Avalanche Forecast Snoqualmie Pass

Date Issued: Valid Until:

Dallas Glass,

Avalanche Forecast

Tue Jan. 22nd · 6:07PM

Alpine

Danger Ratings Considerable

Treeline

Danger Ratings Considerable

Below Treeline

Danger Ratings Moderate

Problems

Storm Slabs Storm Slabs
Loose Wet Loose Wet

Alpine

Danger Ratings Considerable

Treeline

Danger Ratings Moderate

Below Treeline

Danger Ratings Moderate
The Bottom Line: A winter storm will impact the Snoqualmie Pass area Tuesday night and Wednesday morning with warming temperatures, rain, and snow. Avoid open slopes greater than 35 degrees where you find more than 8 inches of new snow, or the wind drifts new snow into thicker slabs.

Snow and Avalanche Danger

We are expecting significant precipitation with warming at Snoqualmie Pass Tuesday night and Wednesday morning. Avalanche danger should peak overnight during the warmest and stormiest periods. As precipitation ends and temperatures cool, the avalanche danger will slowly decrease.

The weak snowpack structure seen near Stevens Pass and the East-Central region does not seem to be as prevalent in the Snoqualmie area--that does not mean it is absent. We were able to find buried surface hoar near Cottonwood Lake/Roaring Ridge last week. An observation from Sunday also suggests an old weak snow layer may be present. This layer could exist 1-2 feet below the snow surface. If you experience any collapsing, or see larger avalanches, it’s time to dial it back and avoid avalanche terrain nearby.

Regional Synopsis

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 form 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.

Weather Forecast

Wed 23rd Jan 14:29 - Dennis D'Amico

Weather Synopsis for Wednesday night through Friday

Post-frontal showers continue to wind down this afternoon and should taper off completely in the early evening. A strong upper level ridge will rebuild offshore Wednesday night and become the dominant weather feature for the next several days.

Some low-level moisture/clouds will likely be trapped along the west slopes of the Cascades Thursday under the ridge. While cloudier conditions are expected along central-west and northwest Cascades, afternoon cloud cover should decrease for areas further south including the south Washington Cascades and Mt. Hood. The east slopes of the Cascades should enjoy mostly sunny skies on Thursday. Freezing levels will be on the rise in general, but will make for a tricky forecast with warm air struggling to make inroads for the north and central Cascades. Higher freezing levels are a better bet over the Olympics and the Mt. Hood area. 

A dry shortwave trough passing through in NW flow aloft will temporarily flatten the ridge and bring an increase in mid and high level clouds Thursday night. On Friday, the upper level ridge will quickly rebuild, but we will left in a repeat scenario with the central and north Cascades struggling to take advantage of the warmer air to the west and south. Also, NW alpine winds will become moderate to strong following the trough's passage on Friday. 

Problems

Storm Slabs

Storm Slabs

Above the rain-snowline, we expect upside-down storm snow caused by warming temperatures, heavy precipitation rates, and moderate winds. In these locations, you will be able 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. Simple observations will help you identify if storm slabs formed overnight. 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? If you answer yes, storm slabs are nearby.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Likely

Expected Size

2 - 2

Loose Wet

Loose Wet

Below the rain-snow line a wet snowpack will develop overnight. You may trigger loose wet avalanches on steep slopes greater than 35 degrees. Even small loose avalanches can be harmful if they carry you into trees, over cliffs, or bury you in a gully. As temperatures cool and the snowpack dries, wet avalanches will become more difficult to trigger.

Aspects: All aspects.

Elevations: Below Treeline.

Likelihood

Possible

Expected Size

1 - 1