Avalanche Forecast
Issued: Mar 19th, 2017 11:03AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Wind Slabs and Loose Wet.
NWAC,
Northwest Avalanche Center
The avalanche danger should decrease further on Monday. E-SE winds may distribute shallow amounts of recent snow onto westerly aspects near and above treeline. Increasing clouds should lower the likelihood of small loose wet slides.
Summary
Detailed Forecast
Increasing cloud cover should quickly spread from south to north over the Cascades on Monday. Precipitation may develop for the southern Washington Cascades Monday afternoon but hold off until the evening for areas further north. Offshore gradients will create locally breezy east winds near the Cascade Passes.
The avalanche danger should decrease further on Monday. Light to locally moderate E-SE winds may distribute shallow amounts of recent snow onto westerly aspects near and above treeline in the central Cascades. In the southwest Cascades, recent or fresh wind slab will more likely be found above treeline. Older wind slab may linger on NW to SE aspects. Watch for firmer wind transported snow on all aspects, especially in areas of complex terrain.
Increasing clouds should lower the likelihood of small loose wet slides Monday. If solar radiation is more intense than forecast, expect the likelihood for natural and human triggered loose wet avalanches to increase on steeper solar slopes. Watch for surface wet snow deeper than a few inches, rollerballs or increasing small natural releases. Avoid areas with terrain traps where a small loose wet avalanche could have unintended consequences.
It is always a good plan to travel well back from ridges, suspected of cornice formation, or on steep slopes below cornices.
Snowpack Discussion
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.
After a short respite from the active weather pattern on Thursday, another strong low pressure system brought several inches of rain to the west slopes of the Cascades outside the Cascade Passes Friday night. 2.5 to 4 inches of water was seen at Paradise, Crystal and Mt. Baker respectively over the 24 hours ending at 5 PM PDT Saturday, mainly in the form of rain. Stevens and Snoqualmie Pass picked up 6-8 inches of new snow Friday night before changing to rain later Saturday morning.
Rapid cooling with the frontal passage produced about 8 inches of snow at Mt. Baker with lesser amounts seen elsewhere along the west slopes of the Cascades late Saturday morning through Saturday evening. Sunday was cool with light winds with a general clearing trend seen throughout the Cascades.
Recent Observations
North
Early Saturday morning, Mt. Baker pro-patrol reported widespread natural loose wet activity that had occurred Friday night off of Table Mt, Mt. Herman and Shuskan Arm. Very heavy rain overnight had increased the potential for wet slab/glide avalanches in the Mt. Baker area.
NWAC observer Lee Lazarra was in the Ptarimgan Ridge area of the Mt Baker backcountry Sunday. There was widespread evidence of the most recent wet snow avalanche cycle from Friday night with debris at the bottom of many avalanche paths. Crowns from wet slabs were especially large below treeline. Lee found the most recent rain crust very supportable above treeline but still in the process of re-freezing below 4500 feet. Small loose wet slides released naturally and ran well on steeper solar slopes during clearing in the afternoon. About 20 cm (8") of recent storm snow was evenly distributed regardless of elevation. Isolated pockets of shallow wind slab were found above treeline but no obvious loading pattern was discernable.
Central
Alpental pro-patrol reported easy and widespread ski triggered 6-8" storm slabs on the upper half of Alpental Saturday morning with rain reaching the summit. Loose wet avalanches were easily triggered later in the day on steeper slopes. Stevens Pass pro-patrol had similar results Saturday morning with widespread sensitive loose wet slides, up to size 2, natural and human triggered during the warming and switch to rain.
South
Crystal patrol reported a natural cycle Friday night, presumably loose wet, but little to no results during avalanche control work Saturday morning. On Sunday, Crystal pro-patrol reported a stout rain crust on the upper mountain with about 5-8 cm (2-3 in.) of well bonded snow on top of the crust.
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: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Loose Wet
Release of wet unconsolidated snow or slush. These avalanches typically occur within layers of wet snow near the surface of the snowpack, but they may quickly gouge into lower snowpack layers. Like Loose Dry avalanches, they start at a point and entrain snow as they move downhill, forming a fan-shaped avalanche. They generally move slowly, but can contain enough mass to cause significant damage to trees, cars or buildings. Other names for loose-wet avalanches include point-release avalanches or sluffs. Loose Wet avalanches can trigger slab avalanches that break into deeper snow layers.
Travel when the snow surface is colder and stronger. Plan your trips to avoid crossing on or under very steep slopes in the afternoon. Move to colder, shadier slopes once the snow surface turns slushly. Avoid steep, sunlit slopes above terrain traps, cliffs areas and long sustained steep pitches.
Several loose wet avalanches, and lots of pinwheels and roller balls.
Loose wet avalanches occur where water is running through the snowpack, and release at or below the trigger point. Avoid terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.
Aspects: South East, South, South West, West.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Mar 20th, 2017 11:03AM
