Mammoth Basin Snowpack Summary - 2015-04-15 07:08

THIS ADVISORY EXPIRED ON April 18, 2015 @ 7:08 am
Avalanche Advisory published on April 15, 2015 @ 7:08 am
Issued by Sue Burak - Inyo National Forest
Avalanche Character 1: Loose Wet
Loose Wet avalanches occur when water is running through the snowpack, and release at or below the trigger point. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Small loose wet avalanches could be triggered by skiers and riders in steep, rocky areas as the day heats up. Snow stability can change in minutes this time of year.


Avalanche Character 2: Wet Slab
Wet Slab avalanches occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoid avalanche terrain when you see pinwheels, roller balls, loose wet avalanches, or during rain-on-snow events.

The snowpack structure of 30 to 50 cm of moist well sintered snow overlying a thick icy layer is a setup for wet slab avalanches. Unlike dry snow avalanches that fail due to increases in load, wet snow fails due to decreases in strength. Melting snow at the surface will reach the very hard February layer at some point- not today and it's impossible to know exactly when water will concentrate above the February layer. Wet slab avalanches do not occur often but the layering in our current snowpack is the perfect setup for wet slabs to occur.




Snowpack Discussion

After warm days and cold nights created spring snow conditions in the higher elevations of the Mammoth Basin, yesterday's relentless west and southwest winds cooled the snow surface, creating a variety of thin icy crusts on north facing alpine terrain. Today, after a cold night, warming temperatures, moderate north winds and strong sun will soften the snow surface gradually and could produce small wet loose avalanches involving the new snow from last week. Cycles of cold nights and mild temperatures will produce spring-like skiing conditions on most north facing slopes above treeline. Expect to find cold winter snow in steep couloirs. 

The next summary will be posted Saturday and will cover the Tioga Pass area.

There was an avalanche fatality last Saturday in Beehive Basin in Montana. The experienced party followed all standard procedures and did everything right., reduced their risk but you can't reduce the risk to zero. Watch the excellent video on the incident.




recent observations

Warm days and cold nights over the weekend resulted in soft corn snow on north slopes in the Mammoth Basin. The upper layers of cold dry powder that fell last Tuesday week transitioned to spring snow in about 4 to 5 days of strong daytime sun and cold nights. Snow beneath the surface is damp and well bonded rounds that is this week's slab overlying the February layer. I broke the tip off a SP1, the latest high tech instrument designed to measure snow hardness. The SP1 designed to "cut through layers like butter" met its match in the snow pack below the Red Cone Bowl. The profile shows 30 cm of fist hardness new snow over the pencil or harder February layer. The probe broke so the profile depth ends at the February layer at hardness that is off the chart.




CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 23 deg. F.
Max. temperature in the last 24 hours: 26 deg. F.
Average wind direction during the last 24 hours: WSW
Average wind speed during the last 24 hours: 45 mph
Maximum wind gust in the last 24 hours: 73 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 18 inches

Morning temperatures are in the low 20's with brisk north winds 20 to 30 mph, gusting to over 45 mph. Today will be sunny with a few degrees of warming but still below average for this time of year. East winds will develop later today blowing in the 40 mph range in alpine terrain. Daytime highs will be in the mid to upper 40's today with lows in the low 20's. 

An increasing chance of precipitation is forecasted for the Sunday - Monday time period.

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: sunny clear sunny
Temperatures: 32-42 deg. F. 18-22 deg. F. 42-48 deg. F.
Wind direction: N-NE E E
Wind speed: 15-20 10-20 10-20
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Wednesday Wednesday Night Thursday
Weather: sunny clear sunny
Temperatures: 27-32 deg. F. 17-21 deg. F. 24-29 deg. F.
Wind direction: N NE E
Wind speed: 45 30 25
Expected snowfall: 0 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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