Loose, unconsolidated snow has been the trend for over a week creating a soft and unconsolidated upper snowpack with a firm and strong slab below it. In our pit tests Friday, we saw several of these individual storm layers that were only partially bonded to the layers below creating moderate failures in compression but lacking propagation or the energy to spread out over large areas.
Over the last 24 hours, temperatures have been on the rise along with new snow which will have likely made the storm slab that much denser than the snow below it. You could trigger a weakness in one of these layers on steep terrain resulting in a slab a foot deep or more today. Worth noting and watching is a subtle crust created by a freezing mist event that occurred mid morning on Monday. It is buried around 12 inches down in the snowpack. This was one of the layers that is failing in compression and may become more reactive now that we have added new, heavier snow on top.
Recent winds and new snow have created wind slabs in leeward terrain. Winds have been gusty throughout the week and mostly out of the S and SW. North and South Valley areas have both seen the same weather this week with cornices slowly growing and wind loading occurring on mostly E, NE, N and NW facing slopes. We observed active wind loading and scouring on a SW aspect just south of Granite Mountain Friday with loading occurring on the NE throughout the day. These slabs have gotten denser with the added new snow, and increasing temperatures.
See the photos below for a comparison of the difference in the snowpack on a SW facing slope at 7100 feet and a NNE slope at 7600 feet on Granite mountain from Friday. Notice the layering in the SW pit and the depth of the freezing mist crust and new snow between the two pits.
Please let us know what you are seeing in the West Central Mountains. Take the time to submit an observation or send us an email. It's easy and may save a life. If you are having trouble adding photos to your observation, send us the photo at our email address and we will add it to your observation. Click on the observation tab on the advisory page or email us at: firstname.lastname@example.org
|0600 temperature:||14 deg. F.|
|Max. temperature in the last 24 hours:||15 deg. F.|
|Average wind direction during the last 24 hours:||WSW|
|Average wind speed during the last 24 hours:||10 mph|
|Maximum wind gust in the last 24 hours:||18 mph|
|New snowfall in the last 24 hours:||NA inches|
|Total snow depth:||NA inches|
Area Forecast Discussion...CORRECTED
National Weather Service Boise ID
305 AM MST Sun Jan 28 2018
...ADDED INFORMATION ABOUT FOG IN SHORT TERM SECTION...
.SHORT TERM...Today through Monday...Will hold on to the Winter
Weather Advisory through 5 am MST due to continuing snow in the
mountains. McCall has picked up around 5 inches so far, and
Brundage around 7 inches. Fog is becoming widespread as some
clearing occurs in mid and high clouds. The fog is even dense in
patches. Will add this to the forecast and keep a close eye in
case a dense fog advisory becomes warranted. As upper level
ridging builds in from the west, moisture remaining over the
northern mountains will be pushed away to the northeast. Today, we
will still have snow showers there, but by this evening they
should be gone. This ridge will then keep us dry and around 10
degrees above normal for Sunday night and Monday. The ridge axis
will move through early Monday, allowing southwest flow aloft to
return. This will bring an increase in mid and high clouds from
west to east through the day. Southeast winds 10-15 mph will
develop Monday afternoon in the western Magic Valley westward
through the Mountain Home area. Otherwise, winds will be less than
10 mph through the period.
.LONG TERM...Monday night through Sunday...Moist westerly flow
aloft will bring mountain snow and valley rain to our northern
zones Monday night and Tuesday. Tuesday night and Wednesday the
flow will shift into the northwest as an upper level high pressure
ridge builds off the west coast. This pattern change will bring
cooler air, but temperatures are expected to stay a few degrees
above normal. Weak disturbances embedded in the flow will continue
the chance of mountain snow and valley rain, mainly for Baker
County Oregon and central Idaho.
.AVIATION...Areas of snow with IFR and local LIFR conditions will
continue this morning over the mountains of Baker County Oregon
and central Idaho. Snow will decrease in coverage this afternoon
and end by 00z Monday. Expect VFR conditions tonight except for
patchy valley fog and stratus after 06z. Surface winds will be
variable mainly less than 10 kts. Winds aloft at 10k ft MSL will
be northwest 25-35 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.