Avalanche Forecast

Issued: Dec 25th, 2018 10:00AM

The alpine rating is moderate, the treeline rating is moderate, and the below treeline rating is moderate. Known problems include Storm Slabs.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

Plan for deteriorating avalanche conditions later in the day as a fresh round of snow and wind moves in. Dial back your terrain choices by choosing more moderately angled slopes if more than 6 inches of snow accumulates. 

Summary

Discussion

Snow and Avalanche Discussion

On Monday morning, an avalanche professional traveling near 5500' in the Mt. Hood Meadows area reported several natural storm slabs releasing up to 1 ft deep following Sunday's heavy snowfall. He did not find any weak snow in the upper or mid snowpack. The most recent rain crust, buried 12/20, was 2 feet or deeper at this elevation. 

Meadows pro-patrol is still finding weak snow (facets) buried 12/7 in backcountry snowpits between 5500 to 6600 ft on north-facing terrain and roughly 2-3 ft down in the snowpack. No natural or explosive avalanches have released down to this layer, but we'll continue to track this layer as it evolves over time.

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.  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

Problems

Storm Slabs

An icon showing Storm Slabs

You may trigger a storm slab within in the new storm snow if new snowfall piles up quickly in the afternoon and evening. If avalanche conditions begin to deteriorate, dial back your terrain selection by avoiding convex rolls and slopes steeper than 35 degrees.  You might find larger storm or wind slabs in steep wind loaded terrain near and above treeline as the winds begin to move snow around in earnest during the latter half of the day. 

Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.

 

You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.

 

Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.

Aspects: All aspects.

Elevations: All elevations.

Likelihood

Possible

Expected Size

1 - 1

Valid until: Dec 26th, 2018 10:00AM