Avalanche Advisory: Monday - Dec 23, 2019

THIS AVALANCHE ADVISORY EXPIRED ON December 24, 2019 @ 6:49 am
Avalanche Advisory published on December 23, 2019 @ 6:49 am
Issued by Chris Engelhardt - Eastern Sierra Avalanche Center

CONSIDERABLE avalanche danger exists at Upper & Middle elevations on Northerly – Easterly Aspects. MODERATE avalanche danger exists at Lower elevations. Wind Slab & Loose Dry Snow avalanches will be of primary concerns with 5-9” of fresh snow and transporting SW winds. Cautious route finding is important today!

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 wind slabs can be expected adjacent to ridgelines on North through Easterly aspects and specific to terrain features that capture snow. South-Westerly winds were fairly strong during the heaviest accumulation periods yesterday afternoon, and have continued in the moderate to light range as light steady snowfall fell throughout the night with relatively cool temps. New snow depths could certainely be much greater at upper elevations. Todays projected light to moderate southerly winds are optimal velocities for transporting light, fresh snowfall to leeward terrain. Look out for red flags such as active wind loading, recent avalanche activity and shooting cracks while you travel today.

Avalanche Problem 2: Loose Dry
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Loose-dry snow sloughing will be of potential hazard today due to the light/moderate winds and relatively cool temperatures present during this storm.  A multitude of surface conditions were present before this fresh snowfall including temperature crusts, slick wind board, and glassy slopes where wind has eroded the snowpack down to persisting rain crusts. Investigation and conservative decision-making will be needed in determining terrain choice today… especially on slopes over 35degrees. Avoid terrain traps and be critical of steep, confined, and convex slopes with exposure to fall hazards.

advisory discussion

Surface conditions before last nights storm consisted of the full gamut of wind shorn, eroded wind slab to shiny slick rain/temperatures crusts and some residual recycled powder in sheltered areas. Relatively cold temperatures coinciding with this storm will make it interesting to see how well the new snow bonds to all those residing surfaces. Up to 10” of snow fell in the Mammoth area and averaged around 5” across other parts of the zone from north to south. Nearly 1” SWE (Snow Water Equivalent) was recorded in the Mammoth snowfall totals which is a decent amount of new weight being added to the snowpack. We need to be cognizant of how, or if, this new added weight potentially affects some of the weaker snow associated with the Dec 12 rain crust that currently exists in the snowpack. Northerly-Easterly aspects around 9300-10500ft is where this rain crust is buried, averaging around a foot below the surface and in some wind-loaded places much deeper. This rain crust and associated weaker faceted snow either above or beneath it has been showing a propensity for propagation on snow stability tests. This new added weight of snow and a skier trigger could be the right recipe for the snow to "step-down" to these rain crusts and start a slab avalanche.

Be observant today for signs of instability such as recent avalanches, or areas of snow sensitive to skier trigger. If you’re seeing shooting cracks or loose snow moving on your way up, it’s sure to be more active on your way down. Wind slab will be the biggest hazard so be on the lookout for clues such blowing snow, recent cornice growth, and fat, pillowed snow surfaces. Cold temps are not only good for riding conditions, but are also conducive of Loose-Dry Snow Avalanche potential that will be of particular concern if you’re getting into extreme or complex terrain

Early Season Obstacles Exist!  If you’re visiting the area for the first time this season, take note that although there is a decent snowpack in the Mammoth area and high elevations of the forecast zone, a majority of the area still has early-season coverage.

Weather and CURRENT CONDITIONS
weather summary

A winter weather advisory is in effect until 4am Tuesday. Looks like a white Christmas for the mountains! A cloudy day is on tap with chance of snow throughout the day for an additional 1-4” for all elevations. Light to moderate southerly winds will continue with some stronger gusts at ridge top ~30mph. Temperatures should remain below freezing and it should cool to the low teens tonight.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Monday Monday Night Tuesday
Weather: Cloudy. Chance of snow in the morning, then snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 80%. Mostly cloudy. Snow in the evening, then chance of snow after midnight. Snow levels below 7000 feet. Chance of precipitation is 80%. Mostly cloudy then becoming partly cloudy. Slight chance of snow. Snow levels below 7000 feet. Chance of precipitation is 25%.
Temperatures: 24 to 30. deg. F. 13 to 18. deg. F. 22 to 28. deg. F.
Mid Slope Winds: Light winds. Gusts up to 25 mph in the afternoon. Light winds. Southwest around 15 mph with gusts to 30 mph.
Expected snowfall: 70% probability of 1 to 4 inches. 30% probability of 4 to 6 inches. | SWE = up to 0.20 inch. in. 1 to 3 inches. | SWE = up to 0.15 inch. in. up to 1 inch. | SWE = less than 0.10 inch. in.
Over 10,000 ft.
Monday Monday Night Tuesday
Weather: Cloudy. Chance of snow in the morning, then snow in the afternoon. Snow levels below 7000 feet. Chance of precipitation is 85%. Mostly cloudy. Snow in the evening, then chance of snow after midnight. Snow levels below 7000 feet. Chance of precipitation is 80%. Mostly cloudy then becoming partly cloudy. Chance of snow. Snow levels below 7000 feet. Chance of precipitation is 30%.
Temperatures: 18 to 24. deg. F. 8 to 13. deg. F. 14 to 20. deg. F.
Ridge Top Winds: Southeast around 15 mph. Gusts up to 30 mph in the morning. Light winds becoming southwest around 15 mph after midnight. Southwest 15 to 20 mph with gusts to 30 mph.
Expected snowfall: 70% probability of 1 to 4 inches. 30% probability of 4 to 6 inches. | SWE = up to 0.20 inch. in. 1 to 3 inches. | SWE = less than 0.10 inch. in. up to 1 inch. | 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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