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Avalanche Forecast

Archived

Mar 15th, 2018–Mar 16th, 2018

Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.
Alpine
Natural avalanches unlikely, human triggered possible.
Treeline
Natural avalanches unlikely, human triggered possible.
Below Treeline
Natural avalanches unlikely, human triggered possible.

Regions

Stevens Pass.

Spring-time weather will create the potential for loose wet avalanches on steep sunny slopes as warming temperatures and a strong March sun soften surface snow. If you see new loose wet avalanches, observe rollerballs, or experience soft surface snow deeper than your ankle, it’s time to change aspects. A low likelihood-high consequence deep persistent slab continues to linger in the Western Cascades. You can avoid this problem by staying out of large avalanche start zones.

Detailed Forecast

Spring like conditions will prevail Friday around the Western Slopes of the Cascades and Passes. Firm surface snow in the morning will be found in most locations. Be careful if you travel on these slopes before they soften. If you fall, it may be difficult to stop.

The sun will soften the surface snow throughout the day. Expect this to occur first on steep sunny slopes moving from eastern to southern to western aspects as the day progresses. If you see new loose wet avalanches, notice new roller balls, or experience soft surface snow deeper than your ankle, it’s time to change slopes. Use slope aspect to stay ahead of any loose wet avalanche problems. Be aware of high consequence slopes such as above cliffs or creeks where even a small loose wet avalanche may have high consequences.

We have been talking about persistent and deep persistent slabs in the Cascades for a month now. This difficult to assess avalanche problem has resulted in fatalities each of the last three weekends. This is a classic low likelihood-high consequence scenario. Tracks on a slope do not indicate that it is safe. It may take a very large trigger or finding a shallower spot in the snowpack to cause a deep persistent slab avalanche. We continue to receive reports from professionals around the area demonstrating that the snowpack structure for this layer is still present. These same professionals are conveying that they are still avoiding large avalanche start zones as they travel. If you want to avoid triggering a deep persistent slab, stay out of large steep open areas.

Continue to use appropriate travel routes and techniques that minimizes your exposure to large overhead avalanche paths. Do not linger in locations where avalanches may run or stop. 

Snowpack Discussion

In most areas 4-6 inches of new snow accumulated Wednesday and bonded to an underlying melt-freeze crust. The thickness and supportability of this crust varies by location. On southern aspects the crust is typically 4-6 inches thick and growing, while on northerly aspects it is only 1-2 inches thick. Sunny skies Thursday created moist to wet surface snow on steep sunny slopes resulting in roller balls and small loose wet avalanches.

Below this most recent crust a variety of snowpack structures have been reported. Most observations indicate strong layers of dry snow intermixed with various melt freeze crusts. In isolated locations, thin buried persistent weak layers have been reported. You are most likely to find a buried persistent weak layer in shaded locations, at higher elevations, and/or in terrain slightly east of the Cascade Crest.

A more widespread deep persistent layer continues to be found 3-6 feet below the snow surface. A thin layer of weak sugary facets have been observed just above a stout crust (2/8). Some snowpack tests (Compression Test and Extended Column Tests) will not be able to appropriately assess this deep weak layer. Deep Tap tests and Propagation Saw tests will yield the most reliable results. While snowpack tests may demonstrate the presence of a weak layer, they cannot prove its absences.

Observations

Baker

NWAC professional observer Lee Lazzara traveled near Glacier Creek Thursday. Lee found 3 to 8 inches of new snow well bonded to a forming melt-freeze crust. Sun affected snow was limited to steep sunny slopes. The 2/8 layer was down 6 feet in this area.

Stevens

NWAC forecaster Dallas Glass was in the Stevens backcountry Wednesday. Dallas observed 6 inches of new snow bonding well to a semi-supportable and strengthening crust. He observed the 2/8 layer down 4 feet in most locations.

Snoqualmie

NWAC professional observer Ian Nicholson traveled in the Alpental Valley Wednesday. Ian found 6 inches of new snow bonding well to the newest crust. He observed several loose wet avalanches up to D2 in size occurring as the sun affected the new snow.  

South

NWAC professional observer Jeremy Allyn traveled in the Crystal backcountry Thursday. Jeremy found 6 inches of new dry snow moderately bonded to a newly forming crust. He observed limited new loose surface snow concerns on steep sunny slopes. The 2/8 layer was found down 4 feet.

Problems

Loose Wet

Loose Wet avalanches are the 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. 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.

Deep Persistent Slabs

Deep Persistent Slab avalanches are the release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer deep in the snowpack. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage.