Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - East Avalanche Forecast

Jan 11th, 2020 10:00AM

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

Another winter storm will maintain dangerous avalanche conditions, especially in areas closer to the crest. A healthy amount of respect is warranted for steep slopes at upper elevations due to uncertainties with our quickly deepening snowpack. Plan to give yourself options to avoid steep slopes, as snowfall during the day will increase the danger of slab avalanches. 

Summary

Discussion

On Saturday, an observer found very reactive, shallow storm slabs in Icicle Creek. I found 8" of new snow without storm slab concerns on Table Mountain near Blewett Pass, though wind slabs were a concern in isolated locations near treeline. Mission Ridge reported a few wind slabs up to 10" deep with control work, where 4 inches of snow fell at their mid mountain site. Storm totals increase substantially closer to the crest. The Sasse Ridge Snotel site (4,300ft) has gained 22" of snow with 2" of water equivalent since the morning of the 10th. Expect increased danger due to more snow further west in the zone, and perhaps less in the eastern half. There have been isolated reports of a reactive layer of buried surface hoar in the zone, generally near and east of Blewett Pass. We have limited information about this layer, so it may be prudent to dig down to look for deeper weak layers such as these on Sunday.

Reactive, shallow storm slabs were found below treeline in Icicle Creek. January 11, 2020. Ryan Carrasco photo.

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

Storm Slabs

An icon showing Storm Slabs

Likelihood

likely

Expected Size

1 - 1

Storm slabs should become likely during the course of the day, as another round of snow makes its way into the area. More snow is forecast for areas closer to the crest such as Salmon la Sac and Teanaway, so expect deeper and more dangerous slabs in that part of the zone.  Be observant throughout the course of the day, as these tend to be the most reactive during periods of heavier snowfall rates. Step, or ride off into virgin snow once in a while, and look for shooting cracks as you travel. There may not be enough new snow for storm slabs near and east of Blewett Pass. 

If you observe 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. A healthy amount of respect for steep slopes at upper elevations is warranted due to uncertainties with the quickly deepening snowpack. 

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.

Wind Slabs

An icon showing Wind Slabs

Likelihood

likely

Expected Size

1 - 1

As the snow falls and the winds blow, wind slabs will continue to form. This strong over weak layering can be expected on slopes near and above treeline. Wind slabs may become reactive to human triggers, especially during periods of heavier snowfall intensity. These are likely to be more dangerous as one increases elevation and closer to the crest, where much more new snow has been accumulating. 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.

Valid until: Jan 12th, 2020 10:00AM