Avalanche Advisory: Tuesday - Jan 15, 2019

THIS AVALANCHE ADVISORY EXPIRED ON January 16, 2019 @ 7:00 am
Avalanche Advisory published on January 15, 2019 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

MODERATE avalanche danger exists this morning primarily due to wind slabs in exposed mid to upper elevations and a lingering underlying weak snowpack concern.  As snowfall increases this afternoon and evening with south winds, Avalanche danger will increase to CONSIDERABLE due to new wind slab formation, storm slab formation in sheltered areas, and the increasing load on the underlying persistent slab problem.

3. Considerable

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Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

3. Considerable

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Near Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

2. Moderate

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Below Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
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The wind slab problem is a bit complex today.  Small new wind slabs likely formed at upper elevations yesterday and last night from 2-3” of new snowfall and relatively light (30-40mph) but ideal wind-speeds for wind slab formation.  These winds were out of the south, so expect to find these small wind slabs just below ridgelines, the sidewalls of gullies, and around other features that promote drifting in the alpine on E-N-W facing slopes.  Strong E winds over the weekend led to sensitive wind slabs at upper elevations.  2 reported skier triggered avalanches occurred on Sunday, one of which resulted in a skier being swept away and partially burial (click here for details).  As snowfall picks up substantially this afternoon and tonight accompanied by continued moderate south winds, wind slabs will grow and become much more widespread at mid to upper elevations on exposed E-N-W facing slopes.  Be cautious of denser pockets of wind deposited snow this morning, and avoid fresh wind deposits later in the day. 

Avalanche Problem 2: Storm Slab
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As new snow accumulates this afternoon and evening storm slab avalanches will become a concern in sheltered areas that receive half a foot or more of new snow on all aspects. The loose faceted old snow surface as well as surface hoar formation in sheltered areas will make it difficult for this new snow to bond and make for sensitive triggering.  This loose old snow could also become entrained in a slide making it larger than expected. 

Avalanche Problem 3: Persistent Slab
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Layers of weak loose faceted snow have been found deeper (50-100cm down) in the snowpack throughout the forecast area.  Snowpit tests continue to show that this layer is reactive.  Although no recent avalanches have been reported involving these deeper layers, the additional snow load expected this afternoon, tonight, and especially later in the week will make these deeper layers quite concerning.  While a persistent slab avalanche will be hard to trigger, the result will likely be very large and destructive.  Do your own localized assessments of slopes by digging down to check on the deeper snowpack before committing to steeper terrain where this problem may exist.

advisory discussion

The persistent slab problem is a notoriously tricky one.  Just because we have not seen activity occurring on these deeper layers, doesn’t mean they are not concerning.  These kinds of problems are low likelihood, but high consequence. Additional snow load can disrupt their balance and lead them to re-awaken.  Areas in the forecast zone with thinner snow packs are most concerning, such as Rock Creek south to Bishop, but these weak layers have been found even in the deeper snowpack surrounding Mammoth.

recent observations
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-1/13 – Bridgeport – Sawtooth Ridge: Partial burial, skier triggered wind slab avalanche.  

-1/14 – Mammoth – Sherwins: Buried surface hoar found just under today’s several cm of new snow in more open areas between trees, small loose dry activity, poor structure.

-1/14 – Mammoth – Sherwins:  Widespread surface hoar in open areas, Propagating ECT test 90cm down.

-1/14 – Mammoth Lakes Basin: Widespread surface hoar, wind slab failures and persistent slab failures in tests.  

-1/13 – Convict – Morrison Col: Sensitive fresh wind slab triggered from ridgetop.

-1/13 – Mammoth – San Joaquin Ridge: New large cornice growth, patchy wind slabs, faceted near-surface snow leading to small loose dry activity.

Weather and CURRENT CONDITIONS
weather summary

2-3” of new snow fell yesterday afternoon and evening.  A slight chance of flurries this morning will lead to heavier snowfall this afternoon and tonight with up to half a foot likely before dark, and up to a foot+ more tonight.  Snow levels will gradually rise to around 6000’.  Light to moderate south winds are expected, with stronger gusts in the afternoon and tonight.  

After a brief break during the day Wednesday, major snowfall is expected Wednesday night thru Thursday night with strong damaging SW winds.  

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: Mostly cloudy. Slight chance of snow in the morning, then snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 90%. Mostly cloudy. Snow in the evening, then chance of snow after midnight. Snow levels below 7000 feet. Chance of precipitation is 100%. Mostly cloudy then becoming partly cloudy. Scattered snow showers through the day. Snow levels below 7000 feet. Chance of precipitation is 55%.
Temperatures: 26 to 32. deg. F. 22 to 27. deg. F. 31 to 37. deg. F.
Mid Slope Winds: South 10 to 20 mph with gusts to 35 mph. South 15 to 25 mph with gusts to 45 mph. South 15 to 20 mph increasing to 15 to 30 mph in the afternoon. Gusts up to 65 mph.
Expected snowfall: 70% probability of 2 to 4 inches. 30% probability of 4 to 8 inches. | SWE = up to 0.40 inch. in. 70% probability of 5 to 8 inches. 30% probability of 8 to 12 inches. | SWE = 0.40-0.70 inch. in. 80% probability of 1 to 3 inches. 20% probability of 3 to 5 inches. | SWE = up to 0.20 inch. in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Mostly cloudy. Slight chance of snow in the morning, then snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 95%. Cloudy. Snow. Snow levels below 7000 feet. Chance of precipitation is 100%. Mostly cloudy then becoming partly cloudy. Widespread snow showers in the morning, then isolated snow showers in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 75%.
Temperatures: 19 to 25. deg. F. 18 to 23. deg. F. 24 to 30. deg. F.
Ridge Top Winds: South 20 to 35 mph with gusts to 50 mph. South 25 to 35 mph shifting to the southwest 30 to 45 mph after midnight. Gusts up to 65 mph. Southwest 30 to 50 mph increasing to 40 to 55 mph in the afternoon. Gusts up to 85 mph.
Expected snowfall: 70% probability of 3 to 6 inches. 30% probability of 6 to 10 inches. | SWE = 0.35-0.70 inch. in. 70% probability of 6 to 10 inches. 30% probability of 10 to 15 inches. | SWE = up to 0.85 inch. in. 80% probability of 1 to 3 inches. 20% probability of 3 to 6 inches. | SWE = up to 0.20 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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