Lawinenlagebericht

Stevens Pass

Feb 4 – 5, 2019Northwest Avalanche Center
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Lawinengefahr

Wed
Hochgebirge
2 · Mässig
Waldgrenze
1 · Gering
Unterhalb
1 · Gering
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Kernpunkte

At upper elevations and on exposed slopes, wind slabs may be reactive to the weight of a person. The recent inch or two of new snow may hide these features. Strong sun may initiate small loose slides by mid day on very steep, south facing slopes. 

Lawinenprobleme

  • Wind Slabs

    • Hochgebirge
    • Waldgrenze
    • Unterhalb der Waldgrenze
    Wahrscheinlichkeit
    Possible
    Größe
    1

    Recent winds with an inch or two of new snow may have masked older, thicker wind slabs that formed over the weekend. Look for these drifts on leeward sides of ridges and in wind exposed upper elevation terrain. Check for slab structure, strong snow over weaker snow. Look for signs of instabilities such as shooting cracks under the weight of a person with these slabs. Does it sound hollow? Snowpack tests can be a good way to determine how the recent snow is bonding to the older, likely firm snow. If signs lead you to think these slabs may not be well bonded, or the snow is unstable, avoid slopes steeper than 35 degrees. On shaded aspects above 5,500ft, check for a thin layer of weak facets a foot or so beneath the surface. Use extra caution to avoid steep open slopes if you find any slabs over facets. 

    On very steep south facing slopes, the sun may help to initiate small loose wet slides by mid day. If seeing roller balls or signs of recent fan shaped slides, avoid going underneath big, steep terrain where even a small slide may hurt you.

Snowpack Discussion

January, 31, 2019

As we turn the corner to February we're coming out of a week-long high pressure ridge and into unsettled weather. The snowpack survived extremely warm temperatures and sunny skies over the week. This break in the weather allowed for avalanche danger to steadily decline in all regions. Stormy weather starting February 1st once again elevated the avalanche danger and brought a round of storm related avalanches.

We’ve heard a variety of stories from backcountry travelers over the past week. There have been reports of extremely firm slopes creating slide-for-life conditions. Others reported perfect spring like snow. Some encountered difficult breakable crust. And, for a lucky few, softer, drier, mid-winter snow has been found. A common thread in most zones is where precipitation falls as snow, it has landed on slick surfaces. It's time to pay attention to the interfaces formed by our recent rounds of precipitation.

A skier triggered storm slab in the recent snow in a steep southeast start zone at 5700ft. Lichtenberg Mtn. Feb 2. Photo: Josh Hirshberg

North-South:

While a high elevation rain event, around January 23rd, formed surface crust in many regions, it’s the constant melt-freeze cycles from the past week, that caused a divergence in the Northerly and Southerly snowpacks.

Near surface facets developed on shaded snow surfaces in many areas. These facets are from near Mt Baker. Photo: Lee Lazzara

North: On shady slopes, things haven’t exactly been soft. The crust formed at the end of the last storm extends to high elevations (Mt. Hood 7000+ft, South Cascades 6500 ft, Passes and Central Cascades 6000 ft. and West-North 5500 ft.). Only areas in the East Cascades seemed to escape the wrath of this breakable crust. Without the help of the sun, shady slopes haven’t been softening even during this period of warm weather. Instead, the surface crust underwent some weakening. Observations found faceting on top of and below this crust. In some locations, this caused the crust to begin to degrade, becoming less supportive. Surface hoar has also been reported from the typical valley bottoms and sheltered terrain near water sources. At low to mid elevations, rain may have melted any weak snow on the surface. Slopes receiving significant dry snow should be suspect for a poor bond at the interface buried around February 1st.

Roller balls and loose wet avalanches on a sunny, southeast slope following the Feb 1st storm Lichtenberg Mtn. Photo: Josh Hirshberg

South: On sunny aspects, the sun drove warming and melting of surface snow. Long, cool, winter nights allowed for the surface to freeze again. This repeating melt-freeze pattern created a thicker, firmer, and more supportable surface. On many days, weak surface snow, such as near surface facets or surface hoar, melted during the day limiting its development. On cooler days, very firm travel conditions were reported. Crusts may provide a poor bond for any snow falling on them. Following the Feb 1st storm, the sun drove a round of small wet loose avalanches and storm slabs on upper elevations slopes that received enough accumulation.

East-West:While we're tracking persistent weak layers (PWL’s), they haven't been the source of avalanches for over a week. It’s not uncommon for our east-side forecast zones to experience lingering PWL's. This season, we’ve also seen several different PWL’s in our western zones. This break in the weather gave the snowpack time to gain strength in all zones.

West: While you may find some weak snow in the upper few inches of the snowpack, the mid and lower snowpack has been found to be quite strong. Firm rounded grains, stout  crust, and strong frozen melt-forms make up the majority of the snowpack at this time.

Southeasterly winds textured surfaces on the east side of Washington Pass on Feb 1-2nd. Photo Matt Primomo

East: The east-side snowpack continues to be highly variable. You may find deep strong snowpacks closer to the crest or you could encounter shallow weak snowpacks areas further east. While there are number of potentially weak interfaces, there are two more common layers we’ve got our eyes on.

  1. January 22nd surface hoar and small facets. You can find these just under the recent storm snow, about a foot down. .

  2. December 26th surface hoar. This layer can be found from 16” to 40” down and is still producing clean, planar shears with tests.

You are most likely to find these layers to be preserved on wind sheltered, shady, and open slopes above 5,500ft. You can find more defined weak layers where snowpack is less than 4 feet deep and variable especially east of Highway 97. Persistent weak layers have been “dormant” or unreactive during the week of high pressure. The latest storm has not been enough to re-activate theses weak layers. We’ll keep tracking them to watch their progression..

The lower eastern slopes and the Columbia River. Snow exists at low elevations, but snowpack depths are shallow. Photo: Matt Primomo

 

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