Mammoth Basin Snowpack Summary - 2015-02-14 07:01

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THIS ADVISORY EXPIRED ON February 16, 2015 @ 7:01 am
Avalanche Advisory published on February 14, 2015 @ 7:01 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.

As the day and snow heats up, the danger of wet loose sluffs, large enough to catch and carry a person, will increase, especially on steep, sunny slopes above 9,000 ft. Wet sluffs may become more active over the next few afternoons with forecasted light wind, warm temperatures and higher sun angles. Avoid travel on south to east facing slopes above 9,000 ft.especially under cliffs and rock bands. 

The wet snow avalanche problem reflect the realities of a dry, warm winter. Night time lows at the 10,700 and 11,000 ft. elevations have been above freezing (35 to 40 degrees F at Gem Pass and the top of Mammoth Mountain) for the last 3 days. Daytime highs at these high elevation stations have been in the upper 40’s on Mammoth Mountain and the mid 50’s at 10,700 ft.  Mid elevation stations at Mammoth Pass and the ski patrol study plot have warmed to 60 F with lows in the mid 30’s.

The wet snow avalanche problem is related to higher sun angles, longer days, and warm temperatures. Shallow mid elevation snowpacks were saturated with rain, and then froze Sunday night and Monday when strong winds cooled the snow surface.  

Avalanche Character 2: Persistent Slab
Persistent Slab avalanches can be triggered days to weeks after the last storm. They often propagate across and beyond terrain features that would otherwise confine Wind and Storm Slab avalanches. In some cases they can be triggered remotely, from low-angle terrain or adjacent slopes. Give yourself a wide safety buffer to address the uncertainty.

Reports of slope collapses (whumpfing) are clear signs the facets and facet/crust layers that composed the snowpack up until last weekend's storm have been reactivated by heavy loading from rain and dense snow.

The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads, and often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.

Snowpack Discussion

This snowpack summary is for snow conditions in the Mammoth Lakes and June Mountain areas. The next summary will be issued Friday morning, February 20.

 

The period of the year during which there is both a deep snowpack and a low-angle sun is now past. Beginning this week, the sun is beginning to become increasingly caustic to snow, certainly on south aspects and, as we progress into spring, other aspects as well.  There is certainly great skiing to be had outside of this time frame but in the seemingly distant past (years prior to and including 2011),the period of mid-December through the second week in February can be a time  there is both abundant snow cover and the ability to ski most aspects even well after storms.

There have been no avalanches observed or reported this week. However, reports of whumpfing in higher elevation terrain are a clear sign that the facets and depth hoar have been activated by heavy loading of several inches of rain and snow. Several inches of water is a lot of weight on a shallow layer of facets, crusts and depth hoar. Despite warm temperatures and mild night time lows, steep high elevation north and east facing slopes are still colder than the temperatures suggest because of radiation losses during clear nights. A few inches of dry snow resting on a thick rain layer provides a good skiing surface for an otherwise shallow snowpack. Under the rain layer is a multi-layered series of crusts and facets.

Whumpfing is a warning sign that deeper instability exists and triggering a deeper avalanche may be possible if you find the sweet spot on a slope. Triggering a slab avalanche on this layer is a low probability, high consequence event. Hard slabs and slabs failing on persistent weak layers are unpredictable, hard to manage avalanches. They can be triggered from areas where the snowpack is shallow and weak, which describes this winter’s snowpack.  If you hear whumpfing, avoid this terrain and chose slopes gentler than 30 degrees with nothing steeper above or adjacent to you. Remember that test slopes, snowpit tests, previous tracks are unreliable- the best way to manage the risk from persistent slabs is to make conservative terrain choices.  

Extended column tests that do not react in the morning, then later collapse on basal weak facets when the snowpack is wet is a clear indication of wet snow instability. Over the weekend, rain flowed through shallow mid elevation snowpacks, reaching the ground in the snowpits I observed. It’s a spring snow scenario- early morning refrozen snow becomes unstable in the afternoon after the sun melts the snow surface and liquid water fills the pore spaces. I don’t have enough information to evaluate the possibility of a wet slab avalanche problem so for now, the wet snow avalanche potential is based on weather forecasts and a handful of observations..

recent observations

Heading out this morning, you’ll find clear skies, mild temperatures in the 30s, and very light northwesterly winds, with average speeds less than 10 mph. The snow surface had a good refreeze last night, and mid elevations slopes are crusted and icy this morning, but will soften with day time heating. The reports continue to trickle in of a few inches of dry setled powder on upper elevation northerly facing slopes.

Reports of loud whumpfing on Wednesday startled the reporting party. The next day, on the same skin track, whumpfing occurred again, 4 days after the storm. The rain crust was reported to be a few inches thick with fist hardness facets and depth hoar under the rain crust.  

Extended column tests did not react (ECTX) in pits dug on moderate north facing slopes at 9,400 ft. in the Mammoth Basin. Returning in the afternoon during the warmest part of the day, 4 out of 5 extended column tests collapsed on wet facets and depth hoar on the ground during column isolation.  Snowpack depths ranged from 30 to 40 inches. The rain layer was 4 inches thick compared to the 8 inch thick monster found on Tuesday on the Knob.

Reports of wet snow point releases in the Solar Bowl of the Negatives show mid and high elevation snow on high elevation southerly slopes is getting a strong dose of sun. .

The sound of slope collapse (whumpfing) was also reported in the Red Cone area.

 

CURRENT CONDITIONS Weather Observations Between June (10,000 ft.) and Mammoth (11,000 ft.)
0600 temperature: 35 deg. F.
Max. temperature in the last 24 hours: 46 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 10-12 mph
Maximum wind gust in the last 24 hours: 32 mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 23 inches
weather

High pressure will remain over the Sierra through early next week. Record highs are possible today. As high pressure builds, northeasterly winds will develop and cool temperatures a few degrees to the mid 40’s but unseasonably warm temperatures are forecasted to continue.

High elevations above 10,000 ft. will warm to the upper 40’s to mid-50’s with lows around 35 F. Mid elevations warm to the 60’s, then cool off to the mid 50’s by early next week.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8,000 ft. to 10,000 ft.
  Saturday Saturday Night Sunday
Weather: clear clear clear
Temperatures: 50-60 deg. F. 33-39 deg. F. 55 deg. F.
Wind direction: SW N N
Wind speed: 5 5 5
Expected snowfall: 0 in. 0 in. 0 in.
Over 10,000 ft.
  Saturday Saturday Night Sunday
Weather: clear clear clear
Temperatures: 48 deg. F. 29 deg. F. 45 deg. F.
Wind direction: N NNE NNE
Wind speed: 5 5 5
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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