Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - North West Avalanche Forecast

Jan 14th, 2020 10:00AM

The alpine rating is considerable, the treeline rating is considerable, and the below treeline rating is moderate. Known problems include Wind Slabs and Loose Dry.

Shifting moderate to strong winds bring new avalanche concerns with fresh, dangerous and subtle large slabs you easily trigger. In exposed areas, look for evidence of snow transport or a slightly denser layer near the top of the snowpack and avoid nearby slopes steeper than 35 degrees. In wind-sheltered terrain, expect loose dry avalanches may out-pace your descent on slopes steeper than 40 degrees.

Summary

Discussion

An active storm cycle with over 5 ft of snow over the last 4.5 days in the West North zone is winding down. The last several days brought very-low-density powder falling with light winds at unseasonably cold temperatures in 10's, dropping into the single digits by Tuesday afternoon. The last 24 hours added about 6" of snow with very low water content. 

Although widespread natural and human triggered avalanche activity was reported over the weekend with some slides triggered from lower angled slopes (observation), the storm slab was becoming less reactive by Monday (observation). We have no back-country observations from Tuesday, but we think that storm slab related weaknesses were healing without additional loading and have dropped them as a problem for Wednesday. 

With all the recent snow deep snow hazards are real. Before heading out, learn about tree well or snow immersion suffocation risks and travel with a partner (www.deepsnowsafety.org). Also, be prepared for the coldest temperatures seen this winter with winds creating frost-bite conditions and the conditions making rescue far more serious.

Snowpack Discussion

January 12th update: In the past week, there have been two fatal avalanche accidents to the east of NWAC's forecast area. One occurred near Kellog, ID and another outside of Baker City, OR. Local avalanche centers will perform accident investigations including final reports. You can find preliminary accident information at avalanche.org.

January 9th, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

As we said Happy New Year and rang in 2020, snow was turning to rain at many trailheads and lower elevation Passes, not exactly the fresh start winter recreationists had in mind. The snowpack was already looking a little thin throughout the region, especially at lower elevations. Low snow in places like Snoqualmie Pass made backcountry travel difficult and hazardous.  NWAC’s snow depth climatology report was showing snow depths 25-64% of normal to kick off the start of 2020.

Things can change quickly in the Pacific Northwest and they did as we entered an extended storm cycle between January 2nd to January 8th. Strong winds, fluctuating temperatures, and heavy precipitation offered few breaks in the weather over this period limiting observations and hampering travel. Despite periods of rain at lower elevations, most areas saw several feet of new snow with big jumps in total snow depths as a westerly storm track strongly favored the West Slopes of the Cascades and the Olympics for the highest precipitation totals.

Location

Total Snow Depth (in) 1/2/20

Total Snow Depth (in) 1/8/20

Hurricane Ridge

31

51

Heather Meadows Mt Baker

55

95

Washington Pass

49

74

Stevens Pass

41

63

Snoqualmie Pass

22

33

Alpental mid-mountain

44

63

Crystal Mt Green Valley

40

66

Paradise Mt Rainier

54

105

White Pass Upper

43

69

Timberline

36

57

Mt Hood Meadows

36

53

 

We may have started with a shallow snowpack, but most locations increased their snowpack by 70% or more over this storm cycle!

During this extended and impressive storm cycle that included backcountry avalanche warnings, natural avalanches were reported in many areas Jan 6th-7th.

The Stevens Pass area was especially active over the period with over 100(!) avalanche observations made on the 6th and 7th. Professionals reported numerous avalanches in places that they hadn't previously observed avalanches and some paths avalanched multiple times in a 24 hour period. Observers reported a few very large (size D2.5-3) avalanches, originating at upper elevations with deeper crowns that likely formed from wind drifting. Topping off an active couple of days, warming temperatures lead to a widespread loose wet avalanche cycle.

The southern Washington Cascades, the Wentachee Mountains and Mt. Hood either saw less precipitation, warmer temperatures leading to more rain than snow, or some combination of the two and ended up with relatively less active avalanche conditions than areas further north. 

A large natural avalanche on Rock Mountain near Berne along Hwy 2 east of Stevens Pass that released Jan 6th or 7th. Photo: Josh Hirshberg 1/7/20

Many small storm slabs released in the Crystal backcountry 1/6-1/7. Pinwheels in the photo suggest loose wet avalanche activity occurred when temperatures rose above freezing and snow turned to rain.

Another active and colder weather pattern is on it’s way. Enjoy yourself out there and be sure to check the forecast before heading out. Remember, NWAC is a community-supported avalanche center and when you submit an observation you make the forecast better!

-Peter Moore

It’s getting deeper! Photo: Jeremy Allyn

Problems

Wind Slabs

An icon showing Wind Slabs

Likelihood

likely

Expected Size

1 - 1

We are concerned that existing snow and up to a foot of low-density fresh snow anticipated Tuesday night into Wednesday morning will be easy fodder for effective wind transport in conjunction with moderate to strong SW to SSE winds. Large, subtle slabs on a variety of aspects may form near and above treeline. All it takes is a slightly firmer layer of snow near the surface and humans become prime triggers for large and fast-moving avalanches. Use quick hand shears to test for this firmer over weaker snow structure, and pay attention for active transport or where you find evidence of recent snow transport (such as scouring of the snow to the lee of vegetation or rocks). In these areas, avoid slopes over 35 degrees.

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:

Alpine, Treeline.

Loose Dry

An icon showing Loose Dry

Likelihood

likely

Expected Size

1 - 1

You are likely (even very likely) to get non-cohesive fast-moving loose dry avalanches to escort you down any slope approximately 37 degrees and steeper. There is a lot of deep snow out there and we keep adding more. We’re calling this out as a problem because we expect more momentum than you would expect in the upper foot of the snowpack. Try to avoid very long steep slopes or terrain traps (rocks, gullies, dense timber, or cliffs) below.  Keep visual contact with your partner and consider the location of each partner on the slope should one member get caught.

Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.

Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.

 

Loose Dry avalanche with the characteristic point initiation and fan shape.

Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.

Aspects:

All aspects.

Elevations:

All elevations.

Valid until: Jan 15th, 2020 10:00AM