Avalanche Forecast
Issued: Mar 23rd, 2016 11:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs, Loose Dry and Persistent Slabs.
NWAC,
Northwest Avalanche Center
Persistent westerly winds will load lee slopes near and above treeline such that wind slab will be the primarily hazard on Thursday. The new snow available for transport will decrease dramatically further away from the Cascade crest. Loose snow and shallow storm slabs avalanches should be lesser concerns on Thursday.
Summary
Detailed Forecast
Cool NW flow should deliver a taste of winter in late March with light to moderate orographic snow showers along the west slopes that will spill over the Cascade crest into the east slopes Thursday. From late Wednesday night through Thursday, new snow should layer right side up as temperatures cool behind the cold front Wednesday night. Deep post-frontal moisture should prevent widespread solar affects immediately adjacent to the Cascade crest, but daytime warming is likely below treeline and further away from the Cascade crest Thursday afternoon despite the seasonally cool air mass overhead.
Persistent westerly winds will load lee slopes near and above treeline. Wind slab will be the primarily hazard on Thursday. The new snow available for transport will decrease dramatically further away from the Cascade crest. Firm wind transported snow and snowpack cracking can be good signs of wind slab layers.
Loose dry avalanches should be small but likely on steeper non-solar slopes. Be aware of terrain traps where even a small loose avalanche could have unintended consequences in the wrong terrain. Loose wet avalanches will not be listed as the primary loose snow avalanche problem, but loose wet avalanches will be possible on steeper solar slopes especially in areas where the sun makes an appearance.
Storm slabs should be a lower concern Thursday. Shallow storm slabs will be more likely in areas that see subtle warming Thursday. During more intense showers Thursday, watch for graupel layers that can become bed surfaces for storm slabs.
Recently formed cornices have grown large. If traveling along ridgelines, be aware that cornices break much further back than expected and you don't want to go for a ride with a chunk of cornice. Despite the cool weather, cornice releases can be unpredictable during the spring.
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.
Snowpack Discussion
Weather and Snowpack
About 2-3 feet of snowfall accumulated along the east slopes March 9-15th.
A combination of sun, mild temperatures and then some rain was seen for several days ending about Sunday 3/20. This weather caused about 6-12 inches of consolidation of the recent snow along the east slopes. Surface snow conditions became highly variable, ranging from cold snow or wind buffed surfaces on non-solar slopes to morning crusts and wet snow on solar slopes.
Frequent March storms have built large cornices along ridges in many areas.
A low pressure system crossed the Northwest on Monday and Tuesday. NWAC stations along the east slopes accumulated a few inches of snow above about 4000 feet through Wednesday morning.
A spotty layer of surface hoar likely formed and was buried on about 3/12 mainly the northeast Cascades with the terrain most suspect being N through E facing slopes.
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 on this layer are now unlikely.
We are also no longer tracking layers of concern formed earlier this winter in the mid or lower snowpack due to lack of recent activity at these interfaces and confirming field observations.
Recent Observations
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.
The North Cascade Mountain Guides at Washington Pass area Friday to Sunday 3/18-20 reported no results from ski cuts and no signs of instability. On Friday a loose wet avalanche that triggered a small 40 cm deep slab. Cornices in this area area were sagging greatly and snow in the below treeline was moist and heavy in the afternoons.
NWAC pro-observer Tom Curtis tested conditions along the Nason Ridge area to Rainy Pass Saturday, March 19th. No significant layers were found in test pits or on slopes. In general, shallow wet snow was found below treeline with shallow stubborn wind slabs on some isolated features near ridges that were gaining strength.
Reports via guides and a private email to the NWAC indicate 2 recent cornice releases in the Washington Pass area with a large size 3 storm slab and car sized blocks running to the valley floor.
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.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Dry
Release of dry unconsolidated snow. These avalanches typically occur within layers of soft snow near the surface of the snowpack. Loose Dry avalanches start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. Other names for loose-dry avalanches include point-release avalanches or sluffs. Loose Dry avalanches can trigger slab avalanches that break into deeper snow layers.
Loose Dry avalanches are usually relatively harmless to people. They can be hazardous if you are caught and carried into or over a terrain trap (e.g. gully, rocks, dense timber, cliff, crevasse) or down a long slope. Avoid traveling in or above terrain traps when Loose Dry avalanches are likely.
Loose Dry avalanche with the characteristic point initiation and fan shape.
Loose dry avalanches exist throughout the terrain, release at or below the trigger point, and can run in densely-treed areas. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells.
Aspects: All aspects.
Elevations: All elevations.
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 24th, 2016 11:00AM
