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BACKCOUNTRY AVALANCHE FORECAST

EAST SLOPES SOUTH - SOUTH OF I-90 TO COLUMBIA RIVER

ISSUED
Tuesday, February 18, 2020 - 6:00PM Tue, Feb 18, 2020 - 6:00PM
AUTHOR
Peter Moore
THE BOTTOM LINE

Continued sunshine and warm daytime temperatures will maintain the possibility of natural and human triggered wet loose avalanches. Cool East winds will limit the potential for avalanches within the recent snow at middle and upper elevations, however, avalanches may still be possible on steep slopes with direct sunshine. Avoid steep slopes with direct sunshine during the mid-morning and early afternoon. 

AVALANCHE DANGERi

Wednesday, February 19, 2020
Above Treeline No Rating
Near Treeline No Rating
Below Treeline No Rating

OUTLOOKi

Thursday, February 20, 2020
No Rating
No Rating
No Rating

OUTLOOKi

Thursday, February 20, 2020
n n n
Danger Scalei
  • Low (1)
  • Moderate (2)
  • Considerable (3)
  • High (4)
  • Extreme (5)

FORECAST DISCUSSION

Wednesday will be a near repeat of Tuesday with clear skies, sunshine and mid-day warming. As the sun warms the surface, loose wet avalanches will be likely. Tuesday’s warming did not quite active a natural wet loose cycle of avalanches, so cold dry snow remains. Small loose wet slides will have the ability to entrain much of this recent snow and become large enough to bury a person. The most likely place for large loose wet avalanches will be at lower elevations which are sheltered from the wind but are still receiving direct sunshine. At middle elevations, cold East wind will limit the size of potential loose wet avalanches, but they will still be possible on steep slopes with direct sun.

Pay attention to signs of warming that suggest natural and human triggered avalanches could occur. Look for trees shedding snow and dripping water. Moist snow on the surface, rollerballs of snow, and small natural wet snow sluffs are all signs of sun weakening the snow surface. Avoid travel on or underneath steep slopes when you see these clues. Loose wet slides are often surprisingly powerful. Use extra caution around terrain traps like cliffs and creeks.

Rollerballs and pinwheels are a sign that you could trigger a loose wet avalanche. Photo: Andrew Keifer 2019

 

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.

February 13, 2020 (The regional synopsis is updated every Thursday @ 6 pm)

Heart of Winter

The action has been non-stop so far in 2020 with several widespread natural avalanche cycles and a few recent close calls. The active weather pattern has kept us all on our toes, especially January’s barrage of storms bringing seemingly endless precipitation and dramatic snowpack growth. Ongoing snow, wind, and rain continued into February, and a not-so-ordinary atmospheric river event recently left its mark on the region. The second week of February brought the first stretch of high pressure in weeks, allowing the snowpack to gain strength and the avalanche danger to ease between storms. Now, in the heart of winter, we have a deep and healthy snowpack with snow depths throughout the Cascades and Olympics near 100% of normal. Looking ahead, each day brings new changes to the upper snowpack, and a dynamic pattern with direct action events (storm-driven avalanche danger) will likely be par for the course.

Atmospheric River Aftermath 

Model simulation for February 5-6th, 2020 showing an Atmospheric River (AR) with a less than common northwest-southeast orientation as it impacts the region. This orientation allowed for strong westerly winds and more favorable upslope flow than a more typical AR approaching from the southwest. Image courtesy of the Center for Western Weather and Water Extremes, UC San Diego. (Link)

An atmospheric river impacted the region on February 5th-8th, causing a string of notable events. This storm favored the Central Cascades and Stevens Pass in particular, which experienced continuous heavy snow and rain for 86 hours, amounting to almost 70in of snow with about 7.5in of water equivalent. Not surprisingly, atmospheric rivers often go hand in hand with avalanche warnings, which were issued for 3 consecutive days at Stevens Pass from February 5th-7th, along with high danger in all other zones. Heavy rain fell at low elevations and even caused a significant mudslide on SR 410 between Enumclaw and Crystal Mountain, closing the road for 4 days and knocking out communications to 9 mountain weather stations for a week. As the AR exited the Northwest, and natural avalanche activity tapered off, conditions still remained touchy to human traffic on February 8th and 9th. Several triggered avalanches were reported that weekend, most notable of which was a close call near Mt. Baker Ski Area:

On February 8th, a skier was fully buried in an avalanche adjacent to Mt. Baker Ski Area. The avalanche was triggered by a traveler from a different party. Mt. Baker Ski Patrol was on the scene immediately, located the victim quickly, dug them out, and cleared the airway. The individual survived and reported no injuries. The avalanche was about 1ft deep and eventually broke up to 500ft wide. NNW aspect 5500ft. Photo: Mt. Baker Ski Patrol

Clear skies on Sunday, February 9th gave observers a chance to document the widespread avalanche cycle in the Stevens Pass zone that occurred February 5th-8th, including this view of crowns from large natural avalanches in the Berne Camp Chutes with Glacier Peak in the background. Photo: Matt Primomo

High Pressure before President’s Day Weekend

The week of February 10th brought the longest stretch of dry weather so far in 2020. A notable northwest wind event redistributed snow throughout the region and drove an isolated wind slab problem in most zones. Generally, it was the quietest few days avalanche-wise in weeks. However, a significant human-triggered avalanche occurred near White Pass on February 12th. Fortunately, no one was caught or injured. The incident provided a good reminder that even during periods of lower avalanche danger when avalanches are unlikely, outlier events can and do happen. The winter snowpack will always pose some level of uncertainty, and big triggers like cornice fall can produce surprising results.  
 

The crown of a human-triggered avalanche on a northeast aspect at 6700ft in the Hogsback area near White Pass. Two travelers unintentionally triggered a cornice, which dropped onto the slope below and triggered a very large avalanche. 2/12/20 Photo: White Pass Ski Patrol

No Corresponding Mountain Weather Forecast Available