Mammoth Basin Snowpack Summary - 2015-12-15 07:43

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THIS ADVISORY EXPIRED ON December 17, 2015 @ 7:43 am
Avalanche Advisory published on December 15, 2015 @ 7:43 am
Issued by Doug Lewis - Eastern Sierra Avalanche Center
Avalanche Character 1: Storm Slab
Storm Slab avalanches release naturally during snow storms and can be triggered for a few days after a storm. They often release at or below the trigger point. They exist throughout the terrain. Avoid them by waiting for the storm snow to stabilize.

The recent storms have added significant weight and stress to the snowpack, which will require some time to adjust to the new load. Winds have formed wind slabs in higher elevation, steep terrain features that have been loaded with snow by wind. Soft slabs are possible on more protected slopes of 35 degrees and steeper where it is possible to trigger slides that may steep down deep into the snowpack with the potential for producing very consequential avalanches. The underlying early season snowpack is shallow, variable and predominantly weak and is still adjusting to the recent load.

Avalanche Character 2: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.
Snowpack Discussion

Last week's storm cycle brought significant snowfall and strong Southwesterly winds to the region over the course of four days. Some storm totals: Mammoth Mountain 35” and June Mountain 40” with similar amounts reported at Monitor Pass, Poison Flat, Lobdell Lake, Sonora Pass, Leavitt Lake, and Virginia Lakes Ridge. The new snow was deposited on a relatively shallow weak snow pack. This recent storm cycle has added significant weight and stress to the snow pack, when combined with the unseasonably cold temperatures, it will take time for the snowpack to adjust to the added load as witnessed by a report of a skier triggered slide in a closed area at Mammoth Mountain. The strong Southwesterly winds have likely loaded Northwest to Southeast aspects near and above treeline. Below treeline the concern is soft slabs and drifted slopes. The primary slide concerns are loaded leeward slopes of 35 degrees and steeper. Overall, riding is really good on slopes less than 35 degrees but early season conditions still with plenty of rocks and stumps just under the surface. 

recent observations

The strong winds and heavy snow associated with this system has rapidly loaded an already weak and shallow snow pack. This combination generally requires a time to adjust to the new load and for bonds between layers to strengthen. However, the recent cold temps will slow the strengthening process. Areas of current concerns are wind slabs and soft slabs that have formed on leeward slopes and side loaded features in the Mid to Upper elevations. Limited snow data suggests a minimum of a couple of week layers to be aware of. The first is within the new snow, approximately 20 to 30 cm from surface where a density change has formed a weak slab. The second layer continues to be  a preserved layer of facets just below a crust that formed on the old snow surface. Stability Test results (20 degree slope, East Aspect, Elevation 10,066 ‘): ECT 12, Q3 on 2-3 mm facets, 23 cm from surface, ECT 22, Q3 on 2-3mm facets, 50 cm from surface. The two layers together could result in a failure that steps down deeper into the snow pack. 

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 1 deg. F.
Max. temperature in the last 24 hours: 16 deg. F.
Average wind direction during the last 24 hours: NE
Average wind speed during the last 24 hours: 55 mph
Maximum wind gust in the last 24 hours: 66 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 31 inches
weather

The upper level Low that brought plenty of snow to the Sierra Sunday exited eastward followed by plunging temperatures with single digits to below zero readings in the Sierra. Cold temperatures will linger through Wednesday with the potential for strong temperature inversions in the valleys while the mountains warm more quickly. Short wave ridging is expected to hold into Wednesday before a warm front moves through Thursday morning. Highs today will remain well below average after a very cold morning and generally in the 30s. Valley inversions will remain strong with poor mixing that will continue into Wednesday. Temperatures will also be about 10 degrees warmer Thursday for all areas. A weak upper ridge over the Great Basin early Friday will shift east during the day as trough advances into the east pacific. Most model runs continue to show a split as the trough moves inland Sat-Sat night. Model confidence is low as to impacts for the Sierra but there’s a chance of light snow for the Sierra. A better chance for precipitation early next week as a more consolidated jet and storm system is expected to move across the region with chances for snow and valley rain. Details are uncertain although current projections indicate an increasing potential for a period of moderate to heavy snowfall for the Sierra Mon/Tue with the possibility of strong winds and warmer temperatures as a strong southerly gradient develops Sunday afternoon into Monday. 

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: SUNNY CLEAR PARTLY CLOUDY
Temperatures: 12 TO 21 deg. F. 0 TO 10 deg. F. 23 TO 31 deg. F.
Wind direction: NORTH NORTH NORTHWEST
Wind speed: 20 TO 35 MPH WITH GUSTS TO 50 MPH DECREASING TO 15 TO 25 MPH WITH GUSTS TO 40 MPH IN THE AFTERNOON. 15 TO 20 MPH WITH GUSTS TO 35 MPH. 10 TO 20 MPH WITH GUSTS TO 30 MPH.
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Tuesday Tuesday Night Wednesday
Weather: SUNNY CLEAR PARTLY CLOUDY
Temperatures: 9 TO 16 deg. F. 4 TO 14 deg. F. 21 TO 28 deg. F.
Wind direction: NORTH NORTH NORTH
Wind speed: 20 TO 35 MPH WITH GUSTS TO 50 MPH DECREASING TO 15 TO 25 MPH WITH GUSTS TO 40 MPH IN THE AFTERNOON. 15 TO 20 MPH WITH GUSTS TO 35 MPH. 35 TO 40 MPH WITH GUSTS TO 55 MPH DECREASING TO 25 TO 30 MPH WITH GUSTS TO 45 MPH IN THE AFTERNOON.
Expected snowfall: 0 in. 0 in. 0 in.
Disclaimer

This snowpack summary applies only to backcountry areas outside established ski area boundaries. This snowpack summary only describes general avalanche conditions and local variations always occur. This snowpack summary expires 24 hours after the posted time unless otherwise noted. The information in this snowpack summary is provided by the USDA Forest Service who is solely responsible for its content.

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