This avalanche advisory is provided through a partnership between the Tahoe National Forest and the Sierra Avalanche Center. This advisory covers the Central Sierra Nevada Mountains between Yuba Pass on the north and Ebbetts Pass on the south. Click here for a map of the forecast area. This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires 24 hours after the posted time unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.


This Avalanche Advisory was published on April 25, 2010:


April 25, 2010 at 7:00 am

Near and above treeline, the avalanche danger will rise to MODERATE on the sun-exposed E-SE-S-SW-W aspects steeper than 35 degrees due to daytime warming. Below treeline pockets of CONSIDERABLE danger will form on the E-SE-S-SW-W aspects. Pockets of MODERATE avalanche danger may also form on NW-N-NE aspects below treeline.


Forecast Discussion:


The forecast area should see another warm and sunny day due to the high pressure lingering over the area. Winds shifted to the east and northeast last night. They should continue this morning before decreasing and shifting more towards the southwest this afternoon and overnight. By tomorrow, the southwest winds should start to increase ahead of another storm system approaching the region. Some convective cumulus clouds could build over the mountains this afternoon. Tomorrow cloud cover should increase as the storm moves closer to the area.

Observations:

On Incline Lake Peak yesterday, enough free water had accumulated in the upper snowpack by noon for several wet snow avalanches to occur. All of the slides occurred on sunny SE-S-SW aspects steeper than 35 degrees. The slides ranged from skier triggered sluffs to natural point releases to one slide that started as a natural point release which then triggered a small wet slab after running about 100 ft downslope (photos). Several other natural and skier-triggered wet, point release avalanches that most likely occurred on Friday were also observed. Only 2 of these slides looked like they entrained enough snow to bury a person; however, even the smaller ones could knock someone off balance, twist a knee, or push someone somewhere that they did not want to be. The interface between the recent snow and the old melt-freeze crust served as failure layer for all of the slides. In some areas, as much as 1-1.5 ft of recent snow existed on top of this old crust. Skier-triggered whumphing and snowpack collapse also occurred on this layer on lower-angle (25-30 degrees), sun-exposed, SE aspects by 11:30 am. On these warm southerly aspects, sticky, wet, heavy snow existed. On the more northerly aspects, a mix of breakable crusts and sticky wet snow existed on sheltered slopes. On the more exposed slopes, wind scoured crusts dominated the northerly aspects. Remote sensors indicated that temperatures above 7800' dropped below freezing for several hours last night. These cooler temperatures combined with a clear night sky that allowed the snowpack to radiate heat out into space should have allowed for a stronger overnight refreeze above 7800'. Below 7800', a weaker refreeze should have occurred due to overnight temperatures above freezing.

Avalanche Concern #1: Wet snow instabilities

Wet snow instabilities will comprise the primary avalanche concern today. Last night's stronger overnight refreeze and the fact that some drainage channels have formed in the recent snow since Friday will help make the wet snow instabilities a little less widespread today. However, daytime highs in the mid 40's to low 50's at all elevations and intense sunshine should weaken the snowpack enough for wet snow avalanches to happen. Point release avalanches should comprise most of these avalanches, but some wet slab avalanches could still occur today as the snowpack warms up. The sun-exposed E-SE-S-SW-W aspects will hold the best potential for these wet snow instabilities; however, pockets of warming instabilities could also exist on the low to mid elevation northerly aspects. These slides could be large enough to bury or injure a person. Clues like widespread surface instabilities (roller balls, pinwheels), small point releases, and sinking into wet snow up to your shins can help indicate when wet snow avalanches become more likely on certain slopes. When these things start occurring, it is time to more to a more frozen or lower angle slope or maybe just head on down for a nice spring picnic.


The bottom line:

Near and above treeline, the avalanche danger will rise to MODERATE on the sun-exposed E-SE-S-SW-W aspects steeper than 35 degrees due to daytime warming. Below treeline pockets of CONSIDERABLE danger will form on the E-SE-S-SW-W aspects. Pockets of MODERATE avalanche danger may also form on NW-N-NE aspects below treeline.


Andy Anderson - Avalanche Forecaster, Tahoe National Forest


Weather Observations from along the Sierra Crest between 8200 ft and 8800 ft:

0600 temperature: 29-31 deg. F.
Max. temperature in the last 24 hours: 44-50 deg. F.
Average wind direction during the last 24 hours: West southwest shifting to the Northeast
Average wind speed during the last 24 hours: West southwest: 30 mph | Northeast 20 mph
Maximum wind gust in the last 24 hours: 51 mph
New snowfall in the last 24 hours: O inches
Total snow depth: 87-141 inches

Two-Day Mountain Weather Forecast - Produced in partnership with the Reno NWS

For 7000-8000 ft:

  Sunday: Sunday Night: Monday:
Weather: Sunny in the morning with a few clouds developing over the mountains this afternoon. Partly cloudy in the evening becoming clear after midnight. Partly cloudy
Temperatures: 49-56 deg. F. 31-39 deg. F. 48-55 deg. F.
Wind direction: East shifting to the southwest West shifting to the south Southwest
Wind speed: up to 10 mph 10 mph 10 mph increasing to 15-25 mph with gusts to 40 mph
Expected snowfall: O in. O in. O in.

For 8000-9000 ft:

  Sunday: Sunday Night: Monday:
Weather: Sunny in the morning with a few clouds developing over the mountains this afternoon. Partly cloudy in the evening becoming clear after midnight. Partly cloudy
Temperatures: 47-53 deg. F. 32-38 deg. F. 46-52 deg. F.
Wind direction: East shifting to the southwest West Southwest
Wind speed: 10-20 mph with gusts to 30 mph decreasing to 10 -15 mph in the afternoon 10 mph 10-15 mph with gusts to 30 mph increasing to 20-35 mph with gusts to 60 mph in the afternoon
Expected snowfall: O in. O in. O in.

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