Northwest Avalanche Center NWAC, Northwest Avalanche Center

Mt Hood Avalanche Forecast

Jan 15th, 2020 10:00AM

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

Dangerous avalanche conditions persist as Mt. Hood receives another round of heavy snowfall. Choose lower angled slopes and avoid lingering in canyons connected to large avalanche slopes on the upper mountain. On steep slopes in sheltered areas, deep and unconsolidated snow exists and the consequences of even small avalanches are significant, especially near terrain traps.

Summary

Discussion

Evidence of active avalanche activity over the last few days is abundant. With unusually cold temperatures, weak layers are slow to heal keeping the avalanche hazard elevated. Crowns from naturally triggered storm slabs up to 18” and loose dry avalanches are abundant and many can even be seen from Highway 26 and 35. This all adds up to another day of conservative terrain selection at all elevations. Consider making travel plans that minimize time spent in canyons or gullies below large avalanche paths that have seen 5-6” of water in the form of snow over the last 5 days along with strong to extreme westerly winds.

With all this new snow, deep snow hazards exist at lower elevations and less wind affected areas. Before heading out, learn about tree well or snow immersion suffocation risks and travel with a partner (www.deepsnowsafety.org). 

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

Storm Slabs

An icon showing Storm Slabs

Likelihood

likely

Expected Size

1 - 1

Fresh snow Wednesday night will renew the sensitivity of storm slabs. With such deep snow, even a small avalanche can be difficult to escape from. Test how well the recent snow has bonded on small inconsequential test slopes before committing to larger slopes. Steer around steep convexities and avoid unsupported slopes steeper than 35 degrees. Watch for cracking in the snow as a sign that you can trigger an avalanche on a nearby slope. 

Given the cold temperatures and lots of new snow, you will find wind-sheltered areas with deep and uncohesive loose snow. On very steep slopes loose snow avalanches or “sluffs” can be large enough to push you over or knock you into tree wells or open creeks. Be aware of stopping below steep slopes or above hazards like gullies, creeks, and cliffs.

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:

Treeline, Below Treeline.

Wind Slabs

An icon showing Wind Slabs

Likelihood

possible

Expected Size

1 - 2

Slopes steeper than 35 degrees with wind drifted snow are the most likely place to trigger an avalanche. With lighter winds and cold new snow, typical signs of wind slabs like a stiff and textured snow surface may not be apparent. Identify the slopes that have been loaded with recent snow by noticing scoured areas adjacent to slopes with lots of snow. Avoid steep open slopes where you see signs of wind transported snow such as cornices, and pillow-like drifts of snow.

Information on the upper mountain is limited however, Mt Hood Meadows Pro Patrol triggered a very large (D3) avalanche with artillery on Wednesday. This is a clue that other large-very large avalanches are still possible. Be aware that very large avalanches releasing from higher elevations can run long distances to lower elevations. Don't linger in canyons and gullies connected to large avalanche paths.

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 16th, 2020 10:00AM