Avalanche Forecast

Snow and Avalanche Discussion:

On Christmas Eve the skies cleared, but valley fog formed over the east side of Stevens Pass. Above the fog, small loose wet avalanches were observed off steep, rocky south facing slopes. Below the fog, temperatures remained cold. On the 23rd the pass received about 8” of new, cold snow with light winds. Winds were still strong enough to move snow near and above treeline. During the day on the 23rd, skiers were able to trigger avalanches about a foot deep and 100ft wide on the new/old interface at 6,200ft on a Northeast aspect of Jim Hill. It appeared that a few large paths in Upper Lanham Lakes and Henry Creek ran between December 18th and 20th. 

The persistent weak layer that we have been tracking since December 9th has been observed to be rounding, gaining strength, and sintering on Stevens Pass. This layer is still weak in areas further east and in places with a shallow snowpack. It remains in the forefront of our minds and will continue to dictate how we travel. 

Be sure to avoid steep slopes on southerly aspects if the sun comes out. Look for signs of loose wet avalanches such as roller balls, or a moist snow surface. 

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.  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