Avalanche Forecast
Regions: Cascades - East.
A drastic change from the recent weather is underway. Very wet and mild weather should substantially increase the avalanche danger through Wednesday. Natural wet snow avalanches are likely and travel in avalanche terrain is not recommended.
Detailed Forecast
An avalanche warning is in effect through Wednesday afternoon: Expect periods of heavy rain and strong winds to continue Tuesday night through Wednesday afternoon. The wet and milder weather should substantially increase the avalanche danger through Wednesday.
The heavy precipitation with a warming trend through Tuesday night and Wednesday will impact the Washington Cascades. A natural avalanche cycle should occur with the heaviest precipitation.
An abundance of weak and cold snow in the upper snowpack especially near and below treeline 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.Â
The 12/17 PWL and other persistent weak layers closer to the surface have been largely unreactive as of late. These layers may awaken during the expected upcoming warm and wet period with the potential to produce large and destructive avalanches.
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Snowpack Discussion
Weather and Snowpack
A pair of warm fronts moved across the Northwest last weekend, which allowed some relatively milder Pacific air to finally work its way east of the crest Sunday evening and night. 3-8Â inches of snow fell through Monday morning, 1/9 along the east slopes.Â
A weak low pressure system, tracking across Oregon Tuesday morning through Wednesday afternoon produced 1-5 inches of snow in the central east to southeast Cascades with the most seen at the Lost Horse and Potato Hill Snotels in the southeast Cascade zone. E to NE winds increased Tuesday afternoon.
Snow showers deposited another 2-4 inches early Wednesday in the southeast Cascades zone, with little or no snow in the north and central.
An upper ridge over the northeast Pacific Ocean has caused fair weather Thursday through Sunday over the Cascades with mostly light winds and moderating temperatures seen mainly at higher elevations along the east slopes due to strong temperature inversions. Low clouds over Eastern Washington stayed banked up against the east slopes over the weekend while the mountains mostly enjoyed the sunshine.Â
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.
Many small loose-wet slides were seen on solar aspects at higher elevations during the mild, mostly sunny weather Monday, MLK Holiday.Â
Warm, wet and windy weather has returned to the Cascades as Tuesday with the rain intensities increasing Tuesday afternoon, 1/17 to all elevation bands.Â
Recent Observations
NWAC pro-observer Tom Curtis was on Mt Cashmere Wednesday and pulled the plug on continuing a tour due to the conditions observed. The sound of a natural avalanche rang out loudly in the valley Wednesday morning. It was estimated the slide released from a more westerly aspect near tree line. Other concerning observations included finding a reactive buried thin crust with faceted crystals below and a 25-35 cm 4F slab above. Cracks were shooting from skis and snowpit tests also indicated human triggered slab releases would be likely in this area.
The NCMG were at Washington Pass on Wednesday and reported that wind slab from last weekend was primarily seen on E-SE terrain features and was becoming less reactive.
A couple reports via the NWAC Observations page are available for Wednesday. A report from Mt Cashmere indicated a shallow, variable snowpack with evidence of wind loading from last week. A collapse and snowpack cracking was seen on a southeast slope at 5300 feet due to a facet/crust layer at 35 cm down but these layers were not found to be widespread. A skier also reported 14-16 inches of snow and snowpack collapsing on Manastash Ridge on Wednesday.
Tom Curtis was on Diamond Head at Blewett Pass on Thursday reported many wind scoured areas with previous wind transport primarily to W slopes. The 12/17 PWL was found at 30 cm down, but was not reactive. Facets at the base of the snowpack were not giving test results.
Tom Curtis was out again on Friday on Mt Cashmere and on a W aspect near treeline and he found the 12/17 PWL at 55 cm giving a PST 37/100 End. He noted some previous wind transport and cross loading on N-W-S aspects. Tom also found great skiing conditions in the trees.Â
An observation via the NWAC observation page from Saturday continued to identify weak persistent grains near the base of the relatively shallow snowpack in the Mission Ridge area. While these layers were reactive in column tests, no recent avalanche activity has been observed involving these layers.Â
Avalanche Problems
Wet Slabs
Release of a cohesive layer of snow (a slab) that is generally moist or wet when the flow of liquid water weakens the bond between the slab and the surface below (snow or ground). They often occur during prolonged warming events and/or rain-on-snow events. Wet Slab avalanches can be very destructive.
Avoid terrain where and when you suspect Wet Slab avalanche activity. Give yourself a wide safety buffer to handle the uncertainty
A Wet Slab avalanche. In this avalanche, the meltwater pooled above a dusty layer of snow. Note all the smaller wet loose avalanches to either side.
Wet slabs occur when there is liquid water in the snowpack, and can release during the first few days of a warming period. Travel early in the day and avoiding avalanche paths when you see pinwheels, roller balls, loose wet avalanches, and during rain-on-snow events.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Possible
Expected Size: 1 - 1
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: Likely
Expected Size: 1 - 1
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: All aspects.
Elevations: All elevations.
Likelihood: Possible
Expected Size: 1 - 1