Avalanche Forecast
Regions: Cascades - South East.
Rain to treeline and blowing snow at upper elevations will create dangerous avalanche conditions. Limit your exposure to steep slopes where avalanches could start or come down from above. Avoid being on or under steep slopes where the snow is wet, and stay off of slopes at upper elevations with freshly wind drifted features.
Discussion
The observations presented here are derived from other zones east of the Cascade Crest.Â
Very large avalanche near Wa Pass (12/21/2018)
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, avoid avalanche terrain.
Recent avalanches, collapsing, and tests showing cracks spreading through the snow (propagation) are all signs to 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 was obvious that mountains were falling down. 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. Due to the uncertain nature of these avalanches, it's best to minimize your exposure to avalanche terrain.
If you dig you'll find the classic strong over weak snow layers of a slab avalanche. 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.
Snowpack Discussion
Regional Synopsis: December 24, 2018
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. Â 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. Â
Happy Holidays