Avalanche Advisory: Tuesday - Feb 2, 2021

THIS AVALANCHE ADVISORY EXPIRED ON February 3, 2021 @ 7:00 am
Avalanche Advisory published on February 2, 2021 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

SW winds have been honking since yesterday morning and are expected to continue thru tonight. Fresh cornices and Wind Slabs lead to a solid MODERATE avalanche danger at and above treeline on NW-NE-SE facing terrain. Scary MODERATE Persistent Slab avalanche danger continues at all elevations due to the weak sugary faceted snow underlying last week’s storm snow on W-N-E-SE facing slopes.  Avalanche danger may rise to CONSIDERABLE for Wind Slabs at upper elevations tonight for the northern reaches of the forecast zone which could receive several inches of new snow.

*Join ESAC Forecaster Steve Mace for Free Virtual Avalanche Awareness Event - Wed Feb 3rd (6-7:30pm): Avalanches 101.  Click here to access Zoom Event!

2. Moderate

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Above Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Wind Slab
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SW winds have been CRANKING consistently in the 60-70mph range over ridgetops since early yesterday morning and are expected to continue thru tonight. Large cornices and fresh wind slabs can be found on NE-NW-SW facing slopes.  Use clues such as cornice formation, blowing snow, and surface sculpting to help you figure out where new denser sensitive wind slabs may exist.  Human triggered avalanches are possible where wind loaded snow exists especially below ridgelines and side walls of gullies, and could range from sensitive to stubborn. A natural avalanche could occur in an isolated area from today’s loading as well.  It is also possible that a smaller wind slab avalanche could be enough to trigger a deeper persistent slab avalanche.  With new snow fall of a few inches possible tonight mainly for the northern parts of the forecast zone, natural wind slab avalanches may become more possible.  

Avalanche Problem 2: Persistent Slab
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The snowpack underlying all of last week’s storm snow is sketchy in many areas consisting of layers of weak sugary facets.  The many feet of new snow has settled, consolidated and strengthened on-top, but the underlying fragile facets are still there. A great deal of uncertainty exists with this trickiest of all avalanche problems. As time passes and the snowpack adjusts it will become harder and harder to trigger these deep weak layers, but for now thin sweet spots are likely to exist where a rider could trigger a large and deadly slide.  A skier triggered one of these 3 days ago, and was extremely fortunate to get caught-up in a tree with relatively minor injuries rather than taken for a long ride and buried deeply.  Staying off and out from under slopes over ~30° facing SE-E-N-W, or sticking to slopes that you know were bare ground before this storm are sure ways to avoid this problem, otherwise you could be rolling the dice with your life. Dig down deep into the snow and check for yourself to help you make more informed decisions.    

Avalanche Problem 3: Cornice
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Winds have been extremely consistent out of the SW since the huge storm last week, and were very strong since yesterday morning.  Large cornices have been forming at ridgelines. Give the edges of slopes that face NW-NE-SE a wide berth as cornices can break off much further back than expected, as happened yesterday.  Staying out from under large cornices is wise as well, as these could break off as they continue to build with today's continued wind transport.  A large cornice failing could also trigger a wind slab avalanche below.  

advisory discussion

The persistent slab problem that we have is not a common one we see in the Eastern Sierra.  Snow fell in November, and then sat around for nearly 2 months becoming weaker and weaker before 30-100” of new snow (4-8.5” of water) fell last week covering it up.  Add to that a new thinner faceted weak layer that formed near the top of the old snow in some places just before this storm and the situation becomes even more complex.  This top layer of facets seems more sensitive than the thicker older layer, and looks to be the culprit of the avalanche that was triggered on Punta Bardini on Saturday that could have killed the skier who triggered it.  This is a situation where you could get lucky and ride over a dangerous slope unscathed that has a shallow sensitive trigger point, or you may hit that spot and get caught in a large avalanche. Its recommended to keep it reeled in for a while longer and give some more time for confidence to grow in this suspect snowpack. 

