Avalanche Forecast
Issued: Mar 25th, 2017 11:30AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Storm Slabs and Cornices.
NWAC,
Northwest Avalanche Center
Fresh and recent wind slabs will be the main problem Sunday, especially if the snowfall accumulations and strong winds arrive early. Keep terrain selection simple and conservative. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.
Summary
Detailed Forecast
Partial clearing Saturday night along with cool temperatures, should allow any moist to wet snow to re-freeze and form a thin crust layer, especially near and below treeline on all but steep northerly facing terrain.
The next frontal precipitation is expected to arrive late Sunday morning and gradually increase along with ridgetop winds through the afternoon. Shallow new wind slabs may become locally sensitive in areas that receive rapid accumulations Sunday afternoon.
Recent winds have been mostly S-SE, so firmer wind slab should be found mainly on W-N-SE slopes near and above treeline. Expected increasing winds Sunday should be similar, mostly southerly.
Recent cornices are very large. Make sure to avoid areas on ridges where there may be an overhanging cornice as well as travel on steep slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable. See a blog post regarding cornices here.
Snowpack Discussion
Special Note: For more information on the massive natural cornice triggered avalanche on the north side of Ruby Mountain on Sunday 3/19 and general thoughts about low-likelihood/high consequence avalanches, please stay tuned for NWAC's upcoming blog post that will be issued this weekend.
Weather and Snowpack
The first week or so of March was very cool and snowy. NWAC stations along the west slopes of the Cascades piled up about 3 to 8 ft of snow with the most at Mt Baker.
The 2nd week of March was equally active with non-stop Pacific frontal systems pummeling the PNW. Unfortunately, these systems delivered far more rain than snow. At least two regional avalanche cycles occurred during the stretch. Significant snowpack consolidation occurred over this period due to rainfall and warmer temperatures.
A strong low pressure system brought several inches of rain to the west slopes of the Cascades outside the Cascade Passes on Friday, 3/17 and caused another wet snow avalanche cycle in the Mt. Baker area Friday night into early Saturday morning.
This past week has also been active weather-wise, but water amounts/snowfall totals have been slightly lower relative to the extreme wetness of the past few weeks. In the last 48 hrs hours ending at 5 PM Saturday, Mt. Baker and Paradise have picked up 16-18", Crystal 12" with 8-10" or less in the Passes (the top of Alpental's snowboard was likely wind affected.)
Recent Observations
North
NWAC pro-observer Lee Lazzara was in the Mt. Baker backcountry on Thursday. Lee found the newest 10" (25 cm) of snow bonding well with the underlying moist snow. He observed evidence of two natural wind slab releases on east aspects of Mt. Herman near treeline. Storm slabs were possible in isolated areas of steep terrain, but not widespread. Mt. The Baker pro - patrol had minimal results during control work Thursday morning with a few pockets of stubborn wind slab noted. The Baker pro-patrol reported a skier triggered, loose wet avalanche on the Shuskan Arm Thursday. The slide caught and carried the skier 150 yards and into trees, resulting in serious but not life-threatening injuries.
The Mt Baker patrol reported sensitive, but more isolated, controlled storm slab avalanches Saturday, involving the 6-8" of overnight storm snow.
Lee was in the Mt Baker backcountry again Saturday. Storm snow remained quite good on shaded terrain with low density surface snow gaining density with depth. Near and above treeline (above about 4500 feet) on shaded terrain there was about 20 inches of lovely early spring powder. In open terrain, older wind slabs were noted buried under the most recent storm snow, but gained strength through the day with gradual daytime warming and settling. No triggered slides of consequence were observed. Below 4500 feet, moist, heavier surface snow was making shallow, loose-wet slides possible.
Central
The Alpental patrol on Wednesday reported about 2 additional inches of new moist or wet snow on the thick crust with triggered small loose wet avalanches remaining possible.
NWAC's Matt Schonwald was in the Alpental Valley Saturday and identified about 14-18 inches of storm snow over the past few days had a favorable density profile. There was evidence of shallow storm slabs being triggered earlier in the day, but gained significant strength through the day. On solar aspects daytime warming caused shallow, loose-wet slides, mostly D1, up to D1.5.
South
NWAC pro-observer Jeremy Allyn was out in the Paradise area on Thursday and found wind distributed snow variably spread across the compass near treeline. Wind slab was not particularly sensitive but up to 80 cm deep in places. Some windward aspects were scoured to the most recent rain crust. Sunshine on Thursday did not cause loose wet avalanches. Also of note but above our forecast area, serac fall had triggered a larger slab avalanche in the vicinity of the Furher Finger/Wilson Headwall, likely within the last 24-48 hours.
On Friday, as the snow-line crept up during the day natural loose wet avalanches occurred along the Paradise Road above 5000'. NPS rangers estimated the loose wet slides entraining about 8-10" of the recent storm snow.
The Crystal pro - patrol reported sensitive and widespread 8-12" storm slabs Friday, particularly on lee slopes during morning control work. Similar to Paradise, as temperatures warmed Friday skier triggered, loose wet avalanches became the biggest hazard below about 5500 feet on steeper slopes. Frequent NWAC contributor Shane Robinson reported widespread natural activity in the Crystal backcountry seen during a brief clearing in the afternoon, consisting of natural loose wet and storm slab activity on steep slopes.
Jeremy Allyn was in the Crystal BC Saturday, in Lakes Basin and near Norse Peak. Recent storm snow varied widely based upon elevation and aspect with up to 20 inches of storm snow or more near and above treeline on non-solar terrain, over the hard rain crust from last weekend. Many steep north aspects within the terrain had been skied both earlier and on Saturday with no triggered avalanches noted. Test profiles and snowpack tests at 6400 feet on a NW aspect below ridgeline, identified weak faceted snow,both above and below the rain crust buried about 20 inches in that location.
The main takeaway in this area is that cornices are very large and storm snow amounts are highly variable. Pay attention to your local snowpack conditions as there is a lot of snow available to be involved in a slide in some areas, such as a small triggered slide releasing a deeper pocket making large avalanches possible. A cornice failure could also trigger a large and dangerous avalanche.
Problems
Wind Slabs
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: North, North East, East, South East, West, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Storm Slabs
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
Expected Size
Cornices
Cornice Fall is the release of an overhanging mass of snow that forms as the wind moves snow over a sharp terrain feature, such as a ridge, and deposits snow on the downwind (leeward) side. Cornices range in size from small wind lips of soft snow to large overhangs of hard snow that are 30 feet (10 meters) or taller. They can break off the terrain suddenly and pull back onto the ridge top and catch people by surprise even on the flat ground above the slope. Even small cornices can have enough mass to be destructive and deadly. Cornice Fall can entrain loose surface snow or trigger slab avalanches.
Cornices can never be trusted and avoiding them is necessary for safe backcountry travel. Stay well back from ridgeline areas with cornices. They often overhang the ridge edge can be triggered remotely. Avoid areas underneath cornices. Even small Cornice Fall can trigger a larger avalanche and large Cornice Fall can easily crush a human. Periods of significant temperature warm-up are times to be particularly aware.
A corniced ridgeline. A large cornice has formed at the top of the ridge. A smaller cornice has formed to the left of the trees from crossloading.
Cornices are easy to identify and are confined to lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth (new snow and wind), rapid warming, and during rain-on-snow events. Cornices often catch people by surprise when they break farther back onto flatter areas than expected.
Aspects: North, North East, East, South East, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Mar 26th, 2017 11:30AM
