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West Slopes Central - Skagit River to South of I-90

Issued: 6:08 PM PST Tuesday, January 22, 2019
by Robert Hahn

New and thick storm slabs should form overnight and into the morning as a moisture-laden winter storm impacts the West - Central region. You will be most likely to trigger avalanches in locations above the overnight rain/snow line where more than 8 inches of snow accumulated, or the wind drifted the snow to greater depths. You can avoid triggering an avalanche by staying away from open slopes greater than 35 degrees.

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Snow and Avalanche Discussion

A winter storm is impacting the West-North area Tuesday night into Wednesday morning. We don’t know whether the impressive forecasted ~1.5” of overnight snow water equivalent will materialize. We expect avalanche danger to peak overnight during the heaviest precipitation and warmest temperatures which are expected to occur Tuesday night. Avalanche danger should then slowly decrease throughout the day as precipitation ends and temperatures cool.  However, avalanche danger may increase locally during periods of heavier snowfall under an anticipated convergence zone which may develop near or north of Stevens Pass.

Two small skier triggered avalanches were reported to the north of this zone in the Mt Baker backcountry Tuesday. These small slides highlight the presence of a firm crust below the recent snow. Avalanches Wednesday could slide on this old crust allowing them to run farther and faster.

Avalanche Problems for Wednesday

Storm Slabi

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We expect new snow to fall with warming temperatures and moderate winds. This should create upside-down storm snow. If this occurs, you will be to trigger avalanches on open slopes greater than 35 degrees. Avalanches will grow larger and be easier to trigger as you go up in elevation, or venture into areas where the wind drifted the new snow. You can use small slopes to test the storm snow. Has the area received more than 8 inches of new snow? Do you see cracking? Can you feel stronger snow over weaker snow? Do you see signs of wind drifted snow? When you answer yes, storm slabs are nearby.

 

Loose Weti

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This warm system will bring rain up into the near treeline band. The Loose Wet avalanche concern will peak overnight or early in the day as rain and warmer temperatures continue to add liquid to the snowpack. These can ruin your day particularly if they push you into a trap or obstacle or over a cliff. Cooling temperatures should limit this problem to below treeline during the daytime hours.

 

January 20, 2019

The recent weather pattern of lower accumulation storms (by NW standards) and longer stretches of calm weather should continue as we move into late January. Since January 17th, incremental snow accumulations punctuated with rising freezing levels favored the south and eastern parts of the region. Storm instabilities have risen with storms and gradually subsided.

A storm slab at Mt Baker.

New Snow Problems

Storms over the past week have brought a range of layers from rain crusts, to heavy moist snow, to stiff drifts, to light dry powder. Some storm days, like the 18-19th, saw reactive, but very short-lived avalanches caused by heavy precipitation and wind. Even the longer-lasting avalanche problems, wind slabs, haven't persisted for more than a few days. Where the recent snow is stressing underlying weak layers, more dangerous avalanche conditions have prevailed.

Surface hoar in the East Central zone

Old Snow Problems

Persistent weak layers (PWLs) have been a constant in the eastern zones of the Cascades this winter. As usual, they have been much less problematic at the Passes and west of the Cascade Crest. The latest PWL is a layer of surface hoar, buried around January 17th and found generally east of the Cascade Crest. Buried surface hoar is an active weak layer in the eastern zones and can be found to a limited extent on the eastern edge of the Stevens and Snoqualmie Pass zones. There few, if any, avalanches have been reported on the buried surface hoar. It may be most problematic in open, wind-sheltered terrain, especially well above the valley floor.

You are most likely to find other layers of old weak snow the further you move east from the Cascade crest. Here snowpacks are shallower, more variable, and generally weaker. In some locations, weak snow near the ground can still be found. These basal facets have hung around all season. Digging profiles and using snowpack tests is the best way to gain information about these old persistent weak layers. However, snowpack tests are just one piece of the puzzle. Your terrain decisions shouldn't hinge on any given test result. Because of the size of our forecast zones and the variability in the snowpack, it's important to make snow observations as you travel. We’ll keep watching these old layers, but let us know what you see while you are in the mountains.

No Corresponding Mountain Weather Forecast Available