Bollettino valanghe

East Slopes North

Northwest Avalanche Center
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Pericolo valanghe

Tue
Alta quota
2 · Moderato
Limite del bosco
2 · Moderato
Sotto il limite
1 · Debole
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In sintesi

Don't be lulled to sleep during a storm pattern that's likely to feature incremental loading over the next 2 days.  The low-likelihood/high consequence deep persistent slab threat may re-awaken, particularly in the Harts Pass portion of the zone. New wind slabs will likely develop on open slopes near and above treeline for areas closest to the Cascade crest. 

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Possible
    Dimensione
    1–1.5

    Saturday night's storm didn't deliver, but there's higher confidence that the incoming frontal system will bring several inches of snow (2-6" with higher totals closer to the crest) along with a warming trend and moderate ridgeline winds to the East North zone Sunday night and Monday.  

    Use caution before engaging with steep open slopes below ridgelines where you find winds have drifted fresh snow.  The new snow may bond poorly to steep slopes with a sun crust of varying thickness or to shady aspects at higher elevations with weak faceted snow.  Look for cracking in the new snow and check the bonding of the new to old snow interface on small test slopes.  

    Small loose wet avalanches may be possible on very steep slopes below the rain-line, especially where rain falls on colder drier snow. 

    While any type of avalanches within the new snow should be small, think about avoiding terrain where small avalanches may overlap with the deep persistent slab problem. 

  • Deep Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Unlikely
    Dimensione
    2–3

    While it's unlikely you'll trigger a deep slab avalanche in much of the zone, the area near Harts Pass may be the most likely place in the East North where the persistent weak layer is about 3' below the surface.  What can you do to manage this low-likelihood/high consequence problem?   You can limit your exposure to large start zones and very steep slopes, especially those with big overhanging cornices (see photo).  Also, avoid complex and unsupported terrain, shallow, rocky areas, and unsupported slopes (see photo in Drew's ob).   The bottom line with these avalanches is while there are unlikely, you should assume they'll be large enough to kill you.  

    On March 3rd and then again on the 9th, backcountry travelers in the Harts Pass area triggered large to very large avalanches on northerly aspects near treeline.  A cornice-triggered avalanche may have also occurred around the 8th or 9th on McGregor Mtn. You can find the late January weak layer (facets sandwiched between two crusts) 3-5' below the surface. Snowpack tests can help you identify the layer of concern. Observers continue to report sudden or propagating test results even as avalanches have become difficult to trigger. East of Washington Pass, the snowpack becomes shallower, and this interface may be nearer to the snow surface.

    (3/10/2022)

    In the center of the photo is a recent D2.5 deep slab, likely triggered by a cornice fall. 

    To the right of the photo, you can see a similar-sized avalanche that likely occurred during the natural cycle of 2/28-3/1.

    McGregor Mtn, NNE, 7,200ft.

    Photo: Matt Primomo

Avalanche Discussion

We'll see if the storm cycle gives us more clarity surrounding the deep persistent slab issue.  At the very least, it will usher in a period of more active weather and fresh snow at mid and upper elevations.  Since the atmospheric river event ended on 3/1 , the Washington Pass precipitation gauge has only recorded about half an inch of water.   

On March 9th, observers near Harts Pass remotely triggered a large, R3-D2 avalanche from 20-30 feet away. The avalanche occurred roughly around 6,600ft on a north aspect and failed on facets above a crust.  However,  a crown profile revealed a uniquely shallow snowpack suggesting previous avalanches may have already ran on this interface. 

On March 3rd, a size D2.5 avalanche was triggered nearby on Tatie Peak on a northeast aspect at 6,800ft. Fortunately, no one was caught in these avalanches. Another avalanche likely occurred on the late January weak layer with cornice fall around March 8-9th on McGregor Mtn, NNE, 7000ft. Observers reported other deep crowns in the Hairpin Valley and near Azurite Pass after a widespread avalanche cycle on 2/28-3/1. This is evidence that the late January facets could produce more avalanches under the right conditions.

Travel conditions may be challenging, especially at lower elevations where you'll find plenty of firm avalanche debris and firm crusts following the 2/28-3/1 cycle. The rain formed a crust up to 5,500- 6,500ft depending on the location in the zone.  

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