Avalanche Forecast

Issued: Jan 10th, 2019 6:05PM

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

Matt Primomo,

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The Bottom Line: The avalanche danger is slowly dropping, but the hefty new load of snow is still a concern. You may trigger avalanches in areas where the wind has formed firm slabs or in locations where weak old snow layers exist. You can stay safe by avoiding slopes greater than 35 degrees.

Summary

Snow and Avalanche Discussion

The recent upslope storm revived winter in the lowlands of the eastern slopes. It also added a hefty load to the upper elevation snowpack in some places. Snowfall totals appear to vary widely, from 5" to 14", likely more at higher elevations. The snow came in cold and warmed up as it fell, creating a density inversion that may still be noticeable. A common structure to be found out there will be storm snow (with or without a wind slab) over a crust. On the 10th, avalanche workers were able to trigger a number of wind slab avalanches in closed areas at Mission Ridge with explosives. These were from 6" to 16" deep and from 60' to 125' wide, running on the crust. One of these broke into older, weak snow near the ground. This one was on a Northwest aspect at 6300ft. 

Weather Forecast

Fri 11th Jan 14:51 - Robert Hahn

Weather Synopsis for Friday night through Sunday
A high-amplitude ridge is centered over Idaho on Friday afternoon. The high pressure will cut off late Friday and will expand slightly westward, increasing the already very warm temperates at mid and upper-levels Saturday night and Sunday over our region.  The ridge and subsequent high will block weather systems from moving eastward, but mid and high-level clouds will remain with us at times into Saturday as a week low skirts northward offshore, brushing the Olympics with some lowering clouds and a possible sprinkle.

Moderate easterly flow will be sustained throughout the forecast period with cooler air at low levels to the east of the Cascade Crest warming as it descends to the Puget Sound lowlands.

A low-level inversion has developed as evidenced by the two stations at Washington Pass where the upper station is above freezing on Friday afternoon. In general, colder air sinks into the deeper valleys, and without wind to stir it up, low clouds and fog will predominate day and night. The easterly flow will suck the low clouds and fog through the lower mountain gaps such as Snoqualmie Pass keeping the passes cool.

More sunshine is expected by Saturday afternoon with mostly clear skies through Sunday as the inversion and moderate east flow continue.

Regional Synopsis

Thu 10th Jan 09:00

January 10, 2019

We’re about a month into our avalanche forecasting season, and what a wild month it has been. We’ve seen the snowpack grow from near non-existent to over a 100 inches deep at Mt Baker. We’ve seen avalanche warnings blanket our coverage area, had several widespread large natural avalanche cycles, and seen persistent weak layers plague nearly every zone. While the active weather pattern has been a welcomed sight, it’s hard to catch your breath.

It looks like this weekend, and extending into next week, we are entering a calm period in the weather. High pressure over Idaho and Montana will keep Pacific storms at bay for at least the short term. As we enter this period of nice weather, there are a few things on our mind here at the avalanche center.

Old Persistent Weak Layers

A high elevation rain event on January 3rd did us a favor in many locations. Besides heavily stressing old weaklayers, the rain created a very strong met-freeze crust, making it extremely difficult to trigger deep avalanches.

There are a few exceptions, and they can be found mainly in the eastern forecast zones. The East-North zone around the Methow Valley and WA Pass did not experience significant rain above 5500 ft, potentially leaving some older weak layers in tact. Areas further east including, but not limited to, Icicle Creek and the Wenatchee Mountains did not receive enough rain to form a strong thick crust. In all of these area, older persistent weak layers are still suspect.

An avalanche on a persistent weak layer in the East-Central zone. Photo: Matt Primomo


Higher Elevations and Remote Trailheads
Stormy weather over the last month has been keeping most of us closer to common trailheads and recreating at near and below treeline. With nice weather on tap, some of you may be thinking about heading into higher elevation terrain and exploring around more remote trailheads. If you are heading out, take time to consider a few key pieces of information.

  • At higher elevations the snowpack is very deep and wintery. This stands in stark contrast to the lack of snow at lower elevations. The snowpack you’ve seen on previous outings this winter, doesn’t represent what you will encounter at higher elevations.

  • We have not received much information about the above treeline nor remote location snowpacks. That means we have a high degree of uncertainty in these areas.

  • You will need to make observations as you travel to confirm if the avalanche forecast is applicable the the area where you are traveling. When observations lineup with the information in a forecast, its valid. If you see observations that don’t lineup, it’s time to take a step-back.

  • If you are heading into our regions highest alpine terrain, remember, NWAC forecast apply to areas below the main Cascade crest, or about 8000 ft. So, if you plan to travel to higher elevations on Mt Baker, Mt Shuksan, Mt Rainier, Mt Hood, or similar areas, you will need the skills and information to evaluate the terrain and snowpack.

If you’re heading into higher elevation terrain, remember we don’t have much information about these snowpacks, and therefore have a higher degree of uncertainty. Photo: SimonTrautman


We’d like to thank our NWAC Community for all the wonderful public observation this year. If you are heading out into the mountains, we appreciate you taking the time to let us know what you saw. This is a great way to contribute to the creation of your avalanche forecast. You can submit observations here.

Enjoy the sunshine!

Problems

Wind Slabs

An icon showing Wind Slabs
Recent winds formed slabs in some unusual locations like well below ridgelines, on the sides of gullies, and in places that are normally scoured. Other places have no wind slab whatsoever. Though you may not see the wind actively loading slopes anymore, the slabs that were built the past few days may take a bit longer than normal to heal. These slabs are resting on lower density snow and sitting over a stout crust, a perfect recipe for avalanches. You will need to use your eyes and sense of touch to locate these dangerous areas as you travel. Feel the snow under you as you travel. Does it feel more firm in one location than another? Does it seem hollow? If you see or feel any of these observations, slabs may be nearby. You can stay safe by traveling on ridges,  scoured areas, and any slope less than 35 degrees.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Likely

Expected Size

2 - 2

Persistent Slabs

An icon showing Persistent Slabs
This snow and wind loading event has stressed older weak layers in some locations. We have a high amount of unertainty regarding how these persistent layers are responding to the new load. What we do know is we have a weak snowpack structure in some locations and we just put more snow on top of it. Persistent slabs can be difficult to assess and predict. If you see any obvious signs of avalanche danger like recent avalanches, cracks in the snow, or feel sudden collapses, stay away from all slopes greater than 30 degrees.

There are two primary layers of concern in the snowpack. You may encounter one or both depending on where you travel. You are most like to encounter persistent slab at higher elevations and in areas towards the eastern edge of the forecast zone.

A layer of buried surface hoar may be found 1-3 ft below the snow surface. This layer is mainly a concern above 6000ft. Above 7,000ft we have virtually no observations, but would be most suspect of this layer up there. 

A layer of weak sugary facets can be found near the ground, particularly in the eastern areas of the forecast zone. If an avalanche failed on this layer, it could be more than 4 feet deep. Use caution if you are traveling in areas where the snowpack is thinner.

Aspects: All aspects.

Elevations: Alpine, Treeline.

Likelihood

Possible

Expected Size

3 - 3

Loose Wet

An icon showing Loose Wet
If the sun breaks out, expect loose wet avalanches to become common. Steep slopes that face the sun are the most likely places these will happen. If you see roller balls or small loose slides that begin entraining snow, get off of steep slopes. These may pack a punch tomorrow.

Aspects: South East, South, South West.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Jan 11th, 2019 5:00PM