Avalanche Advisory: Thursday - Jan 25, 2018

THIS AVALANCHE ADVISORY EXPIRED ON January 26, 2018 @ 7:00 am
Avalanche Advisory published on January 25, 2018 @ 7:00 am
Issued by Josh Feinberg - Eastern Sierra Avalanche Center

CONSIDERABLE avalanche danger in the form of new sensitive wind slabs will exist today for areas north of Bishop that received 2-6+” of new snow last night and today due to strong SW to W winds.  Avoid being on or under steep wind loaded slopes where human triggered avalanches will be likely, and natural avalanches possible.  It is also possilbe that in some areas the persistent sugary weak layers deeper down in the snow could fail with this added slab of new snow and the right trigger (which could be a smaller wind slab avalanche, or a human hitting a sweet spot) and that a resulting avalanche could be much larger than expected.     

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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Fresh dense wind slabs sensitive to human triggering began forming yesterday at upper elevations due to strong S to SW winds even before new snow started to fall after midnight last night.  A fresh supply of 2-6+” of new snow which fell overnight and early this morning, and another couple of inches possible thru today combined with continued strong SW winds expected to last thru most of today has resulted and will continue to result in the formation of dangerous wind slabs.  These will most likely be found in leeward terrain below ridgelines, on the sidewalls of gullies, across slopes, and around other features that promote drifting.  Avoid being on or under steep areas of smooth dense new snow.  And remember that winds can deposit snow MUCH faster and deeper than snow simply falling from the sky.

Avalanche Problem 2: Persistent Slab
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The shallow snowpack throughout much of our area has resulted in the formation of loose sugary layers of faceted snow deeper down in the snow.  Snow-pit tests have continued to show throughout most of the winter that these layers have the potential to fail cleanly, but have lacked an overlying slab of snow significant enough to cause avalanches.  2-6+” of new snow, especially if it is wind deposited into greater depths at greater densities, may very well be the added ingredient that has been missing for a sizeable dangerous avalanche to be triggered.  Areas of greatest concern are non-southerly facing slopes at tree-line and below.  A human hitting the sweet spot, or the force of a smaller wind slab avalanche could be enough to act as a trigger resulting in a much larger than expected avalanche, potentially failing far above the trigger.  This is a tricky problem that is hard to detect without digging, so it is best to avoid steep areas where it may be present that have a new slab of snow on top.  Pay attention to signs such as whoomphing on lower angle terrain as indications that a deeper weak layer has failed, and that if you were on a steeper slope, an avalanche could have resulted. 

advisory discussion

Avalanche concern can be at its greatest during and soon after significant loading events such as heavy snowfall, or wind blowing and depositing snow.  While the 2-6+” of low density new snow that this latest storm has brought in may not be a significant load in and of itself, strong SW winds depositing this new snow into much deeper and denser drifts certainly is.  Add to this the underlying shallow and weak snowpack that exists in many areas at tree line and below, and the avalanche problem becomes complex.  Pay attention to signs such as the sound of whoomphing on lower angle terrain as indications that these deeper weak layers could be failing, and on steeper slopes could result in sizeable dangerous avalanches.  

Weather and CURRENT CONDITIONS
weather summary

A fast moving cold Pacific storm is moving thru our area early this morning, with peak snowfall occurring before dawn dropping 1-7” of snow (greatest around mammoth, lesser to the north, and much less south toward Bishop).  Another few inches may fall this afternoon as another wave moves in.  June area may receive some continued Mono lake effect light snowfall thru the day.  Strong SW winds at upper elevations will decrease to moderate levels in the afternoon, and will continue to decrease into the evening and thru Friday out of the west. 

The weekend will be dry, with average temperatures, and mostly sunny skies. 

At the moment long-term does not look good for Sierra snowfall as a high pressure ridge will set in next week which models are showing will likely remain in place thru the first week of February, and possibly beyond. L

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy then becoming partly cloudy. Snow showers. Mostly cloudy then becoming partly cloudy. Scattered snow showers through the night. Mostly cloudy.
Temperatures: 20 to 28 deg. F. 7 to 12 deg. F. 29 to 35W deg. F.
Wind Direction: W W Light
Wind Speed: 15 to 25 mph. Gusts up to 50 mph decreasing to 35 mph in the afternoon. 10 to 15 mph with gusts to 35 mph. Light winds. Gusts up to 25 mph in the morning.
Expected snowfall: 1-3 in. up to 2 in. 0 in.
Over 10,000 ft.
Thursday Thursday Night Friday
Weather: Mostly cloudy. Snow showers. Mostly cloudy then becoming partly cloudy. Widespread snow showers in the evening, then isolated snow showers after midnight. Mostly cloudy.
Temperatures: 13 to 18. deg. F. 2 to 7. deg. F. 23 to 28. deg. F.
Wind Direction: SW W W
Wind Speed: 0 to 45 mph with gusts to 75 mph becoming west 20 to 30 mph with gusts to 45 mph in the afternoon. 20 to 30 mph with gusts to 45 mph. 10 to 15 mph with gusts to 30 mph.
Expected snowfall: 1-3 in. up to 2 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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