Avalanche Forecast Stevens Pass

Wednesday 23rd January 2019

Avalanche Danger Ratings Wed 23rd Jan 6:06PM Danger Ratings Alpine: Considerable Danger Ratings Treeline: Considerable Danger Ratings Below Treeline: Moderate Storm Slabs Storm Slabs Persistent Slabs Persistent Slabs Loose Wet Loose Wet

Forecaster: Matt Primomo

Date Issued:

Valid Until:

The Bottom Line: Snowfall and precipitation totals are impressive from Tuesday and Wednesday. The snowpack will take time to adjust to this heavy new load. Uncertainty remains with a deeper weak layer. Enter the backcountry with your best thinking cap on, and treat steep slopes with caution.

Snow and Avalanche Discussion

Stevens Pass racked up over 2” of snow water equivalent in about 13” of snow Tuesday night and Wednesday. Snow accumulated rapidly as heavy snow fell and the temperatures warmed up, this caused a widespread natural avalanche cycle throughout the Cascades. Many large paths ran in the area, entraining wet snow as the debris ran through the water saturated lower elevation snowpack. It appears it rained up to around 5,000ft, but uncertainty remains regarding how far down the water percolated into the snowpack.

Weather Forecast

Thu 24th Jan 13:53 - Kenny Kramer

Weather Synopsis for Thursday night through Saturday

An upper-level ridge of high pressure is centered offshore and extends to the Gulf of Alaska, diverting Pacific storms well north of the area. A weak disturbance will move over the ridge and pass to the north of the area Thursday night, mainly causing increasing high clouds across the north and central part of the forecast area with no threat of precipitation. The passing disturbance will enhance cooler NW flow and temporarily lower freezing levels Thursday night. 

The ridge gains strength Friday through Saturday and moves closer to the coast. This will maintain mostly fair weather with a warming trend, especially at higher elevations.

Moisture will continue to be trapped at lower elevations under the building high pressure, maintaining low clouds and fog in the lower elevations, valleys and passes with some afternoon clearing expected.

Dry conditions with mostly sunny days at higher elevations will continue under high pressure through Saturday and into the weekend with high freezing levels as the ridge remains along the coast.

Regional Synopsis

Sun 20th Jan 09:00

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.

Storm Slabs Storm Slabs

Likelihood

Possible

Expected Size

1 - 2

A substantial amount of new snow is to be found at elevations above 5,000ft, where precipitation fell as all snow. The snow fell cold initially, and temperatures warmed up as the snow accumulated. At upper elevations, the snow may be dry, and deep drifts are likely to be encountered. What is this new snow sitting on? Is there strong over weak? Is the new snow cracking, or do you see recent avalanches? Storm slab instabilities should decrease during the day as time helps settle these out. The deep new coat of snow will take time to bond, however. Travel with a conservative mindset, giving the snowpack another day to heal.

Aspects: All aspects.

Elevations: Alpine, Treeline.

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Persistent Slabs Persistent Slabs

Likelihood

Possible

Expected Size

3 - 3

A layer of surface hoar that was buried on January 17th should be 2 to 3 feet down from the surface. We just added a heavy new load to this layer. You're most likely to find it in wind-sheltered, shaded, and open terrain, possibly up to ridges, and east of the Cascade Crest. On the 23rd, observers continued to report signs of instability, with an extremely loud WHUMPH, that in turn triggered another WHUMPH on a Northwest aspect at 4800ft near Grace Lakes. Observations show this layer is present at Grace Lakes, Yodelin, Lichtenberg Mountain, Smithbrook, and Jove Peak areas. At lower elevations, the snow surface has become wet, and water has made its way through. The snowpack will take time to refreeze, but probably won’t do so by tomorrow. As such, it’ll be best to treat the terrain like this layer could still fail. Small and large snowpack test can help you identify this weak layer and gather information. Lack of evidence in one profile or snowpack test shouldn't be a reason to travel in more consequential terrain.

The size of the potential avalanches has just increased with the new coat of snow. As such, any avalanche on this layer would be very large and surprising.

Aspects: All aspects.

Elevations: All elevations.

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Loose Wet Loose Wet

Likelihood

Possible

Expected Size

1 - 2

If and when the sun comes out, expect loose wet avalanches on steep, sunny slopes. These could entrain a large amount of heavy, wet snow if they make it down to lower elevations. If you see roller balls, recent loose avalanches, or notice you are sinking in past your ankle in wet snow, it's time to get off of steep slopes. 

Aspects: South East, South, South West.

Elevations: All elevations.

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