Avalanche Advisory: Wednesday - Jan 24, 2018

THIS AVALANCHE ADVISORY EXPIRED ON January 25, 2018 @ 6:52 am
Avalanche Advisory published on January 24, 2018 @ 6:52 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center

Wednesday (1/24) - primary avalanche concern will focus on Wind Slabs from near treeline and above. Moderate to strong South to Southwesterly winds today and a good supply of transportable snow will form another round of Wind Slabs in the mid and upper elevations on W-NW-N-NE-E-SE aspects where the avalanche danger will begin to climb from MODERATE to CONSIDERABLE with the onset of precipitation this evening. Blowing snow, recent cornice formation, and fresh drifting on leeward slopes suggests new Wind Slabs.

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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A fast moving Pacific storm system will move into the region this afternoon and early evening with moderate to strong South to Southwesterly winds and up to 5 – 10” of new snow by Thursday. Even though snowfall is not forecasted to begin until later today into this evening, there is still plenty of snow from the previous storm system (1/19 – 1/20) to form fresh Wind Slabs primarily on W-NW-N-NE-E-SE aspects in favored locations, from near treeline and above during the day, keeping the hazard at MODERATE, and climbing to CONSIDERABLE as snow begins to accumulate this evening. This new round of Wind Slabs in the mid to upper elevations will only add to an already complex snowpack. Wind Slabs can be encountered on the Leeward side of ridgelines, under cornices, and cross-loaded terrain features in steeper upper elevation 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 has weakened 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. Currently, this layer is not widely reactive but it will need to be monitored as additional snowload is applied by the approaching 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 high consequences.

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

 

advisory discussion

Another storm begins to move into northern California today (Wed) with moderate to strong South to Southwesterly winds and up to 5” to 10” of new snow possible by Thursday AM. Mammoth Mountain has seen steady moderate to strong Southwesterly winds overnight (Tues) and is forecasted to continue thru Wednesday night into Thursday AM. With the forecasted moderate to strong winds, downwind fetches still have enough transportable snow to produce another round of Wind Slabs in the mid to upper elevations on W-NW-N-NE-E-SE aspects today and bump the hazard from Moderate to Considerable by Thursday AM as new snow accumulates and begins to form additional Wind Slabs more broadly throughout mid and upper elevations. Watch for signs of blowing snow and wind loading (snow banners, recent cornice formation, and fresh drifts).

Treeline and below (especially in the Mammoth area) - due to the recent cold temperatures the persistent weak layer that has plagued the region through much of the early part of the season is showing signs of further weakening with possible failure in isolated locations from ~9,000’ to ~10,500’ , 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. Currently, this layer is not widely reactive but it will need to be closely monitored as the snowpack adjusts to the new snowload from the approaching storm system. Currently the possibility remains LOW: Natural and triggered avalanches are unlikely but not impossible and the resulting avalanche could be large. Whumphs (sudden collapse) are a strong sign of instability. Do your own stability assessments, especially as you enter steeper or complex terrain.

There is an off chance that areas that receive the upper end of the forecasted snowfall (~10” +) may see an elevated concern for localized storm slabs, primarily in the mid elevations.

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

 

Weather and CURRENT CONDITIONS
weather summary

Wed thru Thursday - Leading edge of precip band will surge into the region this afternoon and evening as upper trough axis and strong Pacific jet reach the CA coast then swing inland. The short duration of this event will see heavy snowfall rates peaking at 2-3"/hour. Winds will continue increase ahead of the precipitation with gusty conditions beginning by early evening in Mono County with gusts of 50-60 mph. Initial snowfall looks to arrive around 8-9 pm for western Mono County. Snow through early Thursday morning with the heaviest snow most likely to occur between 9 pm-1am (Wed). The band loses some of its intensity and coverage as it pushes south, which will limit snow amounts to about 3 - 6 inches west of US-395, 5- 10 inches along the Sierra crest. Gusty winds combined with heavy snow could produce periods of zero visibility in white out conditions. The snow shower activity will wind down later Thursday evening after the shortwave passes to the east.

Friday thru Saturday - generally dry conditions as flat ridge builds into CA-NV.

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Partly cloudy then becoming mostly cloudy. Cloudy. Snow. Mostly cloudy then becoming partly cloudy.
Temperatures: 38 to 44 deg. F. 16 to 22 deg. F. 22 to 30 deg. F.
Wind Direction: South Southwest Southwest
Wind Speed: 20 to 30 mph increasing to 25 to 40 mph in the afternoon. Gusts up to 75 mph. 30 to 45 mph with gusts to 80 mph decreasing to 20 to 30 mph with gusts to 65 mph after midnight. 15 to 25 mph. Gusts up to 50 mph decreasing to 40 mph in the afternoon.
Expected snowfall: 0 in. 2 to 6 in. 1 to 3 in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: Partly cloudy then becoming mostly cloudy. Cloudy. Snow. Mostly cloudy. Scattered snow showers in the morning, then widespread snow showers in the afternoon.
Temperatures: 30 to 37 deg. F. 8 to 13 deg. F. 15 to 21 deg. F.
Wind Direction: South shifting to the Southwest Southwest Southwest
Wind Speed: 35 to 55 mph, 45 to 60 mph in the afternoon. Gusts up to 90 mph. 50 to 70 mph with gusts to 105 mph decreasing to 40 to 60 mph with gusts to 95 mph after midnight. 30 to 45 mph with gusts to 75 mph decreasing to 20 to 30 mph with gusts to 45 mph in the afternoon.
Expected snowfall: 0 in. 2 to 6 in. 1 to 3 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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