Avalanche Advisory: Saturday - Dec 15, 2018

THIS AVALANCHE ADVISORY EXPIRED ON December 16, 2018 @ 5:19 am
Avalanche Advisory published on December 15, 2018 @ 5:19 am
Issued by Chris Engelhardt - Eastern Sierra Avalanche Center

Human triggered wind slab avalanches may be possible in isolated areas on N-NE-NW-E-SE aspects today. Yesterday’s violent wind event and unpredictable loading patterns potentially formed wind slabs primarily in mid to lower elevations where topography slowed the transport of blowing snow. Evaluate snow and terrain carefully, especially convex rolls and unsupported slopes.  Early season obstacles exist!

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Avalanche Problem 1: Wind Slab
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Winds were blowing consistently in the 50-60 mph range from the S and SW yesterday with gusts into the 90s. S/SW winds were blowing through Friday night and will continue today although with lower velocities. The extreme winds on Friday stripped entire hill sides of snow and moved it elsewhere, generally much further down in elevation. There was evidence of recent wind slab avalanche activity yesterday in the Convict Lake area (See 12/14 Convict Lake Observation) with a small D1 slab releasing on a convex roll adjacent to a terrain trap. Although the prevailing wind direction was from the S and SW, when these gale force winds hit the dramatic mountain topography it can cause them to be quite erratic in nature with terrain specific loading patterns. Identify features of concern today (terrain traps, convex rolls, unsupported slopes) and travel both up and down hill wisely.

advisory discussion

The primary force affecting the snowpack this week was the wind which has scoured and stripped snow from nearly every aspect in the mountains. We had a lengthy period of N and NE winds on Wednesday 12/12 that stripped the shady aspects and then Friday 12/14 an extreme wind event that blew from the S and SW with incredible velocities. All told a lot of the high elevation snow has been removed and taken down slope or eliminated due to sublimation in the atmosphere. Observations in the alpine/upper elevations yesterday of the Convict Lake area, showed snowpack depths ranging from 30-60cm with the remaining snow faceting, decomposing and becoming weaker with every day. Layers of thin wind board are dispersed throughout the area, but are not uniform in coverage. Thin snow coverage is most dramatically affected by temperature gradients between the ground ~0C and varying air temperatures which after prolonged time periods pulls the moisture out of the snow. Snow depths in the more heavily timbered areas and wind sheltered zones are much deeper, more stable and have been found to be from 90-120cm deep in the Mammoth area.

Weather and CURRENT CONDITIONS
weather summary

Overcast conditions will persist today and SW winds will continue to blow although there will be a decrease in velocity from yesterday’s gale force conditions. Wind speeds are expected to be in 10-20 mph range for middle elevations and 20-35 mph range at ridge tops. 50 mph gusts are expected up high. Temperatures remain mild with mid 30s to low 40s at middle elevations to mid 20s to mid 30s at the upper elevations. SW winds are expected again to ramp up a bit more Sunday and with it the potential for snow Sunday evening into Monday. The storm track looks to be possibly taking a more southerly track and with it, better potential for increased accumulation for our area.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
Saturday Saturday Night Sunday
Weather: Partly cloudy. Partly cloudy then becoming mostly cloudy Partly cloudy
Temperatures: 33 to 43. deg. F. 22 to 27. deg. F. 33 to 43. deg. F.
Mid Slope Winds: Southwest 10 to 20 mph with gusts to 45 mph. South 10 to 20 mph. Gusts up to 40 mph increasing to 50 mph after midnight. South 15 to 25 mph with gusts to 60 mph
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
Saturday Saturday Night Sunday
Weather: Mostly cloudy Mostly cloudy Partly cloudy
Temperatures: 25 to 35 deg. F. 18 to 23 deg. F. 25 to 35 deg. F.
Ridge Top Winds: Southwest 20 to 35 mph with gusts to 50 mph Southwest 20 to 35 mph. Gusts up to 55 mph South 30 to 50 mph with gusts to 75 mph
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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