Avalanche Forecast
Issued: Feb 4th, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
The West-North zone receive some very light, dry snow Sunday and Monday. This new snow may be hiding some older wind slabs, especially at higher elevations. As the sun comes out Tuesday, you may find that avalanches become easier to trigger on steep, sunny slopes.
Summary
Discussion
The Mt Baker area picked up 8 inches of very light, dry snow Sunday night into early Monday morning. This new snow fell with very light winds and cold temperatures. Consequently, the most recent snow is generally unconsolidated and may be poorly bonded to the old snow surface. On Monday, this resulted in several small loose dry avalanches. Cold temperatures and clear skies, should keep the snow cold and dry on any slope not receiving direct sunshine.
Snowpack Discussion
January, 31, 2019
As we turn the corner to February we're coming out of a week-long high pressure ridge and into unsettled weather. The snowpack survived extremely warm temperatures and sunny skies over the week. This break in the weather allowed for avalanche danger to steadily decline in all regions. Stormy weather starting February 1st once again elevated the avalanche danger and brought a round of storm related avalanches.
We’ve heard a variety of stories from backcountry travelers over the past week. There have been reports of extremely firm slopes creating slide-for-life conditions. Others reported perfect spring like snow. Some encountered difficult breakable crust. And, for a lucky few, softer, drier, mid-winter snow has been found. A common thread in most zones is where precipitation falls as snow, it has landed on slick surfaces. It's time to pay attention to the interfaces formed by our recent rounds of precipitation.
A skier triggered storm slab in the recent snow in a steep southeast start zone at 5700ft. Lichtenberg Mtn. Feb 2. Photo: Josh Hirshberg
North-South:
While a high elevation rain event, around January 23rd, formed surface crust in many regions, it’s the constant melt-freeze cycles from the past week, that caused a divergence in the Northerly and Southerly snowpacks.
Near surface facets developed on shaded snow surfaces in many areas. These facets are from near Mt Baker. Photo: Lee Lazzara
North: On shady slopes, things haven’t exactly been soft. The crust formed at the end of the last storm extends to high elevations (Mt. Hood 7000+ft, South Cascades 6500 ft, Passes and Central Cascades 6000 ft. and West-North 5500 ft.). Only areas in the East Cascades seemed to escape the wrath of this breakable crust. Without the help of the sun, shady slopes haven’t been softening even during this period of warm weather. Instead, the surface crust underwent some weakening. Observations found faceting on top of and below this crust. In some locations, this caused the crust to begin to degrade, becoming less supportive. Surface hoar has also been reported from the typical valley bottoms and sheltered terrain near water sources. At low to mid elevations, rain may have melted any weak snow on the surface. Slopes receiving significant dry snow should be suspect for a poor bond at the interface buried around February 1st.
Roller balls and loose wet avalanches on a sunny, southeast slope following the Feb 1st storm Lichtenberg Mtn. Photo: Josh Hirshberg
South: On sunny aspects, the sun drove warming and melting of surface snow. Long, cool, winter nights allowed for the surface to freeze again. This repeating melt-freeze pattern created a thicker, firmer, and more supportable surface. On many days, weak surface snow, such as near surface facets or surface hoar, melted during the day limiting its development. On cooler days, very firm travel conditions were reported. Crusts may provide a poor bond for any snow falling on them. Following the Feb 1st storm, the sun drove a round of small wet loose avalanches and storm slabs on upper elevations slopes that received enough accumulation.
East-West:While we're tracking persistent weak layers (PWL’s), they haven't been the source of avalanches for over a week. It’s not uncommon for our east-side forecast zones to experience lingering PWL's. This season, we’ve also seen several different PWL’s in our western zones. This break in the weather gave the snowpack time to gain strength in all zones.
West: While you may find some weak snow in the upper few inches of the snowpack, the mid and lower snowpack has been found to be quite strong. Firm rounded grains, stout crust, and strong frozen melt-forms make up the majority of the snowpack at this time.
Southeasterly winds textured surfaces on the east side of Washington Pass on Feb 1-2nd. Photo Matt Primomo
East: The east-side snowpack continues to be highly variable. You may find deep strong snowpacks closer to the crest or you could encounter shallow weak snowpacks areas further east. While there are number of potentially weak interfaces, there are two more common layers we’ve got our eyes on.
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January 22nd surface hoar and small facets. You can find these just under the recent storm snow, about a foot down. .
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December 26th surface hoar. This layer can be found from 16” to 40” down and is still producing clean, planar shears with tests.
You are most likely to find these layers to be preserved on wind sheltered, shady, and open slopes above 5,500ft. You can find more defined weak layers where snowpack is less than 4 feet deep and variable especially east of Highway 97. Persistent weak layers have been “dormant” or unreactive during the week of high pressure. The latest storm has not been enough to re-activate theses weak layers. We’ll keep tracking them to watch their progression..
The lower eastern slopes and the Columbia River. Snow exists at low elevations, but snowpack depths are shallow. Photo: Matt Primomo
Problems
Wind Slabs
It may be difficult to spot older wind slabs with the new snow on the surface. Additionally, wind patterns over the past few days originated from several different directions. This may have resulted in wind slabs in some unusual locations. You’ll need to use large clues such as fresh cornices, or big snow drifts to indicate the wind transported the snow. You can also feel the snow as you travel. Does it change from place to place? Does it feel hollow or firm? All of these observations may suggest wind slabs may be in nearby terrain. If you suspect there are wind slabs, it’s best simply to avoid open slopes greater than 35 degrees.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
While you may be able to trigger loose avalanches on any aspect, they will be easier to trigger and more consequential on slopes receiving direct sunshine. Very cold temperatures can limit the sun’s effect on the surface snow, but we don’t think they will eliminate it. You may find wet snow on steep, rocky, sunny slopes. When this occurs, expect natural loose avalanches. Don’t let these small avalanches catch you off guard, especially if you are traveling around higher consequence terrain such as gullies or cliffs.
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: East, South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 5th, 2019 10:00AM
