Avalanche Forecast
Issued: Jan 11th, 2019 6:00PM
The alpine rating is , the treeline rating is , and the below treeline rating is Known problems include Persistent Slabs and Wind Slabs.
Matt Primomo,
The Bottom Line: You may be able to trigger life threatening avalanches in areas where weak old snow exists. The wind has formed firm slabs in some locations that are difficult to trigger, but still present danger. You can stay safe by avoiding slopes greater than 30 degrees.
Summary
Snow and Avalanche Discussion
With good weather in store for the weekend, it is likely that folks will be getting up high into the mountains. It is important to realize that there is much uncertainty with our persistent weak layers above 6,000ft, and a high amount of variability within the zone. The recent upslope storm revived winter on the eastern slopes and added a hefty load to the upper elevation snowpack. Snowfall totals vary widely, from 5" to 14", with more at higher elevations. A common structure to be found out there will be storm snow (with or without a wind slab) over a crust. Recent wind slab avalanches at Mission Ridge were from 6" to 16" deep and from 60' to 125' wide, running on this crust. One that started on a Northwest aspect at 6300ft broke into older, weak snow near the ground. At low elevations and on southerly aspects, the snow surface has a thin crust from light rain or sun, and a number of loose wet avalanches were observed.
Weather Forecast
Sat 12th Jan 14:40 - Dennis D'Amico
Weather Synopsis for Saturday night through Monday
A ridge of high pressure anchored over the Intermountain West will lead to a dry weekend. A weak and splitting frontal system offshore has spread some high clouds over the area today.
There is some nuance to the freezing level forecast with offshore easterly flow continuing to draw colder air and low clouds through the Cascade Passes while much milder air can be found at higher elevations and along the west slopes of the Cascades and Olympics.
The cold pool along the east slopes of the Cascades will slowly erode and moderate through Monday with freezing levels on the rise, but low clouds and cooler temperatures will persist along the lower slopes.
Light to moderate easterly flow through the Cascade Passes and at ridge-crest for places like Crystal Mountain will also persist into Monday despite the overall warming trend.
Regional Synopsis
Thu 10th Jan 09:00
January 10, 2019
We’re about a month into our avalanche forecasting season, and what a wild month it has been. We’ve seen the snowpack grow from near non-existent to over a 100 inches deep at Mt Baker. We’ve seen avalanche warnings blanket our coverage area, had several widespread large natural avalanche cycles, and seen persistent weak layers plague nearly every zone. While the active weather pattern has been a welcomed sight, it’s hard to catch your breath.
It looks like this weekend, and extending into next week, we are entering a calm period in the weather. High pressure over Idaho and Montana will keep Pacific storms at bay for at least the short term. As we enter this period of nice weather, there are a few things on our mind here at the avalanche center.
Old Persistent Weak Layers
A high elevation rain event on January 3rd did us a favor in many locations. Besides heavily stressing old weaklayers, the rain created a very strong met-freeze crust, making it extremely difficult to trigger deep avalanches.
There are a few exceptions, and they can be found mainly in the eastern forecast zones. The East-North zone around the Methow Valley and WA Pass did not experience significant rain above 5500 ft, potentially leaving some older weak layers in tact. Areas further east including, but not limited to, Icicle Creek and the Wenatchee Mountains did not receive enough rain to form a strong thick crust. In all of these area, older persistent weak layers are still suspect.
Higher Elevations and Remote Trailheads
Stormy weather over the last month has been keeping most of us closer to common trailheads and recreating at near and below treeline. With nice weather on tap, some of you may be thinking about heading into higher elevation terrain and exploring around more remote trailheads. If you are heading out, take time to consider a few key pieces of information.
- At higher elevations the snowpack is very deep and wintery. This stands in stark contrast to the lack of snow at lower elevations. The snowpack you’ve seen on previous outings this winter, doesn’t represent what you will encounter at higher elevations.
- We have not received much information about the above treeline nor remote location snowpacks. That means we have a high degree of uncertainty in these areas.
- You will need to make observations as you travel to confirm if the avalanche forecast is applicable the the area where you are traveling. When observations lineup with the information in a forecast, its valid. If you see observations that don’t lineup, it’s time to take a step-back.
- If you are heading into our regions highest alpine terrain, remember, NWAC forecast apply to areas below the main Cascade crest, or about 8000 ft. So, if you plan to travel to higher elevations on Mt Baker, Mt Shuksan, Mt Rainier, Mt Hood, or similar areas, you will need the skills and information to evaluate the terrain and snowpack.
We’d like to thank our NWAC Community for all the wonderful public observation this year. If you are heading out into the mountains, we appreciate you taking the time to let us know what you saw. This is a great way to contribute to the creation of your avalanche forecast. You can submit observations here.
Enjoy the sunshine!
Problems
Persistent Slabs
We are entering the timeframe where obvious clues to avalanche danger like cracks in the snow, or sudden collapses may not be observed. This is why persistent slabs are difficult to assess and predict. We do know that we have a weak snowpack structure in some locations and there is plenty of snow over these layers to act as a slab. The snowpack is slowly adjusting to the new load, but rapidly rising freezing levels and the sun are something to consider. It may influence the reactivity of these slabs. In any case, slides on these deeper weak layers could be very large and life threatening. You can choose to avoid high consequence terrain by staying away from all slopes greater than 30 degrees.
There are two primary layers of concern in the snowpack. You may encounter one or both depending on where you travel. You are most likely to encounter persistent slabs at higher elevations and in areas towards the eastern edge of the forecast zone.
A layer of buried surface hoar may be found 1-3 ft below the snow surface. This layer is mainly a concern above 6000ft and where the crust from January 3rd is nonexistent. Above 7,000ft we have virtually no observations. Be suspicious of this layer up there. Recorded avalanches have been on North through Southeast aspects.
A layer of weak sugary facets can be found near the ground, particularly in the eastern areas of the forecast zone. If an avalanche failed on this layer, it could be more than 4 feet deep. Use caution if you are traveling in areas where the snowpack is thinner.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood
Expected Size
Wind Slabs
It has been a few days since the wind has actively loaded the slopes, and the slabs that were built the past few days are becoming more stubborn to initiate. They are quite thick, however, and may still make large avalanches. The recent winds formed slabs in some unusual locations like well below ridgelines, on the sides of gullies, and in places that are often scoured. Some locations have no wind slab at all. Where slabs are found, they may be resting over lower density snow and sitting on a crust, an ideal recipe for avalanches. Approach any steep slopes with caution, and be especially careful of steep, unsupported slopes. Use your eyes and sense of touch to locate areas of wind slab as you travel. Does it feel more firm in one location than another? Does it seem hollow? You can stay safe by traveling on ridges, scoured areas, and low angle slopes.
With warming temperatures and sun poking through the fog, loose wet avalanches may become common. Steep slopes that face the sun are the most likely places these will happen. If you see roller balls or small loose slides that begin entraining snow on any aspect, get off of steep slopes.
Aspects: All aspects.
Elevations: All elevations.
Likelihood
Expected Size
Valid until: Jan 12th, 2019 5:00PM
