Avalanche Forecast
Issued: Jan 6th, 2020 10:00AM
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
Rising temperatures, strong winds, and heavy snowfall have created very dangerous conditions at upper elevations where very large avalanches may release naturally or by human trigger.. Avoid all areas above treeline or anywhere large avalanches can start, stop, or run. At lower elevations, large Loose Wet avalanches may release where you find wet, sticky snow on slopes steeper than 35 degrees.
Summary
Discussion
Very dangerous avalanche conditions exist at mid and upper elevations as Wind Slabs continue to build while Storm slabs gradually heal.
The combination of warming, heavy dense snowfall, and the wind has been building Slabs very effectively and were creating very dangerous conditions on Monday, particularly at mid and upper elevations while below 5000 ft, Loose Wet avalanches formed where snow changed to rain.
By late Monday, areas above 5500 ft picked up 10-20” of new snow during a major storm event. Below 5000 ft, areas have seen a changeover to rain with the upper elevations of Mt. Hood Ski Bowl (5000 ft) changing to rain early Monday morning. Winds have averaged greater than 30 mph at the top of Mt. Hood Meadows since 9 AM on Sunday.
Monday night and Tuesday, Mt. Hood should remain south of the firehose of precipitation aimed at Washington State, but light rain and snow should continue along with strong winds.
Snowpack Discussion
Happy New Year!
The pacific northwest rung in the new year with a winter storm cycle that brought changing conditions to our region. This storm exhibited three characteristics: warm, wet, and windy. Let’s take a look at the end of 2019, the beginning of 2020, and where we can go from here.
Wrapping up 2019
The last few days of December were generally quiet and cold. The snowpack seemed to enjoy this break in the weather. Lingering unstable snow from the pre-Christmas storms gained strength, persistent weak layers appeared to stabilize, and avalanche hazard decreased in all forecast zones. All in all 2019 ended quiet and uneventful… until the New Year’s Eve weather party showed up …
Quiet weather led to lower avalanche danger during the last week of 2019. Hogsback, White Pass, WA. Photo: Andy Harrington
Blowing into 2020
A warm, wet, and windy weather system blew into the northwest for New Year’s Eve and New Year’s Day. This brought rapidly changing conditions and increasing avalanche hazard to all areas.
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Warm: Unfortunately this system brought with it warm air. Freezing levels measured near the coast reached 9000’ on the afternoon of the 31st. Many weather stations recorded above freezing temperatures during the onset of precipitation.
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Wet: While this system wasn’t as wet as the atmospheric river prior to Christmas, it still produced impressive water numbers in many areas. The bulk of the precipitation seemed to be focused on the Passes and Volcanoes, and water spilled over the crest to places like Washington Pass and Leavenworth. Sadly, when combined with the warm temperatures, this translated to rain well into the near treeline band (or higher) for most areas. The main exception appeared to be in the northeastern cascades, where locations like Washington Pass remained all snow.
HurRidge
MtBaker
WaPass
Stevens
Leavenworth
SnoqPass
MtRainier
MtHood
Precipitation (in)
1.84
3.39
1.42
4.56
1.06
6.19
5.39
5.41
Snow (in)
-
15
-
12
0
3
10
8
Table 1: Precipitation and storm totals from selected weather stations during the New Year’s Eve Storm. “-” 24hr storm snow not measured.
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Windy: While the warm and wet were impressive, it’s the winds that may set this storm apart. Most weather stations recorded very strong and extreme winds during the storm. Alpental exceeded 100mph just after midnight to ring in the new year. Any dry snow at high elevations was redistributed by the wind and snow surfaces were transformed.
Table 2: Wind speeds from New Year’s Eve from selected wind sites. Note the sustained period of winds between 40-60mph.
Eventually, temperatures cooled, the rain turned back to snow, and winds calmed. Many locations picked up additional snow as the storm wound down, but 2020 was already off and rolling with its first major storm.
New Year’s Resolutions
The active weather pattern that started the new year appears to continue. The snowpack and avalanche conditions will continue to change. So, what can your New Year Avalanche Resolutions be?
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Read the forecast. This is a great way to monitor conditions even if you aren’t heading into the mountains.
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Get out in the snow! Enjoy the wonderful mountains in your backyard.
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Submit an observation. Tell the avalanche center what you saw while out in the snow by submitting an observation and sending in a photo.
Thanks for all of your support in 2019 and here’s to 2020!
-Dallas
Problems
Wind Slabs
Strong to extreme winds will continue to build large to very large wind slabs near and above treeline. Don’t let the lower rates of precipitation on Tuesday fool you. There’s plenty of snow to blow around and these slabs will continue to build and once overburdened, they may release naturally or with human trigger. The slabs sit atop lower density (weaker) snow that fell earlier in the storm cycle, making them particularly reactive. Identify and avoid all avalanche terrain, anywhere avalanches could start on steep slopes above 6000 ft and be aware that these slides can run into below treeline terrain.
In more sheltered terrain, look for denser storm slab layers on top of lower density layers. These layers need more time to settle so keep your slope angle less than 35 degrees and avoid unsupported slopes if you travel in dry snow near treeline.
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
Where rain falls and creates wet surface snow conditions (>6” deep), expect natural and human triggered avalanches on slopes steeper than 35 degrees. Given all the recent snow, these avalanches could be large and powerful, pushing you into trees, rocks or other obstacles. Remember that avalanches starting in dry snow can also entrain wet snow at lower elevations where they will entrain all the wet snow available above the most recent crust.
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.
Elevations: Below Treeline.
Likelihood
Expected Size
Valid until: Jan 7th, 2020 10:00AM
