Avalanche Forecast
Issued: Mar 24th, 2017 1:27PM
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
We continue to load and stress our snowpack. Don't push it in consequential avalanche terrain - keep it simple and conservative and wait for this pattern to break. Recent storm/wind slab concerns up high and loose wet potential on steeper solar aspects/lower elevations will challenge us to manage terrain safely. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.
Summary
Detailed Forecast
Showers should decrease late Saturday morning through mid-day and become light and scattered in the afternoon. Sunbreaks are possible in the afternoon, especially for the central and southwest Cascades. The air mass will be cooler, but it's spring-time and solar effects should push temperatures near to above freezing at lower and mid-elevations in the afternoon.
We continue to load and stress our snowpack. Don't push it in consequential avalanche terrain - keep it simple and conservative and wait for this pattern to break. Recent storm/wind slab concerns up high and loose wet potential on steeper solar aspects/lower elevations will challenge us to manage terrain safely. Cornices have recently proven dangerous and unpredictable and capable of triggering very large avalanches.
Recent winds have been mostly S-SW, so firmer wind slab should be found mainly on NW-SE slopes near and above treeline. However, near the Cascade Passes and in the southwest Cascades there have been periods of E-SE winds, so watch for wind slab on all aspects, especially in areas of complex terrain.
New storm snow is expected to bond well due to a cooling trend Friday night but storm slab may still become locally sensitive in areas that receive rapid accumulations Friday night or Saturday morning.
Subtle daytime warming and/or potential sunbreaks Saturday afternoon will increase the potential for loose wet avalanches on steeper solar slopes and at lower elevations. Loose wet avalanches won't be listed as an avalanche problem, but watch for surface wet snow deeper than a few inches, pinwheels and increasing natural releases. Avoid steeper slopes with terrain traps where even small loose wet avalanches could have unintended consequences.
You need to avoid areas on ridges where there may be a cornice and slopes below cornices! Natural cornice releases and resulting slab avalanches are unpredictable and as seen above in the special note, cornices have triggered major slab avalanches as of late. 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 read 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 24 hours ending at 5 PM Friday, Mt. Baker and Paradise have picked up 12-18", Crystal 10" with 6" or less inches in the Passes (the top of Alpental's snowboard was likely wind affected.) A slight warming trend occurred during the day on Friday.
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 to 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. Baker pro-patrol had minimal results during control work Thursday morning with a few pockets of stubborn wind slab noted. NWAC received second hand information that a skier triggered loose wet avalanche on the Shuskan Arm Thursday had caught and carried the skier 150 yds and into trees, resulting in non-critical injuries.
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.
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.
Crystal pro-patrol reported sensitive and widespread 8-12" storm slab 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 contributer 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.
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 25th, 2017 1:27PM
