Avalanche forecast

Mt Hood

Mar 13 – 14, 2019Northwest Avalanche Center
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Avalanche danger

Fri
Alpine
3 · Considerable
Treeline
2 · Moderate
Below
2 · Moderate
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Highlights

Dangerous avalanche conditions will persist Thursday at higher elevations where large wind slabs formed during the recent storm. Avoid wind loaded slopes greater than 35 degrees. Filtered sunshine and warming temperatures may be enough to develop Loose-Wet avalanches on steep sun-exposed slopes. Watch for rollerballs or wet surface snow as signs of potential wet avalanches.

Avalanche problems

  • Wind Slabs

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Likely
    Size
    1

    Strong to extreme winds during the recent storm reshaped the new snow in many locations and created large wind slabs in higher terrain. Mt Hood Meadows patrol observed a very large recent natural slab avalanche Wednesday that ran more than 1000 vertical ft. While the start zone was well above our forecast terrain, it shows how quickly large wind slabs can form in open terrain.

    Steer away from any open slope greater than 35 degrees where you see fresh cornices, drifted snow, or feel firm snow under you. If you find firm wind slabs, give these slopes a wide berth. Hard slabs can lure you on to them before failing, sometimes above your head.

  • Loose Wet

    • Alpine
    • Treeline
    • Below treeline
    Likelihood
    Possible
    Size
    1

    March brings with it a strong sun and freezing levels are expected to reach elevations we haven’t seen since January. It won’t take much to get a loose wet avalanche cycle going. If you see new rollerballs, find new fan-shaped avalanches, or experience moist surface snow, loose wet avalanches may occur. With all the new snow, if the slope is big enough, any loose wet may entrain significant new snow and become large. If you see signs of loose wet avalanches, steer away from steep slopes receiving any sunshine.

Discussion

Snow and Avalanche Discussion

Thursday’s avalanche forecast is a bit tricky. We expect overcast skies, however, it’s March, and the sun is strong. It won’t take many sun breaks, or thinner clouds to cause conditions to change quickly. Make frequent weather observations, be alert to changes in the weather that differ from the forecast, and be ready to alter your plans if observations don’t line-up with the forecast.

The recent storm hit the Mt Hood area the hardest with the greatest new snow and strongest winds. If traveling to higher elevations, be prepared for wildly varied surface conditions as a result of the strong to extreme winds. Large hard wind slabs may be hard to trigger but would be potentially deadly if triggered from a thinner portion. Best to give the higher terrain additional time to allow these wind slabs to heal. 

 

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.

Regions covered