Avalanche Forecast
Regions: Mt Hood.
Avalanche conditions have changed very quickly and may continue to deteriorate throughout the day as snow continues to fall with wind transport. With very dangerous conditions present above treeline, avoid lingering where natural avalanches may start and run and avoid all slopes greater than 35 degrees where storm slabs are expected to be reactive.
Discussion
Snow and Avalanche Discussion
It’s not often that we go from low to high avalanche danger in 24 hours, but when we do it should draw your attention. A significant winter storm will ramp up Monday night and bring moderate to occasionally heavy snow through the day on Tuesday, resulting in sustained or even increased avalanche danger throughout the day. Mt. Hood areas are likely to pick up 1-2 ft of snow in this event and the snow may fall on small grains of surface hoar on northerly slopes.
Winds were transporting snow all day Monday above treeline.
The storm comes on the heels of sunny days on Sunday and Monday with temperatures getting above freezing up to 6500 ft on Monday afternoon before clouds rolled in late in the day. The mild weather has created sun crusts on all sun-affected aspects and elevations. North-facing terrain is well settled and consolidating somewhat.
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.Â
Moving Forward
As we move further into March, there are two points to consider:
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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.
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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.
Avalanche Problems
Wind Slabs
Large wind slabs should be very easy to trigger and very widespread above treeline. Snow transport began Monday above treeline. These slabs will grow larger and more reactive into the day on Tuesday and they will also build in near treeline terrain or possibly into exposed below treeline areas. Travel where avalanches can start, stop or run is not recommended.
Release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind 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.
Wind Slabs form in specific areas, and are confined to lee and cross-loaded terrain features. They can be avoided by sticking to sheltered or wind-scoured areas..
Wind Slab avalanche. Winds blew from left to right. The area above the ridge has been scoured, and the snow drifted into a wind slab on the slope below.
Wind slabs can take up to a week to stabilize. They are confined to lee and cross-loaded terrain features and can be avoided by sticking to sheltered or wind scoured areas.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Very Likely
Expected Size: 1 - 1
Storm Slabs
In wind-protected terrain, rapidly accumulating snowfall will form a slab, that may be especially reactive if it sits on surface hoar if it is buried by the incoming storm. Keep in mind that if a storm slab sits on buried surface hoar, it can release on slopes 30 degrees and greater. Perform simple slope tests and hand shears as the snow accumulates to test for a slab over weak layer structure and avoid all slopes steeper than 35 degrees.
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.
Elevations: Treeline, Below Treeline.
Likelihood: Likely
Expected Size: 1 - 1