THIS AVALANCHE ADVISORY EXPIRED ON December 22, 2018 @ 6:17 am
Avalanche Advisory published on December 21, 2018 @ 6:17 am
Issued by Melissa Hendrickson - Idaho Panhandle Avalanche Center

St. Regis Basin/Silver Valley

bottom line

Overnight another round of snow has added loading to a pack that was still adjusting from the storms earlier in the week.  With winds in the 40's last night, expect to see widespread wind slabs on the leeward slopes and cross loaded gullies.  There is still a persistent weak layer in many locations; it was active in pit tests yesterday.  Assess each location you plan on riding or sliding to see how reactive this layer is. 

How to read the advisory

St. Regis Basin/Silver Valley

How to read the advisory

Overnight another round of snow has added loading to a pack that was still adjusting from the storms earlier in the week.  With winds in the 40's last night, expect to see widespread wind slabs on the leeward slopes and cross loaded gullies.  There is still a persistent weak layer in many locations; it was active in pit tests yesterday.  Assess each location you plan on riding or sliding to see how reactive this layer is. 

3. Considerable

?

Above Treeline
Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.

2. Moderate

?

Near Treeline
Heightened avalanche conditions on specific terrain features. Evaluate snow and terrain carefully; identify features of concern.

1. Low

?

Below Treeline
Generally safe avalanche conditions. Watch for unstable snow on isolated terrain features.
    Dangerous avalanche conditions. Careful snowpack evaluation, cautious route-finding and conservative decision-making essential.
Avalanche Problem 1: Wind Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

The storm system that hit yesterday afternoon and is still hitting the forecast area this morning has wind gusts in the 40's at the higher elevations.  Expect to find well developed wind slabs on leeward slopes.  Wind slabs will often look like smooth, rounded pillows of snow that are chalky white in color.  They may sound hollow as you are skiing over them on more mellow terrain.     

Avalanche Problem 2: Storm Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

New snow overnight deposited a storm slab of up to 9" in the higher elevations.  Give this new layer some time to bond and adjust. 

Avalanche Problem 3: Persistent Slab
  • Type ?
  • Aspect/Elevation ?
  • Likelihood ?
    Certain
    Very Likely
    Likely
    Possible
    Unlikely
  • Size ?
    Historic
    Very Large
    Large
    Small

Buried surface hoar was still found at multiple locations yesterday and was propagating in stability tests.  There are also locations that have weak faceted snow near the base of the pack still.  It is important to check each individual area you plan on riding and sliding because these persistent layers are variable spatially.  Faceted snow will be a weak and sugary layer in the pack or at the bottom next to the ground.  Rain up to 5000' last week stabilized these layers at the lower elevations, but they are still found widely in the pack at the higher elevations. 

recent observations

Yesterday we travelled to 6200' on the Idaho/Montana border to the north of Mullan Pass.  We observed a lot of variability in the aspects, with the persistent weak layers being most reactive on the north and west aspects.  While at our location, the other aspects showed improved bonding and stability, I would still expect to find the buried surface hoar in more sheltered areas.  Every slope seems to have a slight variation right now.  The buried surface hoar was about 80cm down in the pack at our location (total pack of 145cm).  I would also expect this to be much more reactive around trigger points and in shallower pack areas.  In the afternoon yesterday, the winds were very strong out of the southwest and were transporting avaliable snow.  They only increased overnight with the new snow that came in.  

Weather and CURRENT CONDITIONS
Two-Day Mountain Weather Forecast Produced in partnership with the Spokane NWS
For 2000 ft. to 4000 ft.
Friday Friday Night Saturday
Weather: Chance Snow Mostly Cloudy Mostly Cloudy
Temperatures: 31 deg. F. 25 deg. F. 31 deg. F.
Wind Direction: SW SW S
Wind Speed: 8-11, G23 6 5
Expected snowfall: <0.5 in. 0 in. 0 in.
For 4000 ft. to 6000 ft.
Friday Friday Night Saturday
Weather: Snow and Breezy Chance Snow Slight Chance snow
Temperatures: 21 deg. F. 18 deg. F. 22 deg. F.
Wind Direction: W SW SW
Wind Speed: 21-25, G46 15-17, G28 13-15
Expected snowfall: 1-2 in. <1 in. 0 in.
Disclaimer

Avalanche conditions change for better or worse continually. Backcountry travelers should be prepared to assess current conditions for themselves, plan their routes of travel accordingly, and never travel alone. Backcountry travelers can reduce their exposure to avalanche hazards by utilizing timbered trails and ridge routes and by avoiding open and exposed terrain with slope angles of 30 degrees or more. Backcountry travelers should carry the necessary avalanche rescue equipment such as a shovel, avalanche probe or probe ski poles, a rescue beacon and a well-equipped first aid kit.  For a recorded version of the Avalanche Advisory call (208)765-7323.