Moderate Avalanche Hazard exists throughout the West Central Mountains, human triggered avalanches are possible today. New wind slabs formed in the last 24 hours. Older wind slabs from last week’s storms still exist as well and may be overlying a weak layer of facets or in some areas a combination of crust and facets. Watch for visual clues of where these new slabs have formed and where old wind slabs still exist.
Avalanche Problem 1: Wind Slab
Last week’s storm left us with wind slabs on a variety of aspects, some of these slabs are resting on a weak layer of snow above a well preserved crust. This layer has produced human triggered avalanches in the last week. While these slabs are not widespread, their ability to propagate and the unpredictable nature of their distribution should cause you to approach steep, exposed terrain with caution today and through the next few days.
Yesterday we found a combination of wind slabs in wind loaded upper elevation terrain that were overlying soft unsupportable snow that consists mainly of graupel, or pellets. These slabs are most likely to be found on upper elevation North through East aspects today but be aware that other aspects could also be harboring pockets of wind slab on minor terrain features or where crossloading has occurred. Watch for visual clues of wind effect and manage your slope angles as you travel in wind affected terrain. These slabs should begin to bond as the temperatures increase over the next few days.
Avalanche Problem 2: Persistent Slab
A layer of faceted snow exists above the February 4th crust and around a series of sun and melt freeze crusts in some areas. These layers have produced avalanches in windloaded areas over the last 2 weeks, and fresh evidence yesterday on Council Mountain and the surrounding ridgelines. The layer of concern is now buried just about 2-3 feet down in the snowpack and has shown the potential to propagate easily when triggered by a skier or rider which is what happened on the W/SW aspect of Sargent’s Mt. that produced a good sized avalanche last Sunday. In some areas, this layer bonded to the new snow above and the crust has broken down or was very weak to begin with, in other areas. possibly more prevalent in the middle elevations 6500-7500 feet, the crust and facet combo persists and should be taken very seriously. The distribution of the layer is erratic and the only 2 ways you are going to insure your safety are by either digging in and finding out if this layer is present where you are riding or skiing or by avoiding slopes over 35 degrees. You can also help minimize your risk by using good travel protocols and avoiding steep, rocky areas where the snowpack is less uniform and you are more likely to find a shallow trigger point.
Conditions are variable across the forecast area right now. Our advisory is based upon a small number of observations. Please let us know what you are seeing out there whether it is deep stable snow or avalanche activity. Observations can be submitted here.
Yesterday we toured Council Mountain. Natural Avalanches had failed about 2 feet down on a graupel layer that produced moderate to hard results (ECTP25) in our pit tests. There was also a combination on the surface of wind slabs and graupel and surface hoar that were not bonded to each other. This layer could produce big avalanches in the future as it is very well preserved right now, and within the depth that skiers and riders can effect.
Area Forecast Discussion
National Weather Service Boise ID
329 AM MDT Sun Mar 11 2018
.SHORT TERM…Today through Monday…Upper ridge over the interior
western US. Embedded short wave trough has produced clouds over
our CWA this morning, and radar indicates sprinkles or flurries in
eastern Oregon. These may get into Valley County, ID, this morning.
No other pcpn expected. Clouds will shift east and eastern Oregon
will become sunny this afternoon. Central Idaho will stay mostly
cloudy today, with clearing tonight. Models still have 2-3 degree
cool bias on Max temps, and current forecast has adjusted for it.
Today will be 1-3 degrees warmer than Saturday except in central
and south-central Idaho where clouds will hold temps down a little.
Clearing tonight will allow for good radiational cooling, especially
where there is still snow cover. Monday will be several degrees
warmer than today everywhere with sunny skies and increasing east/
.LONG TERM…Monday night through Sunday…Unsettled with a cooling
trend. Models agree on a cold front pushing across our area on
Tuesday ahead of a deep cold upper level trough centered off the
northwest coast. Afternoon thunderstorms are possible as the front
moves through. Widespread precipitation will follow the front
Tuesday night. Snow levels will be between 7000 and 8000 feet,
then lower to around 5000 feet on Wednesday. The trough will remain
nearly stationary through Friday, centered over the northern
California coast. It will generate scattered showers through the
period, with snow levels near 5000 feet. Temperatures will average
5 to 10 degrees above normal ahead of the front, then cool to near
normal by Thursday. As the trough moves inland over the weekend,
temperatures will fall to around 5 degrees below normal.
.AVIATION…VFR with mostly clear skies through Monday. Surface
winds will be variable less than 10 kts, becoming southeast 10-15
kts Monday afternoon. Winds aloft at 10k ft MSL will be west to
northwest 10-20 kts.
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.