Bulletin d’avalanche

Mt Hood

Apr 12 – 13, 2019Northwest Avalanche Center
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Danger d’avalanche

Sun
Alpin
3 · Marqué
Limite forestière
2 · Modéré
Sous la limite
2 · Modéré
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A fresh round of wind and snow arriving the latter half of Saturday and continuing over the weekend will maintain the low-likelihood/high consequence potential for destructive and unsurvivable avalanches that release from above treeline and run far distances. Even at much lower elevations, continue to avoid areas where very large avalanches can run, start or stop. 

Problèmes avalancheux

  • Wind Slabs

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1–2

    Continue to avoid all areas where avalanches can start, stop, and run from above treeline terrain. Strong winds and heavy snowfall over the past week have produced very large and powerful avalanches. These avalanches released from the upper slopes of Mt. Hood and traveled down through all elevation bands. Although you are unlikely to trigger one of these monstrous and deep wind slabs, increasing winds and precipitation later in the day on Saturday will begin to load and stress the same paths that have been a concern during this cycle.

    Understand your exposure to very large avalanche paths (like the major canyons) with start zones located much higher on the mountain. Slab avalanches that start in higher terrain can entrain wet snow at lower elevations and become even more powerful and destructive. From a safe location, take a look around 5000-6000' in Clark or Newton drainage and you should see large debris piles from the recent slides. 

    You can also trigger relatively smaller and more recently formed wind slabs in the upper snowpack on steep lee slopes above treeline Saturday. 

  • Loose Wet

    • Alpin
    • Limite forestière
    • Sous la limite forestière
    Probabilité
    Possible
    Taille
    1

    The potential for wet loose avalanches is less certain on Saturday. If clouds and winds increase rapidly during the morning, the likelihood of triggering a wet loose avalanche will decrease significantly.  If the sun comes out for an extended period Saturday morning, all bets are off.

    It's spring-time which means you will need to monitor snow conditions and look for pinwheels and rollerballs as clear signs the potential for wet loose avalanches is rising. If you see natural wet loose activity or find wet snow deeper than your ankle, make the easy call to stick to lower angled terrain.

    Wet snow avalanches may be large where they gouge down and entrain more recent snow. At lower elevations with less recent snowfall, wet loose avalanches will be smaller but can still be powerful.

Discussion

Snow and Avalanche Discussion

NWAC Spring 2019 Forecast Schedule: Daily avalanche forecasts will end Sunday, April 14th. Look for regional Avalanche and Snowpack Summaries every Friday at noon through May 24th.  Avalanche warnings will be issued as needed throughout the Spring if unusually dangerous avalanche conditions develop.

Very large avalanches have released off the slopes of Mt. Hood over the last week during this epic spring storm cycle; both the Mt. Hood and Timberline weather stations have seen about 11 inches of water in the last 9 days! 

While it's true that the likelihood of these deep wind slabs releasing has decreased with no new natural or artillery triggered avalanches recorded since Wednesday, a new round of wind and snow will begin Saturday afternoon and continue through the weekend. This will load or in some cases reload the same paths capable of producing very large avalanches. Avoid lingering near the recent avalanche debris during this cycle

NWAC pro-observer/Meadows patroller Nick Asher reported more recently formed and sensitive wind slabs released on lee slopes during control work Friday morning above treeline. Powerful wet loose avalanches were easily ski triggered below treeline as conditions warmed. 

Photographs from the recent cycle:

Banana Hammock, Clark Drainage, 04/10/19. HS-AA-D3-R3: Aspect ESE, 7400 ft. starting zone, ran an unknown Distance. Crown Height estimated to be 2 X 10 ft with step downs. Photo Credit: J.P. Bevilaqua

 

Super Bowl Slide, Clark Canyon, 04/09/19, HS-AA-D4-R4, ENE Aspect, 8800 ft. starting zone. Ran a total length of 14,157Ft., total Vertical of 3600 ft. Unknown Crown Size. Photo Credit: J.P. Bevilaqua

 

