Avalanche Forecast
Issued: Jan 2nd, 2020 10:00AM
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
Temperatures will rise dramatically Friday increasing the likelihood of wet, loose snow avalanches. Look for signs of warming such as dripping trees or snow forming roller balls on the surface and avoid slopes leading to cliffs, open creeks or other hazards where you see these signs.
Summary
Discussion
New Year’s Day was a wet one on Mt Hood! Telemetry from the ski areas showed 4.0-4.5 inches of water in that 24 hour period! Most of that fell as rain or freezing rain Tuesday night and extended up to 7000’ or higher before cooling down and switching to snow Wednesday afternoon.
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
Leanenworth
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!
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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
Loose Wet
As temps rise on Friday, so does the likelihood of loose wet avalanches. The crust formed by rain on New Year’s Eve may provide a good bed surface for wet snow to slide. Look for signs of warming such as roller balls on the surface of the snow, trees dripping, or natural wet avalanches. When you see these signs, avoid terrain above hazards such as cliffs and open creeks. Remember that loose wet avalanches can be more powerful than you expect and can push you into trees or other obstacles.
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: Treeline, Below Treeline.
Likelihood
Expected Size
Wind Slabs
Very strong winds have been consistent for several days, transporting snow into dense wind slabs. Reports from near treeline suggest that the winds have scoured many typical start zones near ridge crests and that slabs are building mid-slope. These slabs are dense and may be difficult to trigger, but have the potential to be large and dangerous. Look for signs of wind transported snow such as pillow-like drifts and cornice formation. Avoid slopes steeper than 35 degrees where you see these signs.
With such strong winds and limited visibility, there are no observations from the above treeline zone, but the recipe for very large avalanches does exist: strong winds, heavy precipitation, and warming temps. Avoid large open slopes above 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
Valid until: Jan 3rd, 2020 10:00AM
