Mammoth Basin Snowpack Summary - 2015-04-08 07:46

THIS ADVISORY EXPIRED ON April 10, 2015 @ 7:46 am
Avalanche Advisory published on April 8, 2015 @ 7:46 am
Issued by Sue Burak - Inyo National Forest
Avalanche Character 1: 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.

Widespread areas of unstable snow will be found today on all northwest to northeast and east facing aspects in alpine terrain. North facing slopes in the Mammoth Basin above about 9,500 ft had 2 to 3 feet of snow prior to the storm. Winds continue to blow this morning so wind drifts and slabs are still adjusting to loading. 

Avalanche Character 2: 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.

Storm slabs are possible today. Deposits of stronger snow over a density change  or when upper layers of storm snow settle over less dense layers create unstable conditions within the new snow. 

Snowpack Discussion

Over 18” of fresh cold powder has fallen with gusty winds out of the west and southwest. Wind loading is the main avalanche concern since the snow fell onto frozen melt freeze crusts and firm wind affected snow in alpine terrain. Slopes that had previous snow cover are the slopes that will have wind slab avalanche problems. With snowfall rates of 2 inches an hour, it is likely that natural avalanches occurred yesterday and last night.  Human triggering of wind slab and loose dry avalanches are likely today in steep alpine terrain. Avoid slopes steeper than 30 degrees with fresh wind deposits.

In the Horseshoe Lake area Tuesday mid-morning, only 4-5 inches of snow had accumulated and the cold dry new snow was not bonding well to old crusts. Winds were building thin soft slabs that were quite sensitive to ski cuts.

Today, the winter wind slab drill is in affect –new snow might bond to old surfaces in alpine terrain but be wary about wind loaded slopes, especially in serious unforgiving terrain. Close calls will snap you to attention but you never know if the avalanche you trigger will be the size you think it is, or how deep it will be and if you can escape a slab moving at 60 mph. After weeks of stable conditions, it’s easy to get complacent. 

Avalanches won’t be deeper than the new snow but they could run long distances given the hard underlying crusts and firm surfaces. 

recent observations

New snow covered the partially melted out ice that covers Lake Mary. Traveling across lakes is not recommended. Yesterday morning there was about 4 inches of new snow on the ground in the Horseshoe Lake area with calm winds in the trees and wind transport in open areas on Lake Mary and Horseshoe Lake. 



CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 15 deg. F.
Max. temperature in the last 24 hours: 28 deg. F.
Average wind direction during the last 24 hours: WSW
Average wind speed during the last 24 hours: 45-50 mph
Maximum wind gust in the last 24 hours: 60 mph
New snowfall in the last 24 hours: 18 inches
Total snow depth: 18 inches

Since yesterday mid-morning, 16-18 inches of cold dry snow accumulated in a short period of time- about 9 hours. Intense snowfall of 2 inches an hour occurred from 11 AM to 2 PM. The storm started to wind down last night and partially clearing skies led to cold early morning temperatures in the single digits and low teens.  

Winds were from the south at the June Mtn ski area, which picked up 10-12” of new snow. West southwest winds blew 40 -50 mph over the summit of Mammoth Mountain.

Daytime highs today will reach 30 F today under partly cloudy skies. Northwest winds gusting to 40 mph will transport snow throughout the day.    

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: partly cloudy clear mostly sunny
Temperatures: 35-39 deg. F. 16-24 deg. F. 44-50 deg. F.
Wind direction: NW NE light
Wind speed: 20-25 10-15 light
Expected snowfall: in. in. in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: partly cloudy partly cloudy partly cloudy
Temperatures: 24-30 deg. F. 9-16 deg. F. 35-43 deg. F.
Wind direction: NW NE light
Wind speed: 15-25 10-15 light
Expected snowfall: 1 in. 0 in. 0 in.

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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