Area Forecast Discussion
National Weather Service Boise ID
323 AM MDT Sun Mar 18 2018
.SHORT TERM...Today through Monday...Upper trough situated over
the area this morning continues to support snow showers, mainly in
the Boise/West Central Mountains where a Winter Weather Advisory
is in effect until noon. Additional accumulations of 1 to 3
inches are expected, putting storm total amounts of around a foot
in McCall. Scattered showers will continue across the remainder of
southwest Idaho with little additional accumulation. The Winter
Weather Advisory for the Southwest Highlands and Owyhees has been
cancelled. A weak upper ridge with drier conditions in southeast
Oregon will move into southwest Idaho this evening. Snow showers
may linger in the west central Idaho mountains but little
additional accumulation is expected. Limited moisture within a
northwest flow aloft will be enough for a chance of snow showers
over the higher terrain Monday afternoon, mainly in the north. The
departing trough and weak ridging will result in a warming trend
through Monday, although temperatures will remain slightly below
.LONG TERM...Monday night through Sunday...Our unsettled pattern
continues in the extended model runs. A weak upper level trough
pushes through our northern most zones Tuesday with a much deeper
and more potent low impacting us mid-week. Moist southwesterly
flow out ahead of the main trough axis will begin impacting us
late Tuesday into Wednesday with widespread precipitation. Snow
levels still climb up near 7000 feet MSL leaving snowfall to the
highest elevations. As the main trough axis pushes through Friday,
snow levels plummet down to valley floors. Model agreement after
Friday continues to lack consistency between runs leaving low
confidence in the forecast for the start of the weekend.
.AVIATION...Mostly VFR. Showers continue through mid-morning on
the Idaho side with persistent snow showers at KMYL causing
MVFR/IFR conditions. Isolated showers during the afternoon hours,
mostly in the higher elevations near the ID/NV border and central
Idaho mountains. Surface winds, westerly 5-10kts except from KMUO
southeastward with winds 15-20kts with afternoon gusts 20-25kts.
Winds aloft to 10k feet MSL, westerly 10-20kts.
Yesterday, we toured into Maloney Lake, East of Boulder Lake. Our Test pit at 8,000 feet on a North aspect showed about 1.4 feet of new light density new snow resting on a crust. We were getting moderate results in compression and ended up performing an ECT and got propagation across the block on a crust/facet layer 50cm (1.6 FT) below the surface. Multiple mit pits revealed that the new snow, again, showed better signs of bonding on Northern, cooler aspects. We were able to get some localized storm slabs to release on some rollovers that were steep and un-supported at the bottom. We observed a couple natural storm slabs that pulled out on the steep North face of Jug Mtn above Louie Lake.
The new snow over the last few days came in initially dense, and later lightened up. The denser snow created soft storm slabs that are not bonding well to the old snow surface that developed into a crust, and more pronounced and slicker on aspects that are tilted towards the Sun. Northern aspects stayed a little cooler during last weeks warm up and developed a less stout/thick crust and showed better signs of bonding. Adding almost a foot of new snow today, we could likely see some avalanches go naturally. Human triggered avalanches will be possible, especially on steep solar effected aspects. Avoid steep terrain that is exposed,above cliff bands, terrain traps, or slopes that come to an abrupt end in trees or rocks, and use good, cautious travel protocols today. Yesterday, we observed a couple natural storm slabs that pulled out on the steep North face of Jug Mountain above Louie Lake.
The last couple days have given us a gift of light snow with just enough winds to form wind slabs in exposed, and lee terrain. The wind slabs that are of most concern would be the ones that are resting on crusts that developed from the Sun last week, and are not bonded well.
Yesterday we were able to easily trigger a wind slab on a South aspect near the ridgline above Maloney Lake. The winds at ridgetop were fairly strong, and were filling it in quickly.
Several persistent weak layers exist in the snowpack, which are mostly graupel and crust combinations. A thin ice layer with large facet formation has been found around 2-4 feet down from the snow surface. A separate layer that formed in early February is still lingering 4 feet below the surface. Varying levels of stability have been observed in these layers across the zone. Typically these layers will be difficult to trigger and have demonstrated increased stability with warmer temperatures and recent rains. However, if you trigger this layer avalanches may propagate significant distances across the slope and could result in avalanches large enough to bury a person. Yesterday, in the headwaters of Lake Fork Creek on a North aspect we were able to get this layer to fail, although hard results ECTP28, there still is energy that can propagate a fracture...you are most likely to trigger this layer in a shallow, rocky, thin spot in the snowpack.
|0600 temperature:||17 deg. F.|
|Max. temperature in the last 24 hours:||26 deg. F.|
|Average wind direction during the last 24 hours:||Southwest|
|Average wind speed during the last 24 hours:||7 mph|
|Maximum wind gust in the last 24 hours:||15 mph|
|New snowfall in the last 24 hours:||inches|
|Total snow depth:||65 inches|
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.