Avalanche Forecast

Issued: Mar 13th, 2019 11:25AM

The alpine rating is below threshold, the treeline rating is below threshold, and the below treeline rating is below threshold.

Northwest Avalanche Center NWAC, Northwest Avalanche Center

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You could still trigger recently formed wind slabs Thursday, especially at upper elevations. Avoid steep open slopes below ridges where wind slabs likely exist. Uncertainty exists with the bonding of recent snow and older, weak surfaces. Be alert of quickly changing conditions as the sun and temperatures begin to change the snow surface. Choose terrain that avoids steep, upper elevation slopes, below ridges, convex rollovers, and unsupported features. Watch for developing wet snow conditions during extended sunshine or filtered sunshine.

Summary

Discussion

Snow and Avalanche Discussion

A wide range of snowfall amounts accumulated across the area on Tuesday through early Wednesday. The main question, however, is what did the winds do to the upper elevations? Plenty of low density, cold snow was available for transport prior to Tuesday, as well as the recent new snow. Very strong winds through Tuesday likely built reactive wind slabs at the higher and more exposed areas. 

The recent storm buried a mix of sun crusts on southerly aspects and weak, faceted snow on northerlies. Dangerous conditions may be found where thick wind slabs overlie weak, faceted snow. The sun should make an appearance on Wednesday, and with it, a rapid change in snow surface conditions. Loose wet avalanches may run on steep, south-facing slopes in areas that did pick up over a couple of inches of new snow. 

In shallower snowpack areas, you are more likely to find and potentially trigger a persistent slab avalanche within the top 1 to 2 feet of the snowpack. A public observation of a human-triggered slab avalanche came from a north aspect at 3200' just north of the Columbia River (Stacker Butte). We don't know what layer the slab avalanche failed on and the report is now several days old, but it's a clear reminder of weaknesses that remain in the upper snowpack. If you find signs of instability such as cracking and collapsing in shallower snowpack areas, dial back your terrain selection to lower-angled and well-supported slopes.

Snowpack Discussion

March 10, 2019

February started stormy and cold with very active avalanche conditions on persistent weak layers. The month finished with continued cold temperatures but drier weather allowed avalanche activity to taper off. Persistent weak layers that were buried early in the month (Feb 8th) are now unreactive, though you can still find the grains.

Aside from the series of storms in early to mid-February, most regions have measured relatively light snow accumulation in the past 3 weeks. Snow has stayed soft especially on shaded slopes and faceting and surface hoar have been plentiful. While there has been plenty of sunshine since mid-February, very cold temperatures have kept melt-freeze crust to a minimum on sunny slopes.

A natural loose wet avalanche (D1.5) on a southeast aspect of Lichtenberg Mtn at 5,400ft. 3/10/2019. Photo: Josh Hirshberg

Recent Avalanches

Our attention shifted to the upper snowpack. Recent avalanche problems have largely focussed on wind transported snow and weak snow surfaces on both dry, shaded slopes and sun-exposed aspects. In some locations, a weak layer of surface hoar and facets was buried on March 6th. We’ll keep an eye on this interface as we move into the future.

A skier-triggered storm slab avalanche (D1.5) on Shuksan Arm, north aspect, 5200ft. 03/09/19 Adam U Photo. 

Moving Forward

As we move further into March, there are two points to consider:

  • The strength of the March sun: As the sun creeps higher into the sky and the days grow longer, the sun can have a greater impact on the snow surface. When the sun comes out, expect things to change quickly. You may see avalanches conditions change with natural loose avalanches originating from sunny slopes, surface snow becoming thick and heavy, and slabs taking on a moist to wet snow character.

  • Weak surfaces: facets and surface hoar have blanketed snow surfaces in many zones. Any significant snowfall will bury well-developed and widespread persistent weak layers. Recently, most storms have not delivered enough precipitation to cause a problem.

Valid until: Mar 14th, 2019 11:25AM