Some may have noticed a change in the aspects of concern for this problem over the last couple of days, which is a result of the high level of uncertainty and the limited data we have especially on SE and W facing slopes.  If you are out on one of these aspects, we would really appreciate hearing what you find with your shovel under all this new snow.    

recent observations
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-2/1 – Mammoth – Slopes above 395: High elevations winds, human trigger of large fresh cornice, weak deeply buried facets.

-2/1 – Bishop Creek – Table Mtn: High winds transporting snow, weak basal facets.

-2/1 – Convict – Hippy Trees: High winds, basal facets.

-2/1 – Mammoth – Punta Bardini: Weak basal facets.

-2/1 – Mammoth – High winds transporting snow over surrounding peaks.

-1/30 - Mammoth Area – Punta Bardini: Skier triggered and caught in large persistent slab avalanche

-1/31 – Bishop – Habeggars: Weak basal facets, collapses, propagating test results

-1/31 – Near June – Chicken Wing: Propagating test results on Weak deeply buried near-surface facets

-1/31 – Bishop – South Lake: Whoomphing bellow tree line on west aspect.

-1/31 - Bishop – Bishop Bowl: Weak basal facets, new snow consolidating

-1/31 – Bishop – Bishop Bowl: Weak basal facets, wind effects

-1/31 – Convict – Mini Morrison: Avalanches near end of storm

-1/31 – Convict – Batch Plant Bowls: Strengthening snowpack where bare-ground existed before storm

-1/31 – Bishop – Buttermilk Country: Melt-freeze crust developing on sunny aspects

Weather and CURRENT CONDITIONS
weather summary

The anticipated storm for today has been splitting and weakening for our area.  Still, chances of up to 1-2” of snow are possible this afternoon, and up to 3-7” tonight for the northern portion of the forecast area, with far less likely for Mammoth and south.  Expect partly cloudy skies for today with very strong winds out of the SW gusting into the 80mph range, decreasing ever so slightly for tonight.  Temperatures should reach just below freezing around 10,000’.  

After this cold front passes, dry conditions and mild temperatures are expected at least thru the weekend.

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. Chance of snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 30%. Mostly cloudy. Chance of snow through the night. Snow levels below 7000 feet. Chance of precipitation is 75%. Partly cloudy. Slight chance of snow through the day. Snow levels below 7000 feet. Chance of precipitation is 20%.
Temperatures: 30 to 38. deg. F. 16 to 21. deg. F. 22 to 30. deg. F.
Mid Slope Winds: Southwest 20 to 30 mph with gusts to 60 mph. Southwest 15 to 30 mph with gusts to 50 mph. Southwest 15 to 30 mph with gusts to 50 mph.
Expected snowfall: 80% probability of up to 2 inches. 20% probability of up to 1 inch. | SWE = less than 0.10 inch. in. 70% probability of 3 to 7 inches. 30% probability of up to 3 inches. | SWE = 0.15-0.40 inch. in. 70% probability up to 1 inch. 30% probability no accumulation. | SWE = less than 0.10 inch. in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Partly cloudy. Chance of snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 30%. Mostly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 85%. Mostly cloudy then becoming partly cloudy. Chance of snow through the day. Snow levels below 7000 feet. Chance of precipitation is 30%.
Temperatures: 22 to 30. deg. F. 11 to 16. deg. F. 15 to 23. deg. F.
Ridge Top Winds: Southwest 40 to 60 mph decreasing to 30 to 50 mph in the afternoon. Gusts up to 85 mph. Southwest 30 to 45 mph with gusts to 70 mph. Southwest 35 to 55 mph decreasing to 30 to 45 mph in the afternoon. Gusts up to 85 mph.
Expected snowfall: 80% probability up to 2 inches. 20% probability up to 1 inch. | SWE = less than 0.10 inch. in. 70% probability of 3 to 7 inches. 30% probability of up to 3 inches. | SWE = 0.20-0.45 inch. in. 70% probability up to 1 inch. 30% probability no accumulation. | SWE = less than 0.10 inch. 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 Avalanche Advisory is provided by the Eastern Sierra Avalanche Center, who is solely responsible for its content.

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