Avalanche Forecast
Issued: Mar 10th, 2019 11:07AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs.
NWAC,
Northwest Avalanche Center
Expect increasing clouds, warming temperatures, and strong southwest winds on Monday ahead of an approaching frontal system. Watch for blowing snow and fresh drifts in leeward upper elevation terrain. You can trigger a slab avalanche on convex slopes and recently wind loaded features 35 degrees and steeper.
Summary
Discussion
The weekend weather was gorgeous - sunny skies, calm winds, and highs in the mid-30s at the 5000ft level. The Mt. Baker/Hwy 542 areas experienced high use, and the recent snow was reactive to human traffic in specific areas. Several avalanches were triggered by skiers and riders on Friday and Saturday, while avalanche activity tapered off on Sunday. Avalanches failed on the old/new snow interface, breaking about one foot deep. Most avalanches were small (D1) and occurred in north facing, upper elevation terrain on convex rollovers steeper than 35 degrees. Two reported avalanches on Friday on Shuksan Arm were large enough to bury a person (D2) and broke 200ft wide and ran 500-1000ft downslope. The strong March sun caused small wet loose slides on solar aspects at all elevations. Loose dry sluffs were also triggered by skiers and riders and ran far and fast in steep shaded terrain.
A skier-triggered storm slab avalanche (D1.5) on Shuksan Arm, north aspect, 5200ft. 03/09/19 Adam U Photo.
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:
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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.
Problems
Wind Slabs
Strong and gusty southwest winds are expected on Monday. This will be a change to relatively calm winds over the past several days. Plenty of recent, undisturbed snow is available for transport. Existing slabs will thicken and fresh slabs will form in leeward terrain at mid and upper elevations. Lingering storm slabs may still be possible to trigger in areas unaffected by wind. Be especially cautious on convex upper elevation slopes steeper than 35 degrees - terrain characteristics where nearly all recent avalanches have occurred. Continue to check how the recent snow is bonding to the old snow surface buried about one foot deep. Expect to encounter the most reactive slabs in leeward areas just below ridges and in cross loaded gullies. Watch for blowing snow, fresh cornice formation, and pay attention to subtle changes in the surface snow. Stiffer slabs, textured surfaces, and lens-shaped pillows are good indications that you have found wind slabs. If you experience shooting cracks, collapses, or see recent avalanches, seek out well-supported, lower-angled terrain.
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
Expected Size
Valid until: Mar 11th, 2019 11:07AM
