Avalanche Advisory published on March 16, 2018 @ 7:00 am
Issued by -
bottom line

Spring is arriving in the valleys, but it’s still winter in the mountains. A moderate avalanche hazard exists due to recently formed storm slabs and strengthening, but still lingering persistent weak layers that are buried 1 to 3 feet below the surface. Human triggered avalanches remain possible today and you should steer clear of terrain where the consequences of being caught in an avalanche would be significant, such as cliff bands, terrain traps, or slopes that abruptly end. 

How to read the advisory

  • Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.
Avalanche Problem 1: Storm Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

Over the past couple days several inches of medium density snow fell onto an ice/rain crust that formed during this week’s warm temperatures. The new snow at upper elevations and has not bonded well to the old layer.  Avalanches in this layer will most likely be small in size, but could create a problem in consequential terrain and/or areas with terrain traps. Backcountry users should avoid terrain above cliff bands, terrain traps, or slopes that come to an abrupt end in trees or rocks. 

Avalanche Problem 2: Persistent Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

Several persistent weak layers still exist in the snowpack. A thin ice layer with large facet formation has been found around 12 to 18 inches from the snow surface. A separate layer that formed in early February is still lingering 3 feet below the surface. Varying levels of stability have been observed in these layers across the zone. Typically these layers will be difficult to trigger and have demonstrated increased stability with warmer temperatures and recent rains. However, if you trigger this layer avalanches may propogate significant distances across the slope and could result in avalanches large enough to bury a person. 

advisory discussion

SHORT TERM...Today through Saturday...A large upper low
centered off the Oregon coast will continue to spread moisture
into the area. Scattered showers this morning will become numerous
by afternoon. Snow levels of around 3000 feet will rise to around
4000 feet this afternoon. Light snow accumulations are possible,
mainly in the mountains. The proximity of the low with colder air
aloft will promote isolated thunderstorms this afternoon through
early this evening. The center of the upper low will track
eastward, reaching eastern Oregon tonight and southern Idaho on
Saturday. Precipitation will become enhanced along a cold front
situated east of the low, with a north-south oriented band of
rain/snow setting up east of Boise. The higher elevations will
receive several inches of snow within the band, mainly in the
Boise Mountains and near the ID/NV border, tonight through
Saturday morning. A Winter Weather Advisory is already in effect
for Southern Twin Falls County. The Western Magic Valley is
expected to start off as rain, then change to snow early Saturday
morning for light snow accumulations. The showers will continue
through Saturday afternoon as the upper low moves through the
area. Snow levels will range from 3000-4000 feet for additional
light snowfall accumulations. It will become windy late morning
in the Treasure Valley mainly near Mountain Home. The winds will
diminish by this evening. Temperatures will trend cooler through
the period.

.LONG TERM...Saturday night through Friday...Unsettled continues
to be the trend in the long term forecast. A series of troughs
passing through the region will bring periods of precipitation for
much of the area. The first trough passing through Sunday morning
drops snow levels to right around 3000 feet MSL. Most of SE
Oregon and SW Idaho will see snow showers with this passing system
with the Treasure Valley seeing a mixture of rain/snow. Little to
no snow accumulation is forecasted for the Treasure Valley. A
brief break comes Tuesday into Wednesday as a ridge of high
pressure pushes in from our SW. Precipitation begins to press into
SE Oregon again by early Wednesday afternoon ahead of the next
upper level trough sitting off the coast of California. Snow
levels climb up to near 7000 feet MSL by Thursday afternoon as we
sit in the warm sector of this system. The cold front pushes
through early Friday dropping snow levels significantly to valley
floors by the end of the period.

recent observations

Yesterday we toured near the Crestline Trail, in area west of Box Lake. Good snow stability was observed, but several persistent layers were found in the top 3 feet of the snowpack. Snow stability tests demonstrated that it was very difficult to get any of the persistent weak layers to fail. As the temperatures and amount of sun increased, layers near the surface became slightly more reactive, and small point released pockets and shallow slabs of snow began to move. Snow quality above 7000 feet was surprisingly excellent. 

CURRENT CONDITIONS Today's Weather Observations From the Granite Weather Station at 7700 ft.:
0600 temperature: 17 deg. F.
Max. temperature in the last 24 hours: 26 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: 7 mph
Maximum wind gust in the last 24 hours: 15 mph
New snowfall in the last 24 hours: 1 inches
Total snow depth: 65 inches
Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Friday Friday Night Saturday
Weather: Scattered snow showers. Some thunder is also possible. Partly sunny, with a high near 38. Light and variable wind becoming south southeast 5 to 7 mph in the morning. Chance of precipitation is 50%. Little or no snow accumulation expected. A 40 percent chance of snow showers. Some thunder is also possible. Mostly cloudy, with a low around 25. Light and variable wind. New snow accumulation of less than a half inch possible. Snow showers likely. Cloudy, with a high near 35. Southwest wind 3 to 6 mph. Chance of precipitation is 70%. New snow accumulation of around an inch possible.
Temperatures: 38 deg. F. 25 deg. F. 35 deg. F.
Wind direction: South southeast Southwest
Wind speed: 5 to 7 mph Light 3 to 6 mph
Expected snowfall: 0 in. 0.5 in. 1 in.
Granite Mountain at 7700 feet.
  Friday Friday Night Saturday
Weather: A 50 percent chance of snow showers. Mostly cloudy, with a high near 33. South southeast wind 10 to 13 mph. Total daytime snow accumulation of less than one inch possible. A 50 percent chance of snow showers. Mostly cloudy, with a low around 26. South wind 5 to 9 mph becoming light south southeast after midnight. New snow accumulation of less than one inch possible. Snow showers, mainly after noon. High near 29. West wind 3 to 8 mph. Chance of precipitation is 80%. New snow accumulation of 1 to 3 inches possible.
Temperatures: 33 deg. F. 26 deg. F. 29 deg. F.
Wind direction: South southeast South West
Wind speed: 10 to 13 mph 5 to 9 mph 3 to 8 mph
Expected snowfall: less than 1 in. less than 1 in. 1 to 3 in.

This avalanche advisory is provided through a partnership between the Payette National Forest and the Payette Avalanche Center. This advisory covers the West Central Mountains between Hard Butte on the north and Council Mountain on the south. 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 at midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.