Avalanche Forecast

Issued: Jan 6th, 2019 10:00AM

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

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Pay attention to areas where the wind drifted the new snow into firmer and thicker slabs. It’s these wind affected areas where you will be more likely to trigger an avalanche. If you see drifted snow, fresh cornices, or snow pillows you can stay safe by avoiding nearby wind loaded slopes greater than 35 degrees.

Summary

Discussion

Avalanche Summary

We received reports of several human triggered avalanches in the Mt Baker backcountry Sunday. These avalanches occurred within the new snow and were generally small. However, one larger avalanche on Mt Herman reportedly ran for several hundred feet down the slope. We saw two common themes in these human triggered avalanches. One, they occured in areas where the wind drifted the snow into slightly firmer slabs. Two, many of the avalanches were triggered near convex rollovers in the terrain.

Snowpack Discussion

January 6, 2019

This latest round of storms in early January continues to highlight some of this season’s region-wide themes in our snowpack. I’d describe them as Bottom to Top, North to South, and East to West. Let’s take a look at each of these.

Bottom to Top:

In general this winter has been on the warm side in the Cascades. No where is that more evident than in the lack of low elevation snow. Heavy rain events continue to wash-away and thoroughly wet the lower elevation snowpack. In most regions you will see substantial increase is the height of snow from low elevations (3000 ft), to mid elevations (4500 ft), and again at upper elevations  (5500/6000 ft). Above the typical rain lines (5000-6000 ft) a deep and cold mid-winter snowpack exists.

Jan 5, 2019: The dark timber in the Coast Range highlights the thin snowpack at lower elevations and the deeper winter-like snowpacks near and above treeline: Photo: Simon Trautman

North to South:

With few exceptions, this season’s storm tracks have favored the North Cascades. Sunday, Mt Baker’s Heather Meadows weather station passed 100 inches of snow on the ground. A quick look around the state’s weather stations demonstrates just how deep this snowpack is. Most other locations are reporting between 50 and 75 inches. The massive amounts of snow for the northern forecast zones are reflected in several impressive avalanche cycles spread out over the last month.

NWAC weather station graph of the height of snow on the ground. Mt Baker passed the 100 inch mark Sunday.

East to West:

Several stacked persistent weak layers have been buried in the eastern forecast zones. This isn’t unusual, but it is noteworthy. The thickness of the slabs over these weak layers can vary greatly. Closer to the Cascade Crest, the deep snowpack may more closely resemble Stevens or Snoqualmie Passes. Further east, shallower snowpacks, and significant variability are keeping snowpack assessment tricky.

Canadian sized surface hoar found buried by 18” of snow in Icicle Creek above Leavenworth on December 31st: Photo by Matt Primomo.

So what now? Well, on January 3 and 4 a high elevation rain event impacted areas from Mt Baker to Mt Hood. In many locations this has created a firm and thickening crust. In these locations, this new crust is limiting the impacts of any deeper weak layers. In locations that didn’t receive as much rain like WA Pass, Mission Ridge, Crystal Mt, and White Pass, how the small amount of rain impacted the mid and lower snowpacks remain to be seen.

Rain and rime crust at 6,500ft above Gallagher Head Lake in the East Central Zone, Jan 5th. This layer was 3 inches thick in this location. Matt Primomo photo.

One thing is for sure, we’re not even a month into the winter season, there’s a lot more winter to come.

 

Problems

Wind Slabs

An icon showing Wind Slabs

The snow piled up in the West North zone Saturday night and Sunday. Observations found 1-1.5 feet of new snow. In most areas the wind affected the surface snow forming slightly thicker and firmer slabs. It’s these wind affected areas, that you want to pay close attention to Monday. Look for uneven snow surfaces, snow drifts, or fresh cornices. Sometimes you can feel the wind slabs under you. The snow may feel firm or hollow as you travel. All of these clues will let you know you could trigger wind slabs on nearby steep slopes.

Expect the avalanche danger to be more pronounced at higher elevations. Strong winds likely formed thicker more dangerous slabs in the alpine. Hard slabs can be difficult to assess. They can lure out onto a slope before they fail. If you are traveling in alpine areas, give these wind loaded slopes a wide berth.

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: All elevations.

Likelihood

Likely

Expected Size

1 - 1

Valid until: Jan 7th, 2019 10:00AM