Eastern Sierra Avalanche Advisory - 12/1/17

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THIS AVALANCHE ADVISORY EXPIRED ON December 4, 2017 @ 6:54 pm
Avalanche Advisory published on December 1, 2017 @ 6:54 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
bottom line

Welcome to the start of the 2017-18 winter season. The Center will be producing three advisories on a weekly basis (Sat, Tues, Thursday) thru December. This may increase if conditions warrant. January, the Center will be fully operational and will begin publishing avalanche advisories on a full seven day a week publishing cycle into the spring when we will begin to curtail operations. The local support and enthusiasm for the Center’s mission continues to be truly overwhelming. On behalf of the Eastern Sierra Avalanche Center, I would like to thank the entire Eastern Sierra backcountry community for supporting and contributing to your community avalanche center.

Bottomline - Primary concern thru Monday is moderate to strong Southerly winds producing sensitive wind slabs on NW-N-NE-E-SE aspects in the mid to upper elevations. As the system moves off to the east, winds will likely swing to the North / Northeast, which will shift the concern for Windslabs to include NW-W-SW-S-SE aspects as well in the mid to upper elevations. Windslab are possible thru Saturday becoming likely by Saturday PM thru Monday. Below ~9,500’ the avalanche hazard is minimal due to little or no snow (below threshold).

How to read the advisory

Bottom Line

Welcome to the start of the 2017-18 winter season. The Center will be producing three advisories on a weekly basis (Sat, Tues, Thursday) thru December. This may increase if conditions warrant. January, the Center will be fully operational and will begin publishing avalanche advisories on a full seven day a week publishing cycle into the spring when we will begin to curtail operations. The local support and enthusiasm for the Center’s mission continues to be truly overwhelming. On behalf of the Eastern Sierra Avalanche Center, I would like to thank the entire Eastern Sierra backcountry community for supporting and contributing to your community avalanche center.

Bottomline - Primary concern thru Monday is moderate to strong Southerly winds producing sensitive wind slabs on NW-N-NE-E-SE aspects in the mid to upper elevations. As the system moves off to the east, winds will likely swing to the North / Northeast, which will shift the concern for Windslabs to include NW-W-SW-S-SE aspects as well in the mid to upper elevations. Windslab are possible thru Saturday becoming likely by Saturday PM thru Monday. Below ~9,500’ the avalanche hazard is minimal due to little or no snow (below threshold).

Avalanche Problem 1: Wind Slab
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Wind Slab – Friday the avalanche problem is focused on possible scattered shallow Windslabs in the mid to upper elevations on a variety of aspects due to winds veering from North / Northwest to South / Southwest over the past couple of days. However, forecasted moderately strong westerly to southerly winds over the weekend with light snowfall will increase the potential for shallow wind slabs from possible to likely on NW-N-NE-E-SE aspects near and above treeline. As the system moves off to the east, winds will likely shift to the North / Northeast, which will shift the concern for Windslabs to include NW-W-SW-S-SE aspects as well in the mid to upper elevations. Windslab are possible thru Saturday becoming likely by Saturday PM thru Monday. Below ~9,500’ the avalanche hazard is minimal due to little or no snow (below threshold). Isolated wind slabs prone to triggered releases are possible on slopes of 35 degrees and steeper.

Typically, Wind Slabs may be encountered below ridgelines, in gullies/depressions, and adjacent to terrain features that promote drifting. Use extra caution around freshly formed drifts and hollow sounding slabs on steeper terrain. Though these tender pockets may not be big enough to result in burial, they could entrain a rider and carry them over and into hazardous terrain with potentially high consequences.

