Watch for avalanches in the new snow on steep slopes. Evaluate the bond of the new snow carefully. Use extra caution where there is less than 5 feet of snow on the ground.
Check for how the new snow is bonding to the old snow surface by using tests on small, inconsequential slopes. Watch for areas of more than 6” of recently fallen or drifted snow. Use additional caution where you find wind drifted snow at upper elevations. If the skies clear, expect small loose avalanches in the new snow on steep, sunny slopes.
This is a low likelihood, high consequence problem. The chances of triggering are very low, but the resulting avalanche could be unsurvivable. Few seasoned backcountry travelers have experience surviving avalanches of this magnitude. Use extra caution in the eastern end of the zone (east of Smith Brook) and anywhere with less than 5 feet of snow on the ground. You can trigger deep persistent slab avalanches more easily in these areas. The best way to reduce the risk of the high consequence of deep persistent slab avalanches 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.
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
