Avalanche Advisory: Tuesday - Jan 23, 2018

THIS AVALANCHE ADVISORY EXPIRED ON January 24, 2018 @ 6:30 am
Avalanche Advisory published on January 23, 2018 @ 6:30 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Tuesday (1/23) - primary avalanche concern continues to focus on Wind Slabs from treeline and above. With plenty of transportable snow and light to moderate Southwesterly to Westerly winds Monday and Monday night, anticipate Wind Slabs from treeline into the upper elevations where the avalanche danger remains MODERATE. Blowing snow and recent cornice formation on leeward slopes suggests new Wind Slabs.

Mammoth Area – the Persistent weak layer from early in the season is showing signs weakening with possible failure in isolated locations between ~10,500’ and ~9,000’, though the possibility remains LOW: Natural and triggered avalanches are unlikely but not impossible and the resulting avalanche could be large. Whumphing and cracking are signs of sudden failure.

Below ~9,000’, natural and triggered releases are unlikely due to thin snow coverage (below threshold).​

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.

1. Low

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Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    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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The recent storm (1/19 – 1/20) delivered a welcomed 4”- 7” of snow for Mammoth and 6”- 8” for June and Virginia Lakes. The low-density new snow was deposited with minimal wind effect and uniformly distributed, which left Windward slopes primed with plenty of transportable snow. Since then, light to moderate Southwesterly to Westerly winds have created isolated Wind Slabs primarily on NW-N-NE-E-SE aspects in favored locations, treeline and above. Southwesterly winds are forecasted to increase once again this evening (Tuesday), which will likely produce another round of fresh Wind Slabs. Anticipate Wind Slabs on the Leeward side of ridgelines, under cornices, and cross-loaded terrain features in steeper terrain.  

Avalanche Problem 2: Persistent Slab
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Patchy Persistent Slab issues continue to lurk within the snowpack. Recent cold temperatures have increased the temperature gradient within the upper snowpack. As a result, the upper snowpack is loosing strength over time, leaving the Persistent Slab weakness teetering just below failure in some locations, while other areas will require additional loading to fail. Recent reports of Whumphing (9150’) above Lake George with tests showing fracture propagation highlight this concern. Though observations are limited, this needs to be monitored for further deterioration, especially as additional snowload is applied with the upcoming Wed-Thursday storm system. Perform your own stability tests to assess the snowpack of the slope you want to ride. In isolated locations, large Wind Slab avalanches may overload the persistent weakness and step-down into the snowpack with much high consequences.

Below ~9,000’- below threshold, natural and triggered releases are unlikely due to low snow coverage.​ 

 

advisory discussion

Unseasonably cold temperatures have settled into the region with light to moderate Southwesterly winds as another Pacific Northwest storm begins to take aim at northern California on Wednesday. The most recent storm (1/19 -1/20) yielded 4” to 7” of snow in the Mammoth region, 6” to 8” June Lake and north with little wind effect, leaving a very uniform blanket of light transportable snow. Since then, Monday saw steady southwesterly – westerly/15 to 25 mph as a weak disturbance passes to the north of the forecast region, forming fresh Wind Slabs on NW-N-NE-E-SE-S aspects. Southwesterly winds are forecasted to decrease to 10 to 15 through the day (Tuesday) but pick back up by evening ahead of an approaching storm system due to move into the region Wednesday. Light to moderate winds will continue build wind slabs near and above treeline until downwind fetches are exhausted. Watch for signs of blowing snow and wind loading (snow banners, recent cornice formation, and fresh drifts). Caution is recommended in steep alpine terrain.

Treeline and below - due to the recent cold temperatures the persistent weak layer that has plagued us through much of the early part of the season is showing signs of further weakening in the Mammoth area, renewing concerns for potential deep releases, if this trend continues. A recent report of whumphing in the Mammoth Lakes Basin and tests indicating propensity for propagation highlights this problem layer and the potential for failure. Whumphs (sudden collapse) are a strong sign of instability. Do your own stability assessments, especially as you enter steeper or complex terrain.

Below ~9,000’ the new snow will not be enough to cover the rocks and brush poking up from the surface.

 

Weather and CURRENT CONDITIONS
weather summary

Tues - Flat ridge will keep dry and mild conditions with light winds through today.

Wed thru Thursday - A stronger, colder winter storm system moves in to the region Wednesday into Thursday, bringing strong winds and snow. Winds will increase Wednesday ahead of the cold front, with wind gusts 45-60 mph across the advisory areas. As the cold front moves through the area, a band of moderate to heavy snow will push south along the Sierra into Mono County overnight through Thursday morning. Snowfall rates may peak around 2" per hour in the Sierra. Snow levels will start rather low, near 5000-5500 feet Wednesday afternoon. Heaviest snow will be north of Mono County. Snowfall amounts through Thursday of 2 - 5 inches are likely along US-395 with 5 - 10 inches west of US-395 to the Sierra Crest with continued lingering light snow showers producing minimal additional accumulations.

 

 

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: Sunny Partly cloudy Partly cloudy then becoming mostly cloudy
Temperatures: 39 to 45 deg. F. 21 to 27 deg. F. 37 to 44 deg. F.
Wind Direction: Light winds Light winds becoming southwest South veering to Southwest
Wind Speed: 10 to 15 mph with gusts to 35 mph after midnight. 20 to 30 mph with gusts to 65 mph increasing to 30 to 45 mph with gusts to 80 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Tuesday Tuesday Night Wednesday
Weather: Sunny Partly cloudy Partly cloudy then becoming mostly cloudy.
Temperatures: 33 to 38 deg. F. 17 to 22 deg. F. 29 to 36 deg. F.
Wind Direction: Southwest Southwest Southwest
Wind Speed: 10 to 15 mph. Gusts up to 25 mph in the afternoon. 15 to 20 mph with gusts to 30 mph increasing to 20 to 30 mph with gusts to 50 mph after midnight. 30 to 50 mph with gusts to 80 mph increasing to 50 to 65 mph with gusts to 100 mph in the afternoon.
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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