Avalanche Advisory - Tue, Jan. 31, 2017

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THIS ADVISORY EXPIRED ON February 2, 2017 @ 6:59 am
Avalanche Advisory published on January 31, 2017 @ 6:59 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

bottom line:

For Tuesday and Wednesday Natural and human triggered avalanches are unlikely at low to mid elevations.  Small isolated pockets of wind slab may be found in isolated terrain features at upper elevations that could be sensitive to human triggering.  While it is unlikely that such a release would result in a burial, it could knock a rider or skier off balance resulting in a fall thru undesirable terrain.     

Avalanche Character 1: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

Mostly calm winds over the past 2 days have given windslabs that developed during windier periods last week a good amount of time to stabilize.  With winds increasing out of the southwest today (Tuesday) and tomorrow in the 20-30mph range at all elevations, with gusts into the 50s and 60s over 10,000’, there is a possibility that some new small isolated wind slabs may form in exposed areas where loose snow still exists for transport.  Be especially cautious in steep complex terrain such as couloirs where a small isolated wind slab may be sensitive to human triggering, either lingering from last weeks variable winds or more likely newly formed from current winds.  These could most likely be found on E-NE-N-NW facing slopes above 11,000’.  If you are booting up a couloir and encounter smooth hollow sounding snow, do your own stability assessment with a quick hand pit to see how easily and cleanly the overlying dense slab fails before deciding to continue up.  If you are riding down a slope you haven’t checked out, beware of smooth pockets that could possibly pop on you. 

Snowpack Discussion

Over a week has passed since we have seen any new snowfall, which is the longest dry stretch we have seen yet this year!  The last impressive Atmospheric River (AR) event exited our area early last Monday Jan 23rd, leaving us with avalanche concern over windslabs from strong SW winds and storm slabs in sheltered areas, but finally relieving us of the worry over deep buried weak layers.  The storm was followed by a few day period of very cold calm weather, then a couple of days of moderate winds out of the North and East, which created some new wind slabs at upper elevations.  The weekend thru Monday brought a significant warm-up with spring like weather and calm winds (but extremely cold valley inversions!)  Older wind slabs on all aspects were given a chance to strengthen and stabilize, few new wind slabs formed, but some small wet instabilities were observed on solar aspects.  People started hitting big lines at high elevations, and observations were of mostly stable conditions throughout with occasional isolated areas were small wind slabs still raised concern.

For today and tomorrow (Tuesday and Wednesday), SW winds are again on the increase, but between the previous week’s wind events and the more recent melt-freeze occurring on solar aspects, areas where loose snow still exists for transport will be few and far between.  That being said, small isolated pockets of wind slab may still form that could be sensitive to human triggering.  The breezy conditions and slightly cooler temperatures Tuesday and Wednesday will likely keep even small wet instabilities at bay on solar aspects.

Besides avalanche concern, hazards to watch out for include corniced slopes, hard slick snow surfaces especially on aspects that face the sun (E-SE-S-SW-W), and something we don’t often see in the Sierra, creep cracks!  Unusual creep cracks have been observed throughout the forecast area that are large enough for a person to fall into.  Some of these have been found at high elevations on steep slopes at the base of cliff bands or couloirs, but others have been found at lower elevations mid-slope as well, below cliff bands that have been deeply buried and are no longer visible.  

weather

Tuesday:  High pressure will remain over the forecast area for Tuesday, with above average temperatures reaching the mid 30s to mid 40s above 10,000’, mostly sunny to partly cloudy skies, 20-30mph winds out of the southwest gusting into the 50mph range over 10,000’.    

Wednesday:  Expect high temperatures in the mid to upper 30s above 10,000’, increasing clouds throughout the day, and continued southwest winds in the 20-30mph with gusts increasing into the 60mph range above 10,000’. 

High pressure and valley inversions will last one more day before the ridge begins to break down Tuesday night ahead of another series of Pacific storms that are expected to bring snowfall and high winds beginning Wednesday night through Friday night.  Snowline could be as high as 7500’, dropping to 6,000’ by Friday afternoon, with 2-3ft of dense heavy new snow expected in the mountains by Saturday morning.  Models are beginning to show potential for another storm Sunday thru Monday, but it is too early for details with any confidence. 

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: PARTLY CLOUDY PARTLY CLOUDY PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY
Temperatures: 40 TO 46 deg. F. 21 TO 29 deg. F. 39 TO 45 deg. F.
Wind direction: SOUTHWEST SOUTHWEST SOUTHWEST
Wind speed: 10 TO 20 MPH WITH GUSTS TO 40 MPH 10 TO 15 MPH WITH GUSTS TO 35 MPH 10 TO 20 MPH WITH GUSTS TO 40 MPH
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: PARTLY CLOUDY PARTLY CLOUDY PARTLY CLOUDY THEN BECOMING MOSTLY CLOUDY
Temperatures: 35 TO 43 deg. F. 23 TO 29 deg. F. 33 TO 39 deg. F.
Wind direction: WEST SOUTHWEST SOUTHWEST
Wind speed: 20 TO 35 MPH WITH GUSTS TO 55 MPH 15 TO 25 MPH WITH GUSTS TO 45 MPH 20 TO 35 MPH WITH GUSTS TO 60 MPH
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

This Snowpack Summary 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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