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Archived

Avalanche Forecast

Feb 14th, 2019–Feb 15th, 2019
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
3: Considerable
The avalanche danger rating at treeline will be considerable
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate
Alpine
3: Considerable
The avalanche danger rating in the alpine will be considerable
Treeline
2: Moderate
The avalanche danger rating at treeline will be moderate
Below Treeline
2: Moderate
The avalanche danger rating below treeline will be moderate

East to Southeast winds built firm wind slabs on unusual aspects and cross-loaded slopes Thursday. Look for wind stiffened snow in lower angled terrain as clear evidence you can trigger a large wind slab in steeper terrain on similar aspects. Shallow wind slabs may also be found on steep open slopes below treeline.

Discussion

Snow and Avalanche Discussion

An amazing amount of snow, 4-5 feet, has fallen in the West Slopes South zone since the weekend, with the storm peaking Monday night and Tuesday. Avalanche activity associated this cycle tapered off through midweek. A fresh round of E-SE winds redistributed the recent low density storm snow, wind scouring windward aspects and cross-loading slopes Thursday.  

It's important to note we have had limited field observations over this stretch due to the strong storm conditions. Throughout this cycle, Paradise has been closed for several days due to winter storm conditions. Based on the information we do have, all the recent snow is sitting on a hard crust 2-4 ft down, but unlike in adjacent forecast zones, a weak snow(facet)/crust combo isn't widespread and/or reactive. 

Keep the touchy and dangerous avalanche conditions capable of producing very large avalanches found in adjacent zones in the forefront of your mind Friday while we gather more snowpack information heading into the weekend.  

We've received a lot of snow over the last few days. The risk of tree well and snow immersion suffocation is real in the trees. Don't travel alone and always keep in contact with your partner. You can learn more about deep snow safety here.

Snowpack Discussion

Since February 8th, the mountains (and low elevation cities) of the Pacific Northwest have experienced cold and very stormy weather. Significant snowfall has added up in all forecast zones. Records from Snoqualmie Pass DOT avalanche workers back to 1973 show that February 11-12th set a record for the most snow recorded in a 24hr period at that location. The table below shows storm totals starting February 8th through the morning of the 13th 

5 day totals ending morning of Feb 13th

Water Equivalent (inches)

24hr storm totals

(inches)

Difference in Height of Snow (inches)

Hurricane Ridge

1.97

N/A

+ 30

Mt. Baker

1.94

44

 

Washington Pass

1.66

NA

+ 16

Stevens Pass

 

2.71

49

 

Snoqualmie Pass

3.91

80

 

Mission Ridge

1.86

38

 

Crystal

2.91

59

 

Paradise

4.55

N/A

 

White Pass

N/A

57 (4400ft)

+ 26 (5800ft)

Mt. Hood Meadows

4.70

43

 

Heavy precipitation brought many mountain regions to their tipping point. Avalanches ran readily with a peak of snowfall intensity. For Stevens Pass, Snoqualmie Pass, East Central, West South, Mt Hood, and possibly West Central zones we have good confirmation that this cycle happened from the night of February 11th through the 12th. In other zones, snow totals haven’t been significant enough for widespread avalanche cycles, or we lack data (like in the East South zone).

The high rates of precipitation drove avalanches in the storm snow. Notably, a persistent weak layer of facets and surface hoar was buried in most zones on February 8th. Storms produced a widespread and prolonged cycle of avalanches on the February 8th interface, involving a variety of aspects and elevations. Local ski patrols, highway workers, and backcountry travelers reported extensive avalanching with widely propagating crowns and very sensitive conditions. With less stormy weather, observers have just begun to get a sense of the extent of the avalanche activity. Triggering persistent slab avalanches will be a concern for backcountry travelers in zones where the February 8th weak layer is active for at least the near, if not distant future. Stay tuned for more updates.

Avalanche Problems

Wind Slabs

East winds built wind slabs on unusual aspects and cross-loaded slopes in the West Slopes South zone on Thursday. Winds were strong enough to transport snow onto open slopes below treeline in the Crystal and White Pass areas. New snow and moderate south winds will continue to move snow above treeline Friday. 

To avoid triggering a wind slab avalanche, approach steep unsupported slopes with wind-drifted snow cautiously, feeling for firm or hollow sounding snow as a sign that wind slabs may be present. You can stay safe by traveling on ridges, wind-scoured areas and slopes less than 35 degrees. Remember, firmer wind slabs can be triggered from lower angled slopes well below start zones.

If more than 6 inches of snow accumulates Thursday afternoon and night, shallow storm slabs will be possible in wind sheltered terrain on convex rollovers or steep unsupported slopes. 

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: All elevations.

Likelihood: Likely

Expected Size: 1 - 1