Northwest Avalanche Center NWAC, Northwest Avalanche Center

Mt Hood Avalanche Forecast

Jan 12th, 2020 10:14AM

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

Very dangerous avalanche conditions will continue as strong west winds load the same aspects at mid and upper elevations. Choose lower angled slopes and avoid lingering in canyons connected to large avalanche slopes on the upper mountain.  

Summary

Discussion

Human triggered and natural avalanches were reported over the weekend near and below treeline as the snow piled up. Observations were limited from above treeline due to storm conditions. While the new snow should ease on Monday, expect strong west winds to maintain very dangerous avalanche conditions on the upper mountain. 

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). Also, be prepared for the coldest temperatures of the winter over the next several days.

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

very likely

Expected Size

1 - 2

West winds have been loading the same aspects over the last few days. These same aspects will be loaded continuously with new snow Sunday night and Monday. While any steep open slope with wind drifted snow is the most likely place to trigger an avalanche, 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.

Storm Slabs

An icon showing Storm Slabs

Likelihood

likely

Expected Size

1 - 1

With around 3 ft of new snow over the weekend, storm snow instabilities need time to settle out and stabilize. 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.

Snowfall rates and a subtle warming trend in Monday’s storm may create new storm layers for soft slab avalanches to release on. Natural avalanches are the most direct sign that the new snow is unstable.

With cold temperatures and lots of new snow, you may find wind-sheltered areas with deep uncohesive loose snow. These 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 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.

Valid until: Jan 13th, 2020 10:14AM