enough snow to potentially bury a person, especially where a terrain trap is involved

 

advisory discussion

High pressure entrenched over the Southwest since 3/7/17 has brought spring-like weather to the eastern Sierra. This has rapidly transitioned the snowpack from cold winter-like snow to spring-like snow conditions on most aspects (W-SW-S-SE-E above ~ 8000 to ~11,000, below ~8000 the range expands to include NW and NE aspects) with the accompanying melt/freeze cycles, on solar aspects as the sun races toward the spring equinox (March 21). Warm temperatures Saturday softened the snow on solar aspects with the upper 30 to 45 cm of the snowpack becoming wet with small wet point releases reported up to ~9000 reported. Sunday slightly cooler daytime temperatures slowed surface thawing. The recent warm-up has taken its toll on even northerly aspects below ~ 9000’ where the snow hasn’t fully transitioned to corn snow and can become sticky and thick (aka Manky), especially in sheltered trees. Above ~11000’, due to somewhat cooler temperatures, the sun effected surface snow is more narrowly confined to SW-S-SE aspects and has only began to moisten the surface snow, some light crusting possible.

The last snowfall in the area occurred on Sunday (3/5/17) was accompanied by very strong SW to West winds through Monday forming Wind Slabs on exposed W-NW-N-NE-E-SE aspects at all elevations with the mid to upper elevations most heavily impacted. The Wind Slabs that formed during that storm event have had several days to stabilize with spring-like conditions dominating the region since 3/7/17 with above average temperatures and light winds. As temperatures climbed the avalanche focus shifted toward loose-wet point releases on solar aspects in the mid to upper elevations, all but the most northerly aspects in the lower elevations where the overnight low temperatures are near or above freezing.

With spring conditions comes a firm melt-freeze crust on solar aspects in the AM hours (possibly later) and possibly firm wind-board on more northerly aspects. These snow conditions can make an arrest difficult on steeper terrain if a fall occurs (Slide For Life). If these conditions are encountered, crampons, ice ax, or a whippet maybe required to move through steep terrain safely. Caution in complex terrain with multiple aspects due to differential thawing and freezing. Careful snow assessment is recommended, reevaluate your plan as needed, and be prepared to alter your objective as required.

 

weather

Monday thru Tuesday – A high-pressure ridge remain over NV/CA through Tuesday, with a few high clouds moving over the ridge and into the area. Generally light winds are expected during this time with temperatures remaining 10-15 degrees above normal for mid-March. High temperatures in the Sierra valleys in the upper 50s to mid 60s.

Wednesday thru Thursday – Wednesday a shortwave trough moving into the Pacific Northwest will result in increasing high clouds, otherwise high pressure will continue to dominate with dry and warm daytime conditions prevailing across the area with generally light winds except for some afternoon/evening breezes. Temperatures will push into the 50s and 60s both days with mild nighttime low for this time of year.

 

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: Sunny Clear Sunny then becoming partly cloudy
Temperatures: 52 to 60 deg. F. 26 to 32 deg. F. 51 to 59 deg. F.
Wind direction: Light winds. Light winds. Light winds.
Wind speed: Gusts up to 30 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Monday Monday Night Tuesday
Weather: Sunny Clear Sunny
Temperatures: 47 to 55 deg. F. 22 to 28 deg. F. 44 to 52 deg. F.
Wind direction: Light winds becoming southwest Light winds becoming southwest Southwest
Wind speed: 10 to 15 mph with gusts to 25 mph in the afternoon. 10 to 15 mph after midnight. Gusts up to 25 mph. 10 to 15 mph with gusts to 35 mph.
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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