Avalanche Forecast
Issued: Jan 16th, 2017 10:09AM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Loose Wet and Wind Slabs.
NWAC,
Northwest Avalanche Center
A drastic change in the recent weather is expected. The timing is uncertain, so watch for changing conditions. Very wet and milder weather should substantially increase the avalanche danger late Tuesday or Tuesday night. Natural wet snow avalanches are increasingly likely later Tuesday.
Summary
Detailed Forecast
Light rain and snow should develop for the North Cascades Monday night while only thickening clouds are expected for the central and south Washington Cascades. Mild freezing levels will continue Monday night with temperature inversions in the Passes and sheltered valleys. Alpine winds out of the SW are forecast to increase Monday night.
Light to moderate rain, snow or freezing rain is expected Tuesday with increasing SW winds. This weather will begin an increase in the avalanche danger as shallow wind and storm slabs begin to develop. Warming and rain should cause loose-wet avalanches on many steep slopes.
Possible freezing rain in the passes, especially the Snoqualmie Pass area Tuesday, may build a surface crust and locally limit the avalanche danger.
Avalanche watch issued Tuesday night through Wednesday: A drastic change in the weather pattern to wetter and milder weather should substantially increase the avalanche danger Tuesday night through Wednesday.
Heavy precipitation with a warming trend late Tuesday and Tuesday night should cause a more widespread natural avalanche cycle.
From Stevens Pass and southward to White Pass, precipitation amounts, type and timing are less certain and High avalanche danger may occur late Tuesday or not until Tuesday night.
An abundance of weak and cold snow in the upper snowpack combined with the potential for rapid warming and high precipitation rates would lead to very dangerous avalanche conditions during this period and backcountry travel is not recommended later Tuesday.
Snowpack Discussion
Weather and Snowpack
A pair of warm fronts moved across the Northwest last weekend, with fluctuating temperatures to near or above freezing in areas away from the Passes with milder Pacific air scouring the colder air in the Passes late Sunday night 1/8. This allowed a rain or melt freeze crust to form in some areas below treeline before 5-15 inches of snow accumulated through Monday 1/9.
A weak low pressure system, tracking across Oregon Tuesday morning through Wednesday afternoon, deposited another 3-6 inches of snow with light winds from Stevens Pass and southward, with Paradise adding another 12 inches. A period of strong E to NE winds occurred Tuesday afternoon through early Wednesday.
An upper ridge over the northeast Pacific Ocean has caused fair weather Thursday through Sunday, with clouds increasing Monday over the Olympics and Cascades with mostly light winds and moderating temperatures west of the crest and at higher elevations. Several NWAC stations warmed into the lower 40's by Monday while strong temperature inversions and breezy cold east winds persisted in the Passes.
Surface hoar and near surface faceting has been noted widely throughout the range in sun and wind sheltered locations below treeline. Sun crusts have formed on steeper solar aspects over the last few days. Numerous small loose-wet avalanches have been seen Sunday and Monday as temperatures climbed with sunshine.
Recent Observations
NWAC pro-observers Dallas and Ian Nicholson were in the Crystal backcountry Wednesday to assess the distribution and sensitivity of fresh wind slabs from recent strong E-NE winds. They found that wind slabs were becoming larger as they moved into the near tree line band and exposed ridges. NE-E-SE facing slopes were stripped of recent snow with new reactive wind slabs on N-W-S facing terrain and cross loaded features. In wind sheltered terrain below treeline, the storm snow was right side up and lacked slab structure with excellent skiing conditions.
Some reports are available via the NWAC Observations page for Wednesday and Thursday. in summary skiers triggered wind slab layers on Mt Herman near Mt Baker and in the Slot Couloir on Mt Snoqualmie on Wednesday. Potential wind slab layers were noted at Stevens and at Snoqualmie on Thursday but no avalanches were triggered.
NWAC pro-observer Jeremy Allyn was out in the Alpental Valley on Friday and mainly found that recently formed wind slab in the near and above treeline was settling and strengthening.
Lee Lazarra was taking observations for NWAC in Glacier Creek, NW of Mt. Baker proper on Saturday. Lee found generally good riding conditions in the trees on non-solar aspects. Solar aspects were softening and likely to form sun crusts overnight. Snow surfaces just below and along ridges were variable from last week's winds and Lee found recent wind slabs to be unlikely to trigger in his specific area. Widespread surface hoar growth was noted in the usual spots like creek beds, but surface hoar size diminished higher in the below treeline band.
Ian was east of Stevens Pass on Monday, MLK Holiday near Lichtenberg Mountain. Any older wind slabs near tree line had stabilized and were not present on SW facing slopes near treeline where they would be expected, if present. The steep solar exposed slopes produced dozens of small, size 1 loose-wet avalanches.
Problems
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: All aspects.
Elevations: All elevations.
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, North West.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Valid until: Jan 17th, 2017 10:09AM
