Bollettino valanghe

Bollettino valanghe

Payette Avalanche Center
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Pericolo valanghe

Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
3 · Marcato
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In sintesi

Extreme winds will be building dense slabs 1-2 thick across the higher terrain today. 60 MPH S/SW winds will create dangerous avalanche conditions as well as difficult visibility and the potential for downed trees. Wind slabs will be building atop persistent weak layers, adding the potential to trigger a deeper and more destructive persistent slab avalanche. Avalanche conditions will become dangerous across multiple aspects and elevations today.

Today's avalanche forecast is sponsored by Brundage Mountain Resort

Problemi valanghivi

  • Wind Slabs

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

    Extreme S/SW winds are forecasted today into the evening. Gusts could exceed 60 MPH across mountainous terrain. Winds were  already at work overnight drifting snow into dense slabs. There was moderate amounts of snow available for transport yesterday. 60 MPH winds can transport lots of snow over large distances. Expect to find wind slabs 1-2 feet thick in the mountains today. Wind slabs will be deepest and most reactive as you gain elevation (above 7000'), especially on N, NE, and E-facing terrain. Intense wind events like this can build thick wind slabs in unusual places, such as lower elevations and much farther down the slope than expected.

    Local terrain features can significantly alter prevailing wind patterns. Be on the lookout for fresh wind slab development across multiple aspects today, even as you travel down from ridgetops. Gullies, rollovers, and other micro-terrain features can become wind-loaded farther down the slope. Watch for obvious signs of instability like shooting cracks. Pay attention to clues like drifts, pillows, and fresh cornices. If you feel dense snow overlying weaker, softer snow, you've found a wind slab.

    (2/29/2024)

    This hourly weather forecast graph for Granite Mtn., shows extreme wind gusts exceeding 60 MPH today. You'll also notice precipiation potential, sky cover, and relative humidity near 100% for nearly 48 hours. That means snow and lots of it.

    Photo: Kevin Studley

  • Persistent Slabs

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

    Weak layers formed at snow surfaces during last weekend's dry spell and were buried last Sunday. Since then, we've found melt-freeze crusts, facets, surface hoar, and graupel lurking underneath 1-2 feet of recent snowfall this week. Avalanche activity spiked Monday as new snow overloaded these weak layers. This week, we consistently observed ECTP results in test pits. That indicates weak layers showing the potential to produce avalanches. The video to the right highlights buried surface hoar resting atop a layer of graupel found on West Mountain on Monday. 

    Different weak layers have been found across multiple aspects and all of our elevation bands. Some (such as buried surface hoar) are much more sensitive than others (crusts buried deeper down). New snow will be further overloading weak layers today, creating a thicker slab. Persistent slab problems are notoriously difficult to manage. It's very difficult to know where they are and where they are not. The only sure way to avoid persistent slab avalanches is to avoid avalanche terrain (slopes steeper than 30 degrees).

Avalanche Discussion

EXTREME WIND AND PERSISTENT SLABS

Monday, both George and I found touchy avalanche conditions in two different areas of our forecast zone. You can read both of those observations here. Since then, we've received more snowfall with wind and warming temperatures. The new snow above these weak layers has become dense and formed a slab.

So far, we've found buried surface hoar resting atop a layer of graupel on east-facing aspects between 7000' and 7600'. Yesterday, we found weak snow above a melt-freeze about a foot down on an east-facing slope around 6600'. At 7600' on a wind-loaded NE aspect, this crust/interface was about 2 feet down. On a south aspect at 7600', this recent crust was about a foot down and was failing within the easy-moderate range. This south-facing pit showed the potential for propagation within the 02/01 crust complex two feet down. Although much more difficult to trigger, the weak snow adjacent to older crusts should be on your mind as we move into a major storm cycle.

What does all of this mean?

  • Persistent weak layers have been found across multiple aspects and elevations. Persistent slabs are much trickier to navigate than storm or wind slabs. Persistent slabs fail on persistent grains (i.e. surface hoar) and can propagate across much larger distances.
  • We're expected to see a signifcant loading event with 1-2 feet of new snow by Saturday. This will add much more weight and stress to buried weak layers. 
  • Extreme winds will be building dense wind slabs over the next few days. As you venture out in the mountains over the next 48 hours, not only will you have to contend with dangerous avalanche conditions, but other hazards like downed trees will come into play today.
  • Wind slabs will be building atop persistent weak layers, forming an even larger slab atop weak snow (strong over weak = not good!). 
  • Avalanche conditions will becoming increasingly dangerous throughout the day and into the weekend. Avalanches will become larger in size and more sensitive to triggering, especially on buried weak layers.
  • Developing a travel plan that avoids avalanche terrain would be a wise decision over the next few days.

Don't miss the monthly State of the Snowpack Talk this Friday, March 1st from 6-8 PM at Broken Horn Brewery

Want to help improve the avalanche forecast and enter your name to win an awesome raffle prize?

You can do both by letting us know what you're seeing and submitting an observation. Submitting an observation using our new observation platform is quick and easy to use. Every observation submission gets entered into the FPAC monthly raffle and you could win some great prizes courtesy of the FPAC. Observations (such as these recent ones) can greatly improve information for avalanche forecasts.