Avalanche Forecast

Generally safe avalanche conditions should exist for the majority of the East Slopes South zone. We don't know how high rain extended into the alpine on Friday, so it's possible shallow wind slabs formed on lee slopes at higher elevations. Prior to this storm system, observations from along the Tieton River Road showed no signs of recent avalanche activity in any visible terrain. The snowpack below 4000' on sunny aspects is very shallow or non-existent. Even at higher elevations and on shaded aspects, the snowpack appears relatively shallow. 

A weak snowpack still exists in the eastern portion of the East Slopes Central forecast zone. A similar shallow and weak snowpack may exist in isolated areas of the East Slopes South zone.

Forecast Schedule and No Rating

At this time, we do not have enough specific snowpack information to issue an avalanche hazard rating for the East Slopes South zone. However, even when No Rating is applied, applicable avalanche conditions and backcountry travel advice will be provided throughout the season. When weather systems produce very dangerous avalanche conditions in adjacent zones, NWAC will issue an avalanche warning for this zone as well.

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.

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