Avalanche forecast

Selkirk Mountains

Idaho Panhandle Avalanche Center
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Avalanche danger

Sat
Alpine
2 · Moderate
Treeline
1 · Low
Below
1 · Low
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Highlights

A shallow snowpack with 18 inches of soft recent snow on 2.5 feet of older snow.  Some Persistent weak layers exist in the lower snowpack consisting of older near surface facets and a some surface hoar.  With up to a foot of snow expected and temperatures rising with increased winds expect the layers to become more reactive through the weekend.

Avalanche problems

  • Storm Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    With 120 cm of total snowpack in our first weakness was at 65 cm consisting of some surface hoar that sat atop some old near surface facets at 60 cm with another faceted layers at 40 cm.  The 65 cm layer only propogated about 10 cm with hard result but came apart with more force.  With a foot more snow possibly on the way, these layers will get tested as they adjust to the new weight and may be easier to trigger.  

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1–2

    With increasing winds associated with new precipitation, expect the 20 plus mile per hour winds to cause some windslabs near the upper ridgetops which will make the new storm snow more likely to react.  

Avalanche Discussion

National Weather Service Spokane WA 355 AM PST Fri Dec 18 2020

.SYNOPSIS... The continuation of a series of storm systems arrives Friday afternoon. Precipitation will be light with a mix of rain and snow across the northern mountain valleys and accumulating snow over Stevens and Lookout Pass. A wet, windy and mild period will set up over the weekend into Monday. Periods of moderate rain with melting low elevations snow will result in sharp rise on small streams and creeks with minor flooding possible. Colder and drier weather will arrive by Tuesday heading into the Christmas break.

&&

.DISCUSSION...

Friday and Saturday: This morning will start out dry, aside from lingering snow showers in the Cascades and the higher elevations of NE WA and the ID Panhandle. This break will be very short-lived as an upper level trough sitting offshore will begin to push inland this afternoon, bringing another round of precipitation to the region. Snow levels will start between 2000 to 3000 feet in the northern WA valleys. Communities in these northern valleys, such as Republic, Colville to Northport, Newport to Metaline Falls, and Bonners Ferry will see a rain/snow mix increasing the chances for messy roads this afternoon. The Columbia Basin will see little precipitation as strong westerly flow aloft creates a substantial rain shadow. The Palouse, the Spokane/CdA area, and the L-C valley will see primarily rain with up to 0.10 inches of rainfall expected. The main concern will be across the mountain passes such as Stevens and Lookout which could receive up to 10 inches of additional snowfall tonight. Winds will pick up behind the front tonight into tomorrow morning. Gusts up to 30-40 mph will be possible in the Wenatchee area and the Waterville Plateau across the Columbia Basin into the Palouse and the Spokane/CdA area. Another brief break from precipitation is expected Saturday morning and afternoon before the next system moves inland. Winds will remain breezy throughout the day on Saturday and temperatures will feel quite mild with highs in the 40s.

...Heavy rain and snow melt could create localized flooding issues on small creeks and streams...

Saturday night and Sunday: A strong atmospheric river begins to push onshore Saturday night into Sunday morning. With this, the heaviest precipitation will be late Saturday night into early Sunday morning. Snow levels will rise from around 3000 ft to around 5000 ft as low level southwesterly flow advects relatively warm air into the region. Heavy mountain snow above 5000 ft in the Northern Cascades will be likely with 1-2 feet expected for the Cascade crest. The Mountain passes will most likely be spared from any significant snowfall amounts. They may see a few inches of snow before switching over to a rain/snow mix or completely all rain. Valleys in the east slopes of the Cascades, including the Methow Valley, won`t see as much snowfall accumulations, but will potentially see around 2-4 inches of snowfall. The Cascade Valleys may also see light freezing rain Saturday night into Sunday. There is low confidence in the freezing rain forecast for these areas as temperatures are marginal. Westerly flow aloft will allow for significant rain shadowing with this system as well with whereas QPF values are around a few hundreths of an inch for the Basin and up to 0.20-0.50 inches for the Spokane/CdA area, the Palouse, and the L-C Valley.

Another concern with this system are the winds. However, models continue to trend weaker with the speeds, as gusts are now expected to be quite similar to Friday`s system. Gusts 30-40 mph are possible across the Basin into the Palouse and the Spokane/CdA areas. With both systems, a noticeable cross wind will be possible along Highway 195 and in some areas along US-2. Sunday will be the warmest day this week with highs in the upper 40s and low 50s. VMT

Sunday night through Friday: Starting off Saturday night with a low amplitude ridge with significant moisture fluxing across it delivered via a reasonably maintained subtropical moisture tap. The lower level orientation of the ridge favors warm air advection to keep snow levels above most lowland and valley floors and a good number of the lowland locations and low temperatures above freezing coupled with wet weather and cloudy conditions. A portion of the frontal zone associated with the moisture tap moves northward as a warm front Monday. Ensembles and some deterministic models suggest a deepening and intensifying area of low pressure on the backside of this baroclinic band/warm frontal feature that is in alignment with an upper level trof moving in from the west. This should result in the remainder of the baroclinic band becoming more meridionally aligned before passing through as a cold front. It will be accompanied by a very conditionally unstable airmass behind it and robust windy/gusty southwest winds as the surface low associated with this deepening and intensifying system tracks west to east within close proximity of the Southern British Columbia Border Monday night. This system is moving very quickly and by Tuesday morning it is almost out of the area and being replaced by a drier and cooler northwest flow ahead of an incoming ridge, as such minor pops for light snow remain in North Idaho Tuesday/Tuesday night while elsewhere skies should show considerable clearing and may allow for some late night/early AM fog and temperatures start showing a cooling trend. The ridge axis remains off the coast allowing the drier and cooler northwest flow to linger on into Wednesday allowing the cooling trend to continue. The ridge axis moves inland into Friday and low level heat trof placement off the coast orients low level pressure gradient to surface winds light and from a cool northeast/east trajectory of approach and the subsidence inversion brought about by the ridge axis positioning allows for potentially stagnant weather conditions and forecast temperatures to run much more closer to what would be considered normal for this time of year. /Pelatti

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