Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - North East 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 Deep Persistent Slabs.

You can trigger avalanches in the recently fallen and drifted snow. Conditions are most dangerous at upper elevations where east winds will form fresh drifts. With cold temperatures and significant snow accumulations in the past 5 days, the snowpack needs time to adjust to the heavy load of new snow.

Summary

Discussion

On Tuesday, observers reported drifts and wind slab features formed in the past 48 hours at upper elevations from westerly winds. On Monday, an observer near Vasiliki Ridge reported 2 feet of recent storm snow since January 10th and tests showing lingering instability on the Dec 27th surface hoar, buried over 3 feet below the surface. On Saturday, observers reported triggering small avalanches in recent storm snow above 6,000ft on southerly aspects of Delancy Ridge.

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

East winds will intensify throughout the day becoming strong enough to drift snow. It will be easiest to trigger avalanches at upper elevations. You may find cross-loading on open slopes in lower terrain, especially near passes and divides. Use caution on leeward, wind-loaded slopes 35 degrees and steeper where you see drifted pillows of snow and thick wind features. Use small test slopes to determine if the snow can slide. Watch for clear signs of the danger like recent avalanches, shooting cracks, blowing snow, and collapses.

In wind-sheltered terrain at low elevations, you may be able to trigger small loose dry avalanches. While easy to predict, these avalanches can be dangerous near gullies, cliffs, and rocks. 

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.

Deep Persistent Slabs

An icon showing Deep Persistent Slabs

Likelihood

unlikely

Expected Size

1 - 2

Information from upper elevations is limited, increasing our uncertainty regarding a deeply buried persistent weak layer near and above treeline. What we do know is that at least two large, remotely triggered slabs occurred due to a layer of buried surface hoar, and/or weak snow near a crust. This layer can be found about 5ft below the surface at 6,000ft near Washington pass, but is likely quite a bit deeper at higher elevations and in wind deposited areas. It is becoming deep enough that you probably won't observe feedback such as whumphing or shooting cracks as you travel. New snow and wind drifting may increase the load on these layers.

A healthy amount of respect for steep slopes at upper elevations is still warranted. Give yourself plenty of distance from avalanche terrain above 6,000ft. It may be possible to trigger a deep slab from far away or on connected slopes from below. A small slab or cornice failure on the surface may step down to this deeper layer, creating a much more destructive avalanche.

Release of a thick cohesive layer of hard snow (a slab), when the bond breaks between the slab and an underlying persistent weak layer, deep in the snowpack or near the ground. The most common persistent weak layers involved in deep, persistent slabs are depth hoar or facets surrounding a deeply buried crust. Deep Persistent Slabs are typically hard to trigger, are very destructive and dangerous due to the large mass of snow involved, and can persist for months once developed. They are often triggered from areas where the snow is shallow and weak, and are particularly difficult to forecast for and manage. They commonly develop when Persistent Slabs become more deeply buried over time.

 

Deep Persistent Slabs avalanches can be destructive and deadly events that can take months to stabilize. You can trigger them from well down in the avalanche path, and after dozens of tracks have crossed the slope.

 

A snowboarder triggered this Deep Persistent Slab near treeline, well down in the path.

Deep, persistent slabs are destructive and deadly events that can take months to stabilize. You can triggered them from well down in the avalanche path, and after dozens of tracks have crossed the slope. Give yourself a wide safety buffer to handle the uncertainty, potentially for the remainder of the season.

Aspects:

All aspects.

Elevations:

Alpine, Treeline.

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