Our current avalanche hazard is a bit of a head scratcher. Buried surface hoar and basal facets exist in a growing and strengthening snowpack. Tuesday's storm added another 12-15 inches of snow on top of a house of cards. The layer of buried surface hoar that we have been watching is now 2-3 feet down in the snowpack below a slab that is gaining density and depth but not getting stronger. So far the weight of the new snow has not been enough to tip the scale on a widespread natural avalanche cycle.
Forecasters and savvy backcountry travelers know this problem as the sleeping dragon. We know it is a serious concern and it is very reactive in snow tests, right now it hasn't produced widespread natural avalanches. What we don't know and what makes it tricky to forecast is when it is going to wake up into a serious problem or what the trigger will be. If triggered this layer has the potential to spread out or propagate over large distances.
The layer of basal facets is still present throughout our snowpack as well, it is buried deeply enough in most places that a skier or snowmobiler is going to have a hard time triggering it. It is also starting to warm up slightly and become less sugary BUT in the right area, especially shallow rocky areas you could still trigger an avalanche that would break near the ground. There is also a possibility of triggering the more shallow buried surface hoar and having it step down into the basal facets resulting in a large and dense and very un-survivable avalanche.
Your best and safest bet right now is to avoid protected slopes steep enough to produce avalanches. This lingering problem won't go away until we have enough snowload to crush the weak layers or trigger a natural cycle.
Saw Propagation Test Video with impressive failure in Buried Surface Hoar Layer 12/20/18
3 days of moderate and strong winds have produced wind slabs on multiple aspects as winds have shifted from S to SW and W. Wind slabs of varying thickness and strength can be found on upper elevation terrain on E, N and W terrain. Some of these may be overlying buried weak layers. Watch for visual clues such as ripples, pillows or changes in texture of the snowpack as you travel through the wind affected upper elevations. In addition, the amount of wind transport is allowing a dense and surprisingly deep snowpack to build quickly in the upper elevations.
We were able to trigger the buried surface hoar easily in our test pits yesterday. See the video above in the persistent slab discussion. We also had reports and saw the remnants of a natural avalanche that occurred earlier in the week. This natural cycle was not widespread so that indicates that the buried surface hoar layer is still waiting for a trigger in many areas.
We also observed widespread wind transport throughout the leeward terrain. The photo below shows the buried surface hoar layer marked by the snow saw at 80cm/31 inches down that was very reactive in our tests.
|0600 temperature:||16 deg. F.|
|Max. temperature in the last 24 hours:||26 deg. F.|
|Average wind direction during the last 24 hours:||SSW|
|Average wind speed during the last 24 hours:||16 mph|
|Maximum wind gust in the last 24 hours:||26 mph|
|New snowfall in the last 24 hours:||na inches|
|Total snow depth:||na inches|
SHORT TERM...Today through Sunday...The quick moving cold front
continues to progress through Idaho this morning. Snowfall and
rain showers were left in its wake, along with periods of gusty
wind. Patchy valley fog will be possible through the morning
hours. The front should generally be east of the area by this
afternoon. Periods of precipitation will be possible near the
ID/NV border through this evening as a follow-up disturbance and
its associated moisture move through the area. A dry northwest
flow will be found Saturday morning, with dry conditions expected
through Saturday evening. A fairly potent Pacific system will then
begin to move into eastern Oregon Saturday evening, and continue
into Idaho overnight. Precipitation will be possible across the
entirety of the forecast area on Sunday, with the mountains
remaining the most favorable location to see potentially
significant snowfall. Stay tuned for updates regarding this
upcoming system, especially those traveling for the holidays.
.LONG TERM...Sunday night through Thursday...A wet zonal flow begins
our extended forecast period bringing widespread rain and snow. Snow
levels hover around 3000 to 4500 ft Monday and Tuesday meaning lower
valleys will see all rain. Tuesday morning a northwest flow sets up
which helps lower snow levels down to valley floors. The remainder
of the period stays active, however confidence in timing and
placement is still less than desirable. Precipitation that does fall
Wednesday onward will be in the form of snow for all locations.
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.