This week produced another round of snow with accumulations in the 10-15 inch range above 6500 feet. 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 yesterday with loading occurring on the NE throughout the day. You should expect to see and anticipate reactive wind slabs on upper elevation leeward terrain today. These slabs will continue to build through the day today with and additional 3-7 inches of new snow, increasing temperatures and winds from the S around 17 mph.
The snow that fell throughout the week this week came in small increments of light density snow that did not have a major effect on stability. 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 yesterday 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. It is possible that you could trigger a weakness in one of these layers on steep terrain resulting in a shallow slab in the 6 to 12 inch range today. Worth noting and watching over the next 24 hours is a subtle crust created by a freezing mist event that occurred mid morning on Monday. It is buried between 6 and 12 inches down in the snowpack. This was one of the layers that is failing in compression and may become more reactive as we add more snow to it over the next 24 hours. We found this layer to be fairly widespread yesterday and Thursday in the mid to upper elevations and had reports of it from the South Valley area as well.
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. Notice the layering in the SW pit and the depth of the freezing mist crust and new snow between the two pits.
No new avalanche activitiy has been reported or observed.
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|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|
.SHORT TERM...Today through Sunday...Warm-frontal snow event is
about to begin. A significant moisture plume is moving onto the
coast as the warm front develops and heads our way. The NAM is
still an outlier with high QPF values, but as all other models
continue to trend down with QPF, have lowered snow amounts again
in this forecast. I am concerned that the NAM could end up
verifying best based on high PW values arriving later this
morning, but decided to go with the model consensus. Temps will be
tricky as always in these types of systems, with precip holding
temps down initially, and then the warm frontal influence trying
to increase them later. Have again gone below guidance for most
areas, with little change from previous forecast. Guidance agrees
that a weak impulse will follow the warm front this evening,
producing another, though much lighter, precip event. Snow levels
for this will range from 4000 feet in the north to 6500 feet in
the south, so this will be rain for most folks. Sunday will be
warmer, with highs near 10 degrees above normal. Precip will be
moving off to the northeast, with lingering showers mainly in the
mountains of the north.
No changes to locations/timing of the winter weather warnings and
advisories, but will issue an update to indicate slightly lower
.LONG TERM...Sunday night through Saturday...A pattern change
will bring cooling starting Wednesday as the upper level flow
shifts to northwest. An upper level high pressure ridge will keep
our area dry Sunday night and Monday. Moist westerly flow will
replace the ridge Monday night, spreading mountain snow and valley
rain into Baker County Oregon and the west central Idaho
mountains, reaching as far south as Burns and Boise on Tuesday.
Tuesday night and Wednesday a high pressure ridge building off the
coast will shift the upper level flow into the northwest over the
Intermountain Region. Precipitation will be mainly confined to
our northern mountains, and temperatures will be around 10 degrees
cooler at all locations. This pattern will persist through
Saturday, but precipitation coverage will become more uncertain by
the weekend as models diverge.
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.