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.

The high pressure ridge over the area has started to move eastward. During the next 24 hours cloud cover should begin to increase and the winds should shift to the southwest and increase as a low pressure system approaches the region. Temperatures should also continue to gradually cool down. The forecast calls for highs in the 40's today and the upper 30's and mid 40's tomorrow above 7000 ft. As this storm approaches tomorrow some light precipitation could begin to fall. Below 8000 ft this precipitation would likely fall as rain. The forecast does not call for any significant accumulation until the main part of the storm arrives on Tuesday night and Wednesday. For more information about this storm check in with the Reno NWS.

Recent Observations:

Yesterday 3-6 inches of soft wet snow existed above a supportable layer of melt-freeze snow on the E-SE-S aspects of Mt. Judah and Donner Peak at 3pm. On the most sun exposed SE aspects some areas of shin deep wet snow existed (photo). Ski cuts on steep test slopes did not cause anything more than small roller ball activity. On the northerly aspects a frozen melt-freeze crust existed above still moist snow. The layer that was once the March 6th facet layer has metamorphosed into more rounded snow grains. Due to the water present in this layer it remained soft and weak. Most of the several tests on this layer indicated that fracture propagation along this layer has become unlikely (video). Once this layer refreezes it should gain more strength.

Avalanche Problem #1: Loose Wet Avalanches

Sunshine and warm temperatures should allow enough melting to occur today for wet snow instabilities to form. Last night's strong refreeze combined with slightly cooler daytime temperatures, some expected cloud cover, and the fact that the snowpack has undergone several days of melt-freeze cycles should limit these wet snow instabilities to human-triggered roller balls, pin wheels, and point releases. In areas where last night's refreeze did not penetrate as deeply or where the overnight refreeze completely melts today, these wet snow instabilities could entrain large amounts of snow. The largest and most widespread wet snow instabilities should form on the most sun-exposed E-SE-S-SW-W aspects.

Something to remember: Persistent Slabs

Data collected this week indicates triggering a persistent slab avalanche has become unlikely. In many areas, the warm temperatures and water percolating through the snowpack have changed the sugary snow that comprises the old weak layers into rounded snow grains that can form bonds with one another. In some isolated areas above 8000 ft,  tests still show that in the unlikely event that the persistent weak layers do break, the resulting fracture could still travel through these layers. Whether these layers will support the additional snow load expected to arrive this week or they will fail and allow a persistent deep slab avalanche cycle to occur will depend on both how much they change before the storm and how widespread those changes are.

 

The bottom line:

This morning the avalanche danger is LOW for all elevations and aspects. Pockets of MODERATE avalanche danger may form on the E-SE-S-SW-W aspects in both above and below treeline terrain on slopes steeper than 35 degrees due to daytime warming. Human triggered wet snow avalanches will become possible.

Andy Anderson - Avalanche Forecaster, Tahoe National Forest

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

0600 temperature:	25-35 deg. F.
Max. temperature in the last 24 hours:	38-47 deg. F.
Average wind direction during the last 24 hours:	West
Average wind speed during the last 24 hours:	10-15 mph
Maximum wind gust in the last 24 hours:	37 mph
New snowfall in the last 24 hours:	O inches
Total snow depth:	52-82 inches

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

For 7000-8000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Mostly sunny becoming partly cloudy	Mostly cloudy	Mostly cloudy with a slight chance of rain and snow
Temperatures:	42-49 deg. F.	23-30 deg. F.	44-48 deg. F.
Wind direction:	Southwest	Southwest	Southwest
Wind speed:	10-15 mph with gusts to 25 mph	10-15 mph with gusts to 25 mph	10-15 mph with gusts to 25 mph increasing to 20-25 mph with gusts to 35 mph in the afternoon
Expected snowfall:	O in.	O in.	O in.
For 8000-9000 ft:
	Monday:	Monday Night:	Tuesday:
Weather:	Mostly sunny becoming partly cloudy	Mostly cloudy	Mostly cloudy with a chance of snow
Temperatures:	41-47 deg. F.	20-27 deg. F.	36-40 deg. F.
Wind direction:	West	Southwest	Southwest
Wind speed:	15-20 mph with gusts to 35 mph	15-25 mph with gusts to 35 mph	20-30 mph with gusts to 40 mph increasing to 50 mph in the afternoon
Expected snowfall:	O in.	O in.	O in.