Avalanche Advisory published on March 17, 2018 @ 6:15 am
Issued by George Halcom - Payette Avalanche Center
bottom line

Considerable avalanche hazard exists due to recently formed storm slabs that are not bonding well, but resting on slick crusts. Lingering persistent weak layers are buried 1 to 3 feet below the surface mixed into layers of crusts, graupel, and facets. Added snow load today will increase the possibility of Natural avalanches, and make Human triggered avalanches likely. 

How to read the advisory

  • Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Storm Slab
  • Character ?
  • Aspect/Elevation ?
  • Likelihood ?
    Very Likely
  • Size ?
    Very Large

The new snow over the last few days came in initially dense, and later lightened up. The denser snow created soft storm slabs that are not bonding well to the old snow surface that developed into a crust, and more pronounced and slicker on aspects that are tilted towards the Sun. Northern aspects stayed a little cooler during last weeks warm up and developed a less stout/thick crust and showed better signs of bonding. Adding almost a foot of new snow today, we could likely see some avalanches go naturally. Human triggered avalanches will be possible, especially on steep solar effected aspects. Avoid steep terrain that is exposed,above cliff bands, terrain traps, or slopes that come to an abrupt end in trees or rocks, and use good, cautious travel protocols today.


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

Several persistent weak layers exist in the snowpack, which are mostly graupel and crust combinations. 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 propagate significant distances across the slope and could result in avalanches large enough to bury a person. 

advisory discussion




recent observations

Yesterday, we toured multiple aspects near Black Tip and Squaw Meadows and observed a prominent crust under 6-10 inches of new snow. On solar influenced aspects, the new snow had not bonded well and was easily pushed off of the crust with a ski cut, and in mid turn. The upper part of the new snow was very light , and produced minor sluffing on slopes steeper that 35 degrees. Test pits produced moderate compression and easier shear scores, and revealed the slabbier new storm snow to be resting on yet another well preserved graupel layer. Northern aspects stayed a little cooler and the crust did not develop quite as much, and seemed to bond the new snow better overall. The sun came out briefly, but long enough to produce some shallow point releases near steep Southern rock outcrops. 

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

Area Forecast Discussion...CORRECTED
National Weather Service Boise ID
326 AM MDT Sat Mar 17 2018

.SHORT TERM...Today through Sunday...Band of precipitation near a
surface trough extends NW-SE across southern Idaho early this
morning. The band will shift to the northeast with snow ending
across south-central Idaho from the Western Magic Valley to the
ID/NV border in a couple hours. A Winter Weather Advisory is in
effect for areas along the ID/NV border, which will be cancelled
once the band of snow lifts north. The snow will continue in the
Boise/West Central Mountains today where a Winter Weather Advisory
is also in effect. The upper low centered in Oregon will continue
to move eastward, reaching southwest Idaho this afternoon.
Showers and isolated afternoon thunderstorms will accompany the
upper low. A cold front will move in behind the low tonight
through Sunday morning. Another round of snow showers is expected
along the front, mixing with rain in the lower valleys. Snow
accumulations will be light, except enhanced amounts are expected
in the higher elevations of southwest Idaho. A Winter Weather
Advisory will remain in effect for the Owyhees and Southwest
Idaho Highlands tonight through Sunday morning, along with the
Boise/West Central Mountains. Drier conditions will arrive behind
the front on Sunday. Temperatures will remain below normal through
the weekend.

.LONG TERM...Sunday night through Saturday...An upper level
trough swings in from off the Oregon coastline at the beginning of
the period bringing a slight chance of showers for our northern
zones. By Wednesday a much deeper upper level trough sitting off
the California coastline starts to impact our area. We remain in
the warm sector through early Friday morning which means
widespread precipitation. Snow levels climb up to near 6000 feet
MSL for much of the area so snowfall limited for the highest peaks
only. The cold front is still on track to push through Friday
morning dropping snow levels back down to valley floors. Models
diverge after the main frontal band moves through leaving low
confidence in the forecast to start the weekend.


.AVIATION...Mostly VFR. Showers pushing from the NV/ID border
will continue through early morning. Increased shower activity
Saturday afternoon with chances of thunderstorm activity. MVFR/IFR
within snow showers/heavier rain showers. Surface winds: SE
Oregon, west-northwest 10-20kts. SW Idaho, south-southwest
10-20kts. Winds aloft to 10k feet MSL: SE Oregon, northwest
30-40kts. SW Idaho, southwest 20-35kts.

Two-Day Mountain Weather Forecast Produced in partnership with the NOAA-NWS
McCall Airport at 5021 feet.
  Saturday Saturday Night Sunday
Weather: Snow showers. High near 33. West northwest wind 5 to 7 mph becoming south southwest in the afternoon. Chance of precipitation is 90%. New snow accumulation of 2 to 4 inches possible. Snow showers. Low around 23. South southwest wind 3 to 5 mph. Chance of precipitation is 90%. New snow accumulation of 1 to 3 inches possible. A 50 percent chance of snow showers. Cloudy, with a high near 35. Calm wind becoming west around 5 mph in the afternoon. New snow accumulation of less than a half inch possible.
Temperatures: 33 deg. F. 23 deg. F. 35 deg. F.
Wind direction: West Northwest South South West West
Wind speed: 5 to 7 mph 3-5 mph 5 mph
Expected snowfall: 2-4 in. 1-3 in. 1 in.
Granite Mountain at 7700 feet.
  Saturday Saturday Night Sunday
Weather: Snow showers. High near 24. North northeast wind 8 to 10 mph becoming southwest in the afternoon. Chance of precipitation is 100%. New snow accumulation of 6 to 10 inches possible. Snow showers. Low around 18. West southwest wind 7 to 9 mph. Chance of precipitation is 100%. New snow accumulation of 4 to 8 inches possible. Snow showers likely, mainly before noon. Mostly cloudy, with a high near 24. Northwest wind 7 to 9 mph. Chance of precipitation is 60%. New snow accumulation of 1 to 2 inches possible.
Temperatures: 24 deg. F. 18 deg. F. 24 deg. F.
Wind direction: North Northeast West Southwest Northwest
Wind speed: 8-10 mph 7-9 mph 7-9 mph
Expected snowfall: 6-10 in. 4-8 in. 1 to 2 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.