Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - East Avalanche Forecast

Jan 12th, 2020 10:30AM

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

Dangerous avalanche conditions remain at upper elevations due to recent wind drifting and storm totals. Steer around freshly formed wind drifts and watch for a poor bond in the recent storm snow on slopes steeper than 35 degrees. The storm will finally taper off on Monday and be replaced with very cold temperatures.

Summary

Discussion

On Sunday, Observers reported multiple triggered avalanches in the Wenatchee Mountains. Some of these were big enough to bury or kill a person and at least one was triggered remotely. See this observation from Blewett Pass. On Saturday, observers found reactive, shallow storm slabs below treeline in Icicle Creek. On Table Mountain near Blewett Pass, another observer reported 8" of recent snow with wind slabs forming in isolated locations near treeline. Also on Saturday, Mission Ridge Ski Patrol triggered wind slab avalanches up to 10" deep with control work.

Near to the Cascade Crest and west of Highway 97, you can expect both deep storm totals and wind playing a factor. In the Wenatchee Mountains and eastern foothills near Highway 97, less snow has accumulated in the past three days, but westerly winds may have had even more of an impact in forming slabs.

After significant storm totals in the past week, deep snow alone can be dangerous. Deep snow around tree-wells and open creeks can pose a hazard for suffocating. Make sure to travel with a partner and to be aware of the danger (www.deepsnowsafety.org). Expect the coldest temperatures of the winter so far. 

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 recreationalists 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

Strong westerly winds have drifted snow onto leeward slopes below ridgetops. These avalanches could break around or above you. Slabs will be harder than in wind-sheltered terrain. Avalanches will be bigger and more dangerous as you increase elevation and get closer to the Cascade Crest. Look for wind stiffened snow, shooting cracks, and changing surface textures as you transition into exposed terrain. Watch for signs of instability, and steer around fresh drifts and wind-loaded 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:

Alpine, Treeline.

Storm Slabs

An icon showing Storm Slabs

Likelihood

possible

Expected Size

1 - 1

Storm slab avalanches are lingering but should become less reactive than they were over the weekend. Expect deeper and more dangerous slabs near the Salmon la Sac, Teanaway, and Icicle drainages. Step, or ride off into untracked snow once in a while, and look for shooting cracks as you travel. Avalanches could break within storm layers or beneath the three-day storm total. Check the interface (January 10th) beneath the recent snow. If you find weak snow or facets, use extra caution near slopes 35 degrees and steeper.

Storm totals increase substantially closer to the crest. The Sasse Ridge Snotel site (4,300ft) has gained almost 30" of snow with 3" of water equivalent since the morning of the 10th. By contrast, Mission Ridge has picked up just under 1" of water equivalent in that time.

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Elevations:

Below Treeline.

Valid until: Jan 13th, 2020 10:30AM