Eastern Sierra Avalanche Forecast - 12/24/14

THIS AVALANCHE ADVISORY EXPIRED ON December 25, 2014 @ 7:00 am
Avalanche Advisory published on December 24, 2014 @ 7:00 am
Issued by Sue Burak - Inyo National Forest
advisory discussion

Our part of the eastern Sierra is one of the driest parts in the state. Just 60 miles to the north, the Leavitt Lake snow pillow, located at a similar elevation as Mammoth Pass, is fat with 106% of average snow on the ground. This 106% translates to 57 inches of snow depth in the Leaviit Lake and Leavitt Bowl area- that is almost 5 feet of snow!  Locally, snow conditions are dismal with less than 2 feet of snow above 9,500 ft in the Mammoth Lakes Basin. Since the "big" storm that ended on December 16, snow depths went from 32 inches to 24 inches in a few days. We often see snow depths decrease each day during the spring but the calendar says winter began on Dec 21. 

You could blame it on the sudden deviation of the jet stream from model forecasts on December 15- a deviation that cost us the kind of heavy snowfall that the Leavitt Lake area enjoyed. The real culprit though is the ridiculously resilient high pressure ridge that shunted a huge storm meant for us, over the high pressure ridge into Idaho and the Wasatch; new snow amounts of 2 to 3 feet prompted avalanche warnings. Meanwhile, we are high and dry. Maybe the 6 to 8" of new snow forecasted for tonight is the new eastern Sierra snowfall "dump". Let's hope the New Year brings big, multiday storms. 

Yesterday's observation indicated a shallow but consolidated snowpack. Even though there are no avalanche problems today, other hazards include icy conditions and a shallow snowpack with lots of rocks and other obstacles. The onset of a prolonged period of cold temperatures combined with a shallow snowpack will set up conditions for a "facet factory"- one that transforms rounded grains into facets. By tomorrow morning, the impact of new snow and wind will create a wind slab avalanche problem. Even though there are just a few individual slopes that have enough coverage to ski without hitting rocks, additional wind blown snow will increase the likelihood of human triggered avalanches in steep wind loaded terrain.



recent observations

The past few warm days wreaked havoc on the snow in the Mammoth Lakes Basin. Saturday's mild temperatures, high relative humidity and mid level clouds added a huge shot of energy into the snowpack on all aspects and elevations- even on slopes above treeline. The only settled moist powder snow without a crust was found in the Coldwater Creek drainage. Despite today's forecasted mild temperatures in the upper 40's, expect crusts to remain frozen on north aspects after the snow loses energy during the night. 

Snow depths are similar to last year with around 23-25 inches of snow on the ground. This year is warmer and the recent run of warm days and mild nights have reduced any temperature gradients that might exist in a early winter snowpack to about 1 to 2 degrees C from the ground to the snow surface, a distance of 24 inches or 60 cm. 

Extended column tests done on a north aspect at 9,400 ft and on northeast aspects above treeline at 10,000 and 10,200 ft.  did not propagate. Snow depths were around 24 inches (60 cm). Facets at the base of the snowpack at the 10,000 and 10,200 ft snowpits were rounding. Wind slabs observed last Thursday ( December 18) that had been sensitive to ski cuts, are well bonded to the underlying snow. There is now a 1 inch hard crust on the surface.  

With the onset of cold temperatures that could last for a week, it's easy to picture the shallow snowpack that is now close to 0 degrees C, change character with large changes in temperature between the warm ground and cold snow surface. The generally well bonded snowpack will likely change to a weak shallow pack by New Years.  

weather summary

Tired of daytime high temperatures in the 60's, wet snow, and crusts? Winter returns later today with strong winds and hopefully 6 to 8 inches of snow tonight. By tomorrow, temperatures fall 20 degrees and the warm southwest winds will be a memory. North winds and wind chills of 0 degrees F and colder are expected for Christmas Day.  

Expect one more mild day with mountain temperatures above 10,000 ft reaching the mid 40's. The winds will pick up in the morning and clouds move in by mid day. Expect wind gusts to reach 80 mph along the Mammoth Crest and San Joaquin Ridge. The storm is a quick hitter and will drop whatever snow will reach the ground by early Christmas morning- optimists can hope for the forecasted high end of 8" in wind sheltered areas and more on wind loaded lee slopes. Christmas Day temperatures are going to be cold- from the mid 40's today to a high temperature of 15F on Christmas Day- a drop of 30 degrees! 

Mid -elevations from 9,000 to 10,000 ft. won't be spared the wind- gusts up to 70 mph are expected in the Mammoth Lakes Basin. Snowfall amounts are expected in teh 6 to 8 inch range.  

For the rest of the week, cold north flow will keep temperatures below normal with highs reaching the upper 20's and low 30's above 8,000 ft.  Nights will be cold and reach the single digits above 10,000 ft. After tonight's storm, the outlook for the next 5 days is for dry and cold conditions.

Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 28 deg. F.
Max. temperature in the last 24 hours: 50 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 10-15 mph
Maximum wind gust in the last 24 hours: 51 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 25 inches
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: Clear AM, clouds and snow in the afternoon snow showers clear and windy
Temperatures: 47 deg. F. 12 deg. F. 18 deg. F.
Mid Slope Winds: SW NW N
Expected snowfall: 1-3 in. 3-5 in. 0 in.
Over 10,000 ft.
Wednesday Wednesday Night Thursday
Weather: clear then snow snow clear and windy
Temperatures: 43 deg. F. 9 deg. F. 15 deg. F.
Ridge Top Winds: SW W N
Expected snowfall: 1-3 in. 3-5 in. 0 in.
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