Eastern Sierra Avalanche Advisory - 1/9/18

Please help!  Post your observations to our site under the "Participate" Tab... Click Here to Find Out How

THIS AVALANCHE ADVISORY EXPIRED ON January 11, 2018 @ 6:58 am
Avalanche Advisory published on January 9, 2018 @ 6:58 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

With up to a foot of new snow over the past 24 hours at the highest elevations, and the potential for another 6” during the day today, avalanche danger has risen especially for elevations over 9,500’ due to wind slabs, storm slabs, and the effect of this added weight and slab over the persistent weak layers in the underlying snowpack. 

Tuesday:  Human triggered avalanches will be likely, and natural avalanches possible, especially for areas that received the higher amounts of new snow.  Careful snowpack evaluation, cautious routefinding and conservative decision-making essential.  

Wednesday:  Human triggered avalanches will be possible, and natural avalanches unlikely. 

No Rating

?

Above Treeline

No Rating

?

Near Treeline

No Rating

?

Below Treeline
Avalanche Problem 1: Wind Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Up to 12” of new snow has fallen in the past 24 hours, and 6” more may fall during the day today.  While winds have been much lighter than typical with this storm, moderate SW winds have still been strong enough to blow snow into sensitive wind slabs.  These are likely to found on the leeward side of ridges, the sidewalls of gullies and around other features that promote drifting, especially at higher elevations.  S winds today will gradually shift to W tonight and then to NW tomorrow, meaning wind slabs forming on a variety of aspects.  Be on the lookout for dense smooth snow, and avoid steep terrain at upper elevations where these may exist for today (Tuesday), and do your own assessments on Wednesday to determine how well these wind slabs are stabilizing.

Avalanche Problem 2: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

The existence of weak, sugary, faceted snow in the underlying snowpack is widespread throughout our area.  Snowpit tests have been resulting in propagating failures in many areas.  While prior to this storm little to no actual avalanche activity has been reported on these layers, it is quite possible now with this additional snow load that areas may exist where it is possible for an avalanche to be triggered in these deeper layers.  This risk will not likely decrease for Wednesday. 

Avalanche Problem 3: Storm Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Up to 12” of new snow has fallen in the past 24 hours, and 6” more may fall during the day today.  Temperatures have been relatively warm, meaning storm slabs may be more cohesive and dense than if the storm were colder, increasing the chance of a slab release in sheltered terrain as opposed to mere sloughing.  Be careful especially around steep convex rolls, and remember even small slides that end in a terrain trap can result in a burial.

advisory discussion

A complex snowpack and avalanche situation exists at the moment.  The latest storm did not bring nearly as much snow as anticipated, and snowlines remained quite high for the bulk of it.  While this definitely resulted in less of an increase in overall avalanche danger, it actually added more complexity to the avalanche problem.  Had we gotten 3’+ of new snow, widespread natural avalanches would have been much more likely, hopefully resulting in either the cleaning out of some of the deeper weak layers that have been the focus of recent concern, or burying them deeper where they can begin to start strengthening.  As it turns out, enough snow has fallen likely to have led only to a few isolated natural avalanches, leaving the questionable snowpack even more questionable and perhaps closer to the brink of slope-wide failures.  The wind slab and storm slabs are the immediate and obvious problems for today and tomorrow.  Less obvious is the possibility that one of these smaller slides could be the stress necessary to trigger one of these deeper layers and result in a much larger more destructive avalanche.  As the storm slab and wind slab problems settle, then we are left with overall more weight and slab ontop of the weaker underlying pack, where a person could conceivably find a thinner weaker spot that could propagate a failure across an entire slope.  It will be heads up out there for awhile!

Weather and CURRENT CONDITIONS
weather summary

The Winter Storm Warning initially issued for today has been dropped to a Winter Weather Advisory.  The highly anticipated Low pressure system that was hoped to move in directly over our area has headed further south, resulting in much less snow than anticipated, and snowlines falling much slower than expected, remaining quite high above 8500’ as of this morning. 

For today (Tuesday), another ½” of water could fall at the Sierra Crest, with much less further down-slope.  Temperatures will drop throughout the day into the mid 20s around 10,000’, and moderate S winds will gradually decrease.

For tomorrow (Wednesday), expect a mix of clouds and sunshine, temperatures reaching the upper 30s at 10,000’, and shifted moderate winds out the NW.

Longterm:  High pressure is expected until the 3rd week of January, when the chances of more low pressure moving in and more moisture increases.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Cloudy. Snow showers likely. Mostly cloudy. Snow showers likely in the evening, then slight chance of snow showers after midnight. Mostly cloudy then becoming partly cloudy. Slight chance of snow showers in the morning.
Temperatures: 35 to 40 deg. F. 20 to 25 deg. F. 39 to 45 deg. F.
Wind Direction: SE W W
Wind Speed: 10 to 15 mph with gusts to 35 mph in the morning becoming light. Light winds becoming west 10 to 15 mph after midnight. Gusts up to 30 mph. 10 to 15 mph with gusts to 45 mph.
Expected snowfall: 2-4 in. up to 1 in. 0 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Cloudy. Snow showers likely. Mostly cloudy. Snow showers likely in the evening, then slight chance of snow showers after midnight Mostly cloudy then becoming partly cloudy. Slight chance of snow showers in the morning.
Temperatures: 29 to 34 deg. F. 16 to 21 deg. F. 33 to 38 deg. F.
Wind Direction: S W NW
Wind Speed: 20 to 30 mph with gusts to 50 mph decreasing to 10 to 15 mph with gusts to 30 mph in the afternoon. 15 to 20 mph with gusts to 30 mph increasing to 20 to 30 mph with gusts to 40 mph after midnight. 25 to 35 mph with gusts to 55 mph.
Expected snowfall: 3-6 in. up to 1 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.

ESAC receives support from ...