Bollettino valanghe

Mt Hood

Northwest Avalanche Center
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Pericolo valanghe

Wed
Alta quota
3 · Marcato
Limite del bosco
3 · Marcato
Sotto il limite
2 · Moderato
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Non aggiornato — questo bollettino è scaduto 2 years ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Slabs will be large, widespread, easy to trigger, and difficult to manage as winds redistribute significant fresh snowfall. Note the signs pointing to actively or recently wind-loaded slopes and avoid steep wind-loaded slopes at middle and upper elevations. If storm slabs are reactive on test slopes, lower-angle slopes are your friend.

Problemi valanghivi

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1.5–2.5

    Although we cried wolf on Monday, we're more confident that won't be the case on Tuesday as a more significant storm slams into Mt Hood. Steep open slopes in wind-exposed terrain will be dangerous on Tuesday. Slabs will be widespread and easy to trigger at middle and upper elevations and could be large (up to size 2.5) and difficult to survive. Seek wind-sheltered and lower-angle terrain where you will find the best riding conditions. Shooting cracks, whumphs, or punchy layering all serve as reminders of the large slabs you can trigger on a steep slope nearby. Active wind loading fresh sharp cornices both point to the problem while scoured slopes provide a safer exit option. Gauge the potential size of slabs by feeling for the 3/8 interface which may serve as a bed surface for larger slab activity. You can also trigger smaller slabs at lower elevations. Steer around wind-stiffened snow and areas where the snow has piled up more deeply above the crust in these areas.

    Shooting crack extending from your mode of transport highlights the instability.

    Photo: Joe Dellaporta

  • Storm Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1–2

    We're expecting around 1 ft of snow from Monday evening through Tuesday. This will create widespread storm instabilities on middle-elevation slopes with the potential for slabs to push into the large size range. Expect more short-lived instabilities at lower elevations where the temperatures will be milder. Use small inconsequential test slopes, skin track tests, or hand shears to assess instabilities as the snow piles up frequently and you move through the terrain. When in doubt, avoid open slopes steeper than 35 degrees.

    (2/26/2023)

    Example: A human triggered  storm slab avalanche.

Avalanche Discussion

NWAC will be closed on Tuesday, March 12th, as we gather and grieve following the death of one of our forecasters, Matt Primomo. We will not issue regular avalanche or mountain weather forecasts on Tuesday. We will return to normal operations on Wednesday, March 13th. We appreciate the community's understanding and support during this time.

Discussion:  

Storm cycle totals were in the 4-12" range prior to the final storm in this cycle. The precipitation was generally spaced out enough that the shallow slabs formed had time to heal. That will not be the case with the main event from Monday afternoon through Tuesday. Plan to step back and approach the terrain with caution.

There is a slight chance that some areas may have buried surface hoar on 3/8. Before the storm cycle, observers found surface hoar on middle and lower-elevation slopes with open canopies. Middle-elevations would be the most likely candidates for surface hoar preservation. Any avalanche failing on such an interface could break widely across terrain features. If you encounter warning signs such as whumphs or shooting cracks, dial back your terrain selection to slopes less than 30 degrees as avalanches.

Cornices are growing and sensitive. Tread cautiously as you approach a ridgeline's edge or ascend on the slopes below. Actively loading cornices may fail without warning, potentially triggering an avalanche on the slopes below.

The recent storm cycle follows a warm, warm spring-like weather late last week with freezing levels rising to 8000 ft, putting crusts up to these elevations on all but shaded aspects. Observers reported the crust to be supportable and around 0.75" thick on sun-exposed aspects at middle elevations. We expect that the 3/8 crusts exists at least at lower elevations and perhaps into middle elevations on shaded aspects.  Below the crust, the snow increases in density and is gradually rounding, giving us no cause for concern. Therefore, focus on the snow above the 3/8 interface as you travel for stability concerns, using hand pits and small slope tests to evaluate as you move into increasingly wind-exposed areas.

Photos in the gallery depict conditions prior to the storm.

Outlook (Day 2): 

Winds and snowfall continue into Tuesday night, with a transition to drier weather and increasingly sunny skies on Wednesday. Stability should improve slightly relative to Tuesday but remain dangerous at middle and upper elevations. Large wind slabs will remain reactive at middle and upper elevations and you can still trigger them on open slopes steeper than 35 degrees. Use swales and recently formed cornices to identify and avoid the problem. When in doubt, stick to ridgelines and scoured slopes. Storm slab instabilities should be improving, but wet loose activity will develop on sun-exposed slopes. In areas where the snow above the 3/8 crust is deeper, these could be large and gain significant steam. Stay out from under consequential terrain as the clouds begin to part. Since this forecast is an outlook, there is added uncertainty. Exercise additional caution and prepare to make your own snowpack assessments if you consider engaging with consequential terrain.

If you're out in the snow, share your observations with us and your local backcountry community.

Regioni coperte