Avalanche Forecast
Issued: Jan 18th, 2020 10:00AM
The alpine rating is , the treeline rating is , and the below treeline rating is
NWAC,
Northwest Avalanche Center
Pay close attention to changing conditions Sunday as there’s a high degree of uncertainty as we transition to a milder weather pattern. Slab avalanches are possible on steep slopes at upper elevations while loose wet avalanches are likely on very steep sunny slopes.
Summary
Discussion
Forecast Discussion
We are in the midst of a weather regime change as we leave the cold, windy and dry snow pattern of the last 10 days behind us and move toward more seasonable temperatures.
Rising freezing levels and possible sunshine on Sunday increases the chances for loose wet avalanches on steep sunny slopes. Temperature inversions and cloud cover on lower slopes may limit the loose wet avalanche problem to sunny slopes at upper elevations.
Pay attention to changing surface conditions. If the surface snow becomes wet, you see natural pinwheels or natural wet avalanches, it's time to avoid nearby steep slopes. Loose wet avalanches may surprise you if they become large and powerful by entraining recent cold snow underneath. Be aware of terrain traps like cliff bands or gullies that would amplify the consequences of a loose wet avalanche.
Recently formed wind or storm slabs layered on top of colder weaker snow may still be sensitive to human triggering on Sunday. Steer around steep open slopes steeper than 35 degrees at mid and upper elevations. Slopes with wind drifted snow are capable of producing larger avalanches. Give fresh cornices some space when traveling along ridgelines and avoid traveling on slopes directly below cornices.
Forecast Schedule and No Rating definition
We do not have enough specific snowpack information to issue an avalanche hazard rating for the East South zone. However, even when No Rating is applied, applicable avalanche conditions and backcountry travel advice will be provided throughout the season - this forecast is updated every day, all winter long! When weather systems produce very dangerous avalanche conditions in adjacent zones, NWAC will issue an avalanche warning for this zone as well.
Snowpack Discussion
January 16th, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
In the past week and a half, there have been five avalanche fatalities in three separate accidents in the US. One occurred near Kellog, ID and another outside of Baker City, OR. Local avalanche centers will perform accident investigations including final reports. You can find preliminary accident information at avalanche.org.
From January 9th to 16th the Pacific Northwest slid into deep winter. A cold and snowy regime brought a nearly continuous barrage of storms through the area. Temperatures bottomed out as modified arctic air made its way south from interior Canada, and many stations recorded the lowest temperatures of the season so far. A snowpack has been growing at lower elevations due to some lowland snow on both sides of the Cascades. NWAC’s snow depth climatology report shows most stations have surpassed average depths on the ground for this time of year. Quite the comeback from two weeks ago, when most were at 25-64% of normal.
Location
Total Snow Depth (in) 1/8/20
Total Snow Depth (in) 1/16/20
Hurricane Ridge
51
91
Heather Meadows Mt Baker
95
126
Stevens Pass
63
85
Snoqualmie Pass
33
77
Mission Ridge Mid Mtn
18
28
Crystal Mt Green Valley
66
92
Paradise Mt Rainier
105
138
White Pass Upper
69
110
Timberline
57
118
Mt Hood Meadows
53
98
Snow depths continued to rise. Total snow depths doubled in some locations.
The mountains went through a period of prolonged dangerous to very dangerous conditions as the snow kept coming. Many locations picked up over a foot of new snow per day for a number of days in a row, and storm slab instability was widely experienced across the region. At times, instabilities within new snow layers were very reactive, and you didn’t have to do much to provoke an avalanche. Many people triggered small to large soft slab avalanches, even well below treeline. The cold temperatures tended to preserve these instabilities longer than usual during this time.
Small ski triggered storm slab near Mt Hood Meadows. January 11, 2020. Scott Norton photo.
This cold, low density snow was also susceptible to wind drifting as westerly winds buffeted the alpine zone from the 8th to the 15th. On the 15th the mean winds shifted, and a south and east wind event disturbed the powder on open, exposed terrain near the passes and at upper elevations throughout the region. This created wind slab problems in some unusual locations.
Wind slabs formed over the low density powder snow. Mt Baker Backcountry. January 15, 2020. Zack McGill photo.
Trailbreaking in undisturbed snow was often very deep and difficult. In most places at any point in the week you could step off your skis or machine and sink in up to your chest in deep powder snow. The deep snow presented hazards of its own such as tree wells, and made it very easy to get stuck on a machine or lose a ski. Many folks experienced excellent, deep powder conditions and stuck to conservative terrain choices.
-MP
A cold winter’s day over the Chiwaukum Range, from Stevens Pass. Matt Primomo photo.
Valid until: Jan 19th, 2020 10:00AM