Basalts Slide, Heather Drainage, 04/09/19, HS-AA-D3.5-R4, E Aspect, 7500 ft staring zone.  Ran a total length of 5100Ft., total Vertical of 2000 ft. Unknown Crown Size. Photo Credit: J.P. Bevilaqua

Snowpack Discussion

April 9th, 2019

Spring Conditions

The snowpack and weather have shifted solidly to spring-like conditions. A major warm-up started in mid-March with a prolonged period of strong sun and warm temperatures. This created a major difference between the snowpack on sunny slopes and that on shaded aspects. More recently, warm, wet, and sometimes weak spring storms have brought more rain than snow. The bulk of the precipitation with these storms focused on the southern forecast zones. Even so, mid-elevation rain established a dramatic snow line (about 4-4,500ft) below which the snowpack is minimal to non-existent in most zones. Going into the second week in April, intense snow and wind drove a prolonged period of High danger at Mt Hood.

A crown of a very large avalanche (D3+) above Mt. Hood Meadows resort. 04/08/2019. Photo Credit: Peter Moore.

Challenging Weather Forecasts

The Cascades have been experiencing unsettled spring weather with rain to many low and mid-elevation slopes and snow at upper elevations. Spring weather forecasts in the Cascades are notoriously challenging. With these storms, the weather models have been inconsistent and the accuracy has been limited to 12-24 hours, at best. A trend has been significant precipitation amounts for the Mount Hood area and other south-central Cascade volcanoes. 

Very bare southeast aspects of Rock Mtn/Nason Ridge. April 2nd. Photo: Josh Hirshberg

Shrinking Snowpack

From the peak height of snow in mid to late February through early April, mountain weather stations in the 4,000-5,000ft range showed an average of 27% decrease in height of snow. The percentage decrease ranged from 22-29%. This year's spring snowmelt is much earlier than normal. If you’re traveling in the mountains, the loss of snow coverage is most noticeable on southerly, sun-exposed slopes and below 4,000ft. On northerly aspects and slopes above 5,500ft, the snowpack has seen less dramatic changes and has even maintained some dry layers. On upper elevation shaded slopes there’s still potential for large wet slab avalanches with prolonged warm temperatures or high elevation rain events.

A natural loose wet avalanche (D1), Lichtenberg Mtn, N, 4,850ft. 4/7/2019. Photo: Will Govus

Spring avalanche considerations

As you head into the mountains there are a few questions to ask yourself common to spring avalanche conditions:

  1. Can you trigger avalanches due to new snow?

    1. If so, would they be storm slabs or wind slabs? And where?

  2. Can you trigger avalanches due to warming or rain?

    1. Will recent snow be warmed enough to result in loose wet avalanches?

    2. Will these avalanches be predictable point releases or more destructive wet slabs or gouging loose wet avalanches?

    3. What are the recent high and low temperatures and the forecasted temperatures during the time you’ll be in the mountains?

    4. How is the cloud cover contributing to the melting or freezing of surface snow? Did clear skies allow for a sufficient overnight freeze? Will the sun be strong enough to weaken surface layers?

Debris from a natural loose wet avalanche (D2), Lichtenberg Mtn, SW, 5,000ft. 4/7/2019. Photo: Josh Hirshberg

Other Considerations

In addition to daily avalanche hazard, the early snowmelt has created other travel considerations. Some roads and lower elevation slopes may not have enough continuous snow coverage for travel on snow machines. Holes melted around rocks, trees, and creeks could create a fall hazard. When nighttime temperatures and cloud cover allow for surface freezes, bring appropriate equipment to mitigate slip and fall hazard on steep slopes.

The last daily avalanche forecast for all zones will be issued for April 14th. Statewide mountain weather forecast and weekly avalanche condition advisories will continue through May. The weather station data is available year round. Keep checking the advisories and help us out by submitting observations when you are in the mountains.

Glide avalanches and holes opening up in rocky terrain on an east aspect of Mount Herman. 4/3/19 Photo: Andrew Kiefer

Régions couvertes