Avalanche Forecast
Issued: Feb 1st, 2019 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs.
NWAC,
Northwest Avalanche Center
The latest round of weather has not been enough to raise the avalanche danger. Avalanches are unlikely but not impossible. Beware of places where slabs avalanche conditions may linger such as high, north facing terrain and in areas with a shallow snowpack.
Summary
Discussion
A variety of old snow surfaces exist from weak facets on shaded slopes to very firm, slick crusts on sunny slopes. Any new snow may not stick well to some of these interfaces. Check the bond at the new/old snow interface. Watch for slipping on firm crusts especially in terrain where a fall could have consequences or where stopping a fall may be difficult.
Check out the Regional Synopsis tab for more details.
A somewhat fresh looking slab on the north side of Jack Ridge at 6,700ft in Icicle Creek. Photo on 1/26 by Matt Primomo.
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 with minimal new wet avalanche activity reported. This break in the weather allowed for avalanche danger to steadily decline in all regions. With stormy weather, the danger is once again elevated.
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 likely will be landing on slick surfaces. It's time to pay attention to the new old interface formed by our most recent storm.
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.
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.
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.
Near surface facets developed on shaded snow surfaces in many areas. These facets are from near Mt Baker. Photo: Lee Lazzara
East-West:It’s not uncommon for our east-side forecast zones to experience lingering persistent weak layers (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.
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.
-
January 22nd surface hoar and small facets. You can find these just under the recent storm snow, about a foot down. .
-
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
Problems
Persistent Slabs
Even though persistent slab avalanches are unlikely, it's important to stay vigilant in the variable snowpack of the Eastern Cascades. Watch for cracking in the snow, and listen for collapses. Use the terrain to your advantage, avoiding areas with thick slabs adjacent to shallow, rocky slopes. Use extra caution around steep, extreme terrain and unsupported slopes on shaded aspects.
In the Wenatchee Mountains and east of Highway 97, the snowpack is often less than 4 feet deep and highly variable. The most recent layer of buried surface hoar (January 22) may be found. A pair of thin freezing rain crusts associated with the early January and late December weak layers are still a concern in this area as well. You may find these to be best preserved on shaded, and open slopes above 5,500ft. Further west, the snowpack is generally strong and well settled, but the January 22nd surface hoar layer is worth considering before committing to any consequential slopes. You are most likely to find these above 6,000ft in the area and in the upper foot of the snowpack.
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East, South East, South West, West, North West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Feb 2nd, 2019 10:00AM
