Avalanche Forecast Cascades - South East

Date Issued: Valid Until:

Robert Hahn,

Avalanche Forecast

Wed Dec. 26th · 6:00PM

Alpine

Danger Ratings Below Threshold

Treeline

Danger Ratings Below Threshold

Below Treeline

Danger Ratings Below Threshold

Alpine

Danger Ratings Below Threshold

Treeline

Danger Ratings Below Threshold

Below Treeline

Danger Ratings Below Threshold
The bottom line: You can trigger avalanches due to recent blowing snow at upper elevations. Weak layers, lurking deep in the snowpack, maintain the threat of triggering a large and dangerous avalanche. Use caution on steep slopes and areas of shallow and variable snow depths.

Regional Synopsis

In most parts of the state, a stout melt freeze crust was formed when it rained to high elevations around Thanksgiving. The one exception to this event was in the East North Zone, where the precipitation from the Thanksgiving storm was all snow. A quick storm at the end of November put a small amount of snow above the melt-freeze crust, and preserved the older basal facets in the northeastern areas.

Cold and clear weather dominated the first week in December, with valley fog and very cold temperatures east of the crest. The surface snow sat around and decomposed. Surface hoar grew large on top of this.

The jet stream took aim at the Pacific Northwest in the 2nd week of December.  Most notably, light storms buried and preserved a widespread layer of surface hoar and/or near surface facets on December 9th. From December 9th to December 23rd, storms kept coming. Freezing levels fluctuated, but never moved much above 5000ft throughout the Cascades (although the southernmost volcanoes and Mt. Hood saw rain well above 6000).

Initially, the storm track favored the northern zones. The accompanying avalanche cycle began on December 11th. Most of these slides were soft slabs, but some propagated widely on the December 9th layer. Higher snowfall totals in the West North resulted in very large (D3+) avalanches in the mountains along Hwy 542.

A second, and larger avalanche cycle occurred during heavy snowfall and strong wind events between December 18th and 20th. Although these cycles were once again most prevalent in the northern and eastern zones, big storm totals around Mt. Rainier tipped the balance down south as well. This 2nd cycle was impressive, with very large and destructive avalanches (some D4) reported. The culprit was once again the December 9th surface hoar/facets (and/or the basal facets in the northern and eastern zones).

Today we have a large difference in snowpack depths between the Pacific Crest and the Eastern Slope. This is nothing unusual, as more often than not the west side of the Cascades and the passes get more snow than areas further east. Moving forward, places with a deep snowpack (say greater than 5ft) and warmer temperatures may continue to gain strength. Areas with a shallow snowpack (say less than 3.5ft) may take much longer. In a general and applied sense, this means the avalanche danger/conditions may begin to diverge between the western and eastern zones.  

As the skies clear and we move into high pressure, take note as to which avalanche paths have run large on deep, weak layers, and those which haven’t. Be sure to track surface conditions, as this next period of cold, clear weather may create the next weak layer when the storm track does turn back toward us. As always, please share your photos and experiences with us!

Happy Holidays

Weather Synopsis

A trough centered over the Intermountain West will bring cool N-NW flow to the region as a broad ridge builds offshore on Thursday. The flow will be sufficiently moist to keep low clouds, fog, and a chance of flurries in the forecast. Very weak NW-SE oriented convergence zones will locally enhance snow and cloud in parts of the west slopes Cascades foothills and into the lower passes. Clouds will decrease throughout the day.

Late Thursday night some light snow will arrive in the Olympic mountains from the west as a significant warm front approaches. The Cascades will remain clear and cold.

On Friday, the warm front will be draped parallel to the British Colombia and Washington coastlines. Clouds will lower and thicken with light snow arriving in the Olympics in the morning and in the Cascades by early afternoon. 

Friday night the warm front crosses the region bringing moderate to heavy snow changing to rain at Snoqualmie Pass around midnight as easterly flow in the evening switches to westerly, while Stevens Pass will change to rain after midnight.

Snowpack and Avalanche Discussion

While it’s now harder to trigger persistent slabs, the grave consequences remain. The best way to reduce the risk of this low likelihood, high consequence avalanche problem is to:

-Limit the amount of time you spend on or near large slopes 35 degrees and steeper.

-Put an extra buffer of terrain between where you travel and where avalanches could start, run, or stop.

-Stay away from features where avalanches are commonly triggered like: rocks, steep roll-overs and convexities, unsupported slopes ending in cliffs or steep drops, and areas of shallow, variable snow.

-When in doubt, choose a route of travel that is lower-angle, less exposed to avalanche terrain.

Along the east slopes of the Cascades, Recent avalanches, collapsing, and tests showing cracks spreading through the snow (propagation) all suggest that you minimize your exposure to avalanche terrain. If you were in Icicle Creek, Mission Ridge, or near Washington Pass during the avalanche cycle of Dec 18-20th, it may have been obvious that mountains were falling down around you. Now, your challenge is; both the chances of triggering and the consequences of these avalanches are less obvious. Incremental snow and wind in the past week stressed the weak layers that lurk in the snowpack, maintaining the possibility of triggering persistent slabs.

If you dig you may find the classic strong over weak snow layers of a slab avalanche. Watch for weak sugar-like weak layers near the middle to bottom of the snowpack. A layer of small facets and surface hoar was buried on December 9th and can be found in the lower half of the snowpack on most slopes. At upper elevations, a deeper layer of facets can be found close to the ground. The poor snowpack structure that currently exists along the East Slopes of the Cascades stretching from I-90 to the Canadian border may also exist in the East Slopes South zone.

You are responsible for assessing the snowpack, so evaluate conditions carefully and understand the potential hazard before entering avalanche terrain. The best way to reduce the risk of a potential high consequence persistent slab avalanche is by minimizing your exposure to slopes capable of producing large avalanches. This includes being cognizant at lower elevations if crossing large avalanche paths that can release much higher in the terrain.

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.