advisory discussion

The snow season is off to an interesting start with a complicated snow structure developing. The season started off in earnest with a warm storm in early November, with 7 to 20 inches being reported throughout the region. This resulted in the formation of a combination of snow and melt-freeze crusts above ground that can be found between 9,500’ to 11,000’+. A couple weeks of dry unseasonably warm weather followed these storms, which resulted in the development of small facets above ground and capped by an ice crust above. The snowpack depth receded dramatically as the unseasonably warm temperatures took their toll on the snowpack. The next system to move into the region rolled though on November 27th, with another wave of warm moisture dropping an additional 1 to 10 inches of snow above 9000’ level. The storm system came in very warm with moderate Southerly to Southwesterly winds. Precipitation amounts range from 1 to 10” of snow, initially wet and dense. As the system moved through the region temperatures began to fall and the snow density decreased producing shallow powder skiing above 9,500’. These two storms have combined to produce a supportive ice crust / facet mid snowpack with faceted surface grains. The basal snow is showing some previous faceting but the current temperature gradient within the snowpack is weak, slowing further growth.  The snowpack is fairly thin overall with early season conditions and hazards (rocks, stumps, logs, open water, etc). With little to no snow below ~ 9000’ to 9500’, the likelihood of human triggered releases is unlikely but as you ascend above 9,500’ triggered avalanches are possible. Keep an eye out for wind slabs, shallow unstable pockets, as well as the presence of persistent slab weaknesses in the upper elevations on E-N-NW facing slopes.

South and north of Mammoth, snowfall amounts have been lower, making easily access to skiable and rideable terrain more difficult and long approaches.

Other concerns: early season conditions exists. There are plenty of rocks, stumps, down trees just under the snow surface, use caution while riding and playing in the backcountry. 

weather

Weak high pressure will remain over the area today. The next Pacific storm on tap for this weekend will bring breezy to gusty winds Saturday along with some light rain and higher elevation snow. Dry weather returns next week. Model simulations continue to trend towards a wetter and more unstable for the storm this weekend. Wind forecast is on track with wind gusting to around 40 to 55 mph on Saturday afternoon and evening. NAM and GFS models still vary a bit on the amount of moisture and forcing with the cold front, they are both very similar with the timing of the front. Cross sections for Sunday morning are showing a notable amount of frontogenetical forcing paired with just enough moisture in the dendritic growth zone (-5 to -15 degrees C). Although there isn`t an incredible amount of moisture, it doesn`t take much to produce snow when it is very cold and we will have 700mb temps around -10 to -12 degrees C as the cold front quickly barrels through with 0 to 3 inches of snow. Colder temperatures and brisk northerly breezes will prevail on Sunday afternoon behind the storm, with widespread hard freezes likely by Monday morning.

Extended Forecast - The forecast models...deterministic and ensembles...are in good agreement that a ridge will build over the region through the extended period. The ECMWF is a little slower dropping the upper trough south of the region Monday and develops a closed low off the srn California coast by early Tuesday...but the dry ... colder ... air plunging into the region behind the trough is roughly in line with the GFS and ensembles. The primary impactors during the extended period will be easterly ridge winds from Monday through Wednesday. The easterly winds are expected to strengthen a bit due to the depth of cold air advection east of the Sierra creating a strong thermal gradient from east to west. Areas west of the Sierra Crest that are favored for easterly gusts could see fairly strong winds...especially Tuesday night. The depth of the cold air east of the Sierra will also result in cooler lows and highs in many of the lower valleys than at the mid slopes and ridges.

 

 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Friday Friday Night Saturday
Weather: Partly cloudy. Partly cloudy. Partly cloudy.
Temperatures: 43 to 51 deg. F. 26 to 31 deg. F. 39 to 47 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: Light winds, 10 to 15 mph with gusts to 30 mph in the afternoon. 10 to 15 mph with gusts to 40 mph. 10 to 15 mph with gusts to 40 mph increasing to 20 to 30 mph with gusts to 60 mph in the afternoon.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Friday Friday Night Saturday
Weather: Partly cloudy. Partly cloudy. Partly cloudy.
Temperatures: 39 to 45 deg. F. 23 to 28 deg. F. 33 to 41 deg. F.
Wind direction: Southwest Southwest Southwest
Wind speed: 15 to 25 mph. Gusts up to 25 mph increasing to 35 mph in the afternoon. 20 to 30 mph with gusts to 50 mph. 25 to 35 mph with gusts to 55 mph increasing to 35 to 50 mph with gusts to 80 mph in the afternoon.
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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