Northwest Avalanche Center NWAC, Northwest Avalanche Center

Cascades - South West Avalanche Forecast

Jan 12th, 2020 10:11AM

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

A potent winter storm has dropped over 3 feet of snow in areas of the zone since Friday, creating dangerous avalanche conditions. Avalanches may be large enough to injure, bury, or kill you. Cautious route-finding and conservative decision-making is essential on Monday.

Summary

Discussion

With over 4 inches of water since Friday night all in the form of snow at Paradise, and less in areas such as Crystal, variable is the word for the West South zone. Due to the strength of the storm, observations from the volcanoes have been non-existent, leading to a high degree of uncertainty and a non-standard forecast. With such variability in the zone, you may find locally higher or lower avalanche danger, depending on snow totals, with the highest danger focused on Rainier, Adams, and St. Helens. As Dallas stated yesterday, all areas received significant snowfall, so don’t be lulled to sleep just because you’re not on a volcano. The variability in the zone demands you make conservative decisions and your own observations.

After significant storm totals in the past week, deep snow alone can be dangerous. Make sure to travel with a partner and to be aware of the danger. You can learn more at www.deepsnowsafety.org. The next few days should produce the coldest temperatures of the year, adding another change in the conditions for you to consider.

 

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

Avalanches on Monday may involve new snow, or could even break several feet deep involving all the storm and wind loaded snow from the past few days. Natural avalanche activity is trending downward, but still possible, while human triggered avalanches are a major concern. Unsupported slopes, large terrain features steeper than 35 degrees, and any slopes with evidence of wind-drifted snow have a higher likelihood of avalanches and should be avoided. Shooting cracks, collapses, and recent avalanches are obvious signs of instability that should give you pause.

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 13th, 2020 10:11AM