Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - East Avalanche Forecast

Jan 10th, 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 Storm Slabs.

A cold winter storm cycle continues. Exercise cautious route finding to stay off of, and out from underneath steep slopes if you find signs of instability such as shooting cracks within new and wind thickened snow. Especially dangerous conditions may be found at upper elevations, where more snow and wind continues to pile up.

Summary

Discussion

Mission Ridge reported a few small slabs with control work in previously unskiied terrain on Friday where only a couple inches of new snow fell. Much more snow fell closer to the crest, and at least 10" accumulated at the Sasse Ridge Snotel near the Salmon la Sac. On Thursday observers found a substantial amount of recent wind loading at upper elevations near Blewett Pass and in the Icicle Creek area. A skier unintentionally triggered a small slab (D1) on a northeast aspect at 6,700ft. This was on a convex feature near a rocky outcrop, and about 10" deep. He reported a substantial natural avalanche cycle that likely took place on December 6th and 7th. The most notable was a very large slab with a 6 to 9ft deep crown. This originated on north aspect at 7,200ft in Icicle Creek. Another observer found a reactive layer of buried surface hoar down about 2ft at 5,800ft on an east aspect near Diamond Head. We have limited information about this layer in the area, so it will be prudent to dig down to look for deeper weak layers on Saturday.

Snowpack Discussion

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

At upper elevations and on open slopes near treeline, the wind will continue to drift the new and previous snow into thick slabs, creating strong over weak layering. These may be touchy, and are likely to be more dangerous as one increases elevation. Look for wind stiffened snow, shooting cracks, and changing surface textures as you transition into exposed terrain. If you find signs of instability, steer around fresh drifts that may sit below ridgelines and on open slope features.

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

likely

Expected Size

1 - 1

Instabilities with the new snow are likely to linger with cold temperatures preserving a density change that developed on Friday. Be observant as you travel, step, or ride off into virgin snow once in a while, and look for shooting cracks as you travel. In areas near the crest such as the Salmon la Sac and Teanaway, expect deeper and reactive storm slabs. There may not be enough new snow for storm slabs near and east of Blewett Pass. If you find signs of instability such as recent soft avalanches within deep new snow, be sure to set your boundaries. Communicate openly and intentionally as a group to help choose which slopes you may and may not want to enter.

Observers have found a layer of buried surface hoar above 5,500ft in select locations. New snow and wind drifting may increase the load on this layer, and deeper slabs may be possible. Dig down and perform snowpack tests to look for it. Put a large buffer of terrain between you and any slope that is steep enough to slide if you suspect this layer exists. A healthy amount of respect for steep slopes at upper elevations is warranted due to uncertainties regarding this layer. 

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.

Aspects:

All aspects.

Elevations:

All elevations.

Valid until: Jan 11th, 2020 10:00AM