Avalanche Forecast
Issued: Mar 26th, 2016 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Storm Slabs, Wind Slabs and Persistent Slabs.
NWAC,
Northwest Avalanche Center
A mix of likely dangerous winter and spring avalanche conditions is likely on Sunday especially nearer the crest in the above treeline . The sun is getting much stronger and new snow may be very reactive to solar effects. The greatest danger should be in the afternoon to the end of the daylight hours.
Summary
Detailed Forecast
Southwest flow should carry an energetic front and shortwave to the Northwest Saturday evening and night. Moderate west flow should carry a 2nd short wave, an upper low and a cool unstable air mass to the Northwest Sunday. This should cause southwest to west winds, rain or snow changing to snow showers and lowering snow levels on Sunday. Snow and snow showers should cause up to about 6 inches of snow in the above treeline along the east slopes by the end of the daylight hours on Sunday.
A mix of likely dangerous winter and spring avalanche conditions is likely on Sunday especially nearer the crest in the above treeline . The sun is getting much stronger and new snow may be very reactive to solar effects. The greatest danger should be in the afternoon to the end of the daylight hours.
New wind slab is most likely on lee slopes in the above treeline on Sunday. This is most likely on north to east slopes but possible on other aspects. Watch cracking and firmer or chalky wind transported snow. Snow pits may not be helpful due to variability in the snowpack.
New storm slab layers will be likely in areas where there is rapidly accumulating snow for more than a few hours. Again this is mostly likely nearer the crest. Warming daytime temperatures may contribute to upside down layers and instability. Pay special attention to slope convexities where storm slab is mostly likely to be triggered.
Due to recent sensitive storm slab releases on a suspected spotty 3/12 buried surface hoar layer, we are currently listing it as a persistent slab. Though it may be spotty and not widespread it should nonetheless get attention from backcountry travelers in the NE zone.
Loose wet avalanches won't be listed due to other potential avalanche problems on Sunday but may be possible on steep solar slopes on Sunday especially nearer the crest. Don't overstay your welcome if you start seeing pinwheels or small initial natural loose wet avalanches. Be aware of terrain traps where even a small loose avalanche could have unintended consequences if you venture out on Sunday.
Cornices have grown large recently. Cornices can break much further back on ridges than expected and releases can be unpredictable during the spring.
Snowpack Discussion
Weather and Snowpack
A spotty layer of surface hoar likely formed and was buried on about 3/12 mainly in the northeast Cascades with the terrain most suspect being N through E facing slopes.
Snow accumulations Wednesday to Friday following the last front varied based on proximity to the Cascade crest and higher terrain with about 12 inches at the NRCS Lyman Lake Snotel, 6 inches at Harts Pass and next to nothing at Blewett, Mission Ridge and Berne weather stations. Crest level westerly winds were strong and sustained at the Mission Ridge station through Thursday afternoon.
Frequent March storms have built large cornices along ridges in many areas.
No avalanches have been noted for quite some time on a persistent buried surface hoar layer from February 27th in the Washington Pass area and to a lesser extent the central-east zone. Avalanches releasing on or down to this layer are unlikely.
Recent Observations
The last known human triggered avalanche on the 3/12 PWL in the Washington Pass area was on 3/19.
The North Cascades Heli Skiing operation skied extensively on a non-solar slope in Cedar Creek on Monday 3/21 and did not observe any avalanche activity in nearby terrain.
On Wednesday 3/23 NWAC pro-observer Tom Curtis was in the Blewett Pass/Table Mountain area and found dust on a mostly supportive crust prior to the incoming frontal system. Also of note, many windward and solar slopes were nearly or completely melted out up to about 6000 feet.
Mission Ridge pro-patrol reported strong west winds Thursday morning but little in the way of avalanche concerns in area due to the lack of new snow. On Saturday they reported minor small point release loose wet avalanches and minor rollerballs.
Problems
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: Alpine, Treeline.
Likelihood
Expected Size
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.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Persistent Slabs
Release of a cohesive layer of soft to hard snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slabs.
The best ways to manage the risk from Persistent Slabs is to make conservative terrain choices. They can be triggered by light loads and weeks after the last storm. The slabs often propagate in surprising and unpredictable ways. This makes this problem difficult to predict and manage and requires a wide safety buffer to handle the uncertainty.
This Persistent Slab was triggered remotely, failed on a layer of faceted snow in the middle of the snowpack, and crossed several terrain features.
Persistent slabs can be triggered by light loads and weeks after the last storm. You can trigger them remotely and they often propagate across and beyond terrain features that would otherwise confine wind and storm slabs. Give yourself a wide safety buffer to handle the uncertainty.
Aspects: North, North East, East.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 27th, 2016 11:00AM
