Avalanche Forecast
Regions: Cascades - North West.
Very dangerous avalanche conditions will continue Friday as additional rain and snow overload the snowpack. Natural avalanches could grow very large and run far distances. Travel in or below avalanche terrain is not recommended.
Discussion
Since Wednesday afternoon, the West North Zone received 4in of water. Heavy rain fell on dry snow up to 6000ft. More water (2-2.5in) is on the way over the next 24 hours as temperatures gradually cool with snow levels lowering to 4000ft by Friday afternoon.Â
Avalanche conditions will remain very dangerous as the snowpack continues to get saturated with water. The potential for very large natural wet avalanches will linger Thursday night and into Friday morning. As temperatures cool and rain transitions to snow, fresh slabs of dry snow will build and become increasingly reactive throughout the day.
Snowpack Discussion
January 23rd, 2020 (The regional synopsis is updated every Thursday @ 6 pm)
January to Remember
A parade of storms since the New Year began brought relentless precipitation, and impressive snowpack growth throughout the region. Areas along the west slopes of the Cascades have rarely gone more than 24-48hrs without precipitation. In the past three weeks, the Volcanoes and Passes received 20-28in of water equivalent, translating into incredible snowfall totals - Mt. Baker Ski Area reported 240in of new snow since January 1. The majority of NWAC weather station sites are reporting snow depths well over 120% of normal for this time of year. Ongoing precipitation and fluctuating snow levels have maintained elevated avalanche danger throughout the month, with avalanche warnings issued for nearly all forecast zones on 4 separate days (January 6, 7, 12, 23). Â
Snow depth imagery for the Northwest Region on January 23, 2020. Many areas in the Cascades and Olympics show well over 100 inches of snow on the ground. Image courtesy of NOHRSC Regional Snow Analysis.Â
Location
January 1-23 Precipitation (Water Equivalent)
Hurricane Ridge
14.79â€
Mt Baker Ski Area
28.65â€
Harts Pass
11.2â€
Stevens Pass
20.26â€
Leavenworth
4.01â€
Snoqualmie Pass
25.43â€
Paradise, Mt Rainier
24.13â€
Mt Hood Meadows
22.34â€
Table 1: Precipitation totals for select weather stations January 1-23, 2020. Huge numbers at Baker, Rainier, Hood, and the Passes and there is still another week left this month.
MLK Weekend and a Pattern of Rain on Dry Snow
After a prolonged cold period with arctic air and lowland snow, MLK weekend brought unseasonably warm temperatures and a rapid thaw. High snow levels and rain caused wet avalanche activity throughout the region January 18-19. A few very large natural avalanches occurred at upper elevations where all precipitation fell as snow. A cooling trend followed, creating a pronounced crust that is now buried in almost all forecast zones. Cold, dry snow January 21-22 was followed by a rapid warm-up and the most significant rain on snow event yet, causing avalanche warnings on January 23 for 5 of our 10 forecast zones.
The crown of a very large avalanche around 10,000ft on the Newton Headwall of Mt Hood which likely occurred during the storm late last week. 01/19/20 Photo: Kevin Kayl
The buried MLK crust is widespread throughout the region and a prominent marker in the snowpack. Crystal Backcountry 01/20/20 Photo Jeremy Allyn
One More Week To Go
January isn’t over yet, and the long term forecast continues to show an active and wet weather pattern for the Northwest. The low-snow, drought-like conditions of the early season seem like a distant memory at this point. We’ve certainly made up for lost time in 2020, and this already impressive month looks to end with a bang.
Avalanche Problems
Wet Slabs
We are in the midst of a major avalanche cycle, and the snowpack is still capable of producing surprising wet avalanches. Wet slab, wet loose, and glide avalanches could occur naturally and grow very large. Wet avalanches could run full track to valley bottom, and be destructive enough to break mature trees. These avalanches are difficult to predict and even harder to assess. Build in a wide margin for error if you choose to venture out Friday. Avalanche conditions are dynamic and unmanageable. Being caught in a wet avalanche would have devastating consequences. Even loose wet avalanches could build significant mass as they run downslope and pack quite a punch. Avoid travel in or below avalanche terrain and steer clear of avalanche path run out zones.
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: All elevations.
Likelihood: Very Likely
Expected Size: 1 - 2
Storm Slabs
Incoming precipitation will fall as snow at upper elevations and thicken existing storm slabs. With a cooling trend throughout the day, fresh slabs may begin to form as low as 4000ft. Slabs will be stiffer and more reactive in wind loaded terrain. Avalanches within new and wind loaded snow will be easy to trigger on upper elevation slopes 35 degrees and steeper by Friday afternoon. These avalanches could easily entrain wet heavy snow as they run onto lower slopes becoming more dangerous and destructive. Give the snowpack time to adjust before venturing into avalanche terrain.
Release of a soft cohesive layer (a slab) of new snow that breaks within the storm snow or on the old snow surface. Storm-slab problems typically last between a few hours and few days. Storm-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.
You can reduce your risk from Storm Slabs by waiting a day or two after a storm before venturing into steep terrain. Storm slabs are most dangerous on slopes with terrain traps, such as timber, gullies, over cliffs, or terrain features that make it difficult for a rider to escape off the side.
Storm slabs usually stabilize within a few days, and release at or below the trigger point. They exist throughout the terrain, and can be avoided by waiting for the storm snow to stabilize.
Aspects: All aspects.
Elevations: Alpine, Treeline.
Likelihood: Likely
Expected Size: 1 - 1