Lots of Snow/Rain in Forecast, bad idea to snowshoe right afterwards?

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proxie
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Lots of Snow/Rain in Forecast, bad idea to snowshoe right afterwards?

Post by proxie » December 18th, 2019, 3:21 pm

I have some time off and was looking to hit up some snowshoes either around Bend or Mt. Hood area. Been researching and have some options. However, there is a lot of snow/rain in the upcoming forecast for the next 4-5 days.

This may be an obvious question, but assuming road conditions are doable with chains to get to the start of a trailhead/sno-park, is it a bad idea in general to tackle some snowshoe trips "right after" they get dumped on by rain/snow? I'm thinking that while the fresh dump of snow may make things a lot more beautiful, it is just way too high a risk for stuff like avalanches on steep terrain.

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Bosterson
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Re: Lots of Snow/Rain in Forecast, bad idea to snowshoe right afterwards?

Post by Bosterson » December 18th, 2019, 3:35 pm

It completely depends on what kind of terrain you'd be in. If you're in a mostly flat area, this wouldn't really matter, though the snow could be waterlogged and heavy and generally make snowshoeing more of a pain.
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jessbee
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Re: Lots of Snow/Rain in Forecast, bad idea to snowshoe right afterwards?

Post by jessbee » December 18th, 2019, 4:04 pm

At least bookmark these two sites:

https://www.nwac.us/

http://www.coavalanche.org/

If you're headed up a mountain or below steep, avalanche prone slopes, it's very important to stay on top of current avy conditions.

And if you don't want to deal with breaking trail in wet, heavy snow, wait til Sunday so the trails will have had at least a day of foot traffic packing down the trail :)
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BigBear
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Re: Lots of Snow/Rain in Forecast, bad idea to snowshoe right afterwards?

Post by BigBear » December 19th, 2019, 9:27 am

Location. location. location.

If you're planning to ascend up from Timberline, the avalanche danger would be extreme when a new layer of precipitation lands on existing snow. It takes 48 hours for the new layer "bond".

If your plan is a forested ridgeline like the PCT to Twin Lakes from either Barlow Pass or Wapinita Pass, you are going to be okay (with the exception of that exposed talus slope between the two lakes in the outflow canyon).

Slab avalanches on open slopes would be very likely in the early days of this storm. Snow bombs from high tree limbs very likely if the temperature warms to the freezing point or above.

pcg
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Re: Lots of Snow/Rain in Forecast, bad idea to snowshoe right afterwards?

Post by pcg » December 19th, 2019, 1:50 pm

BigBear wrote:
December 19th, 2019, 9:27 am
Location. location. location... the avalanche danger would be extreme when a new layer of precipitation lands on existing snow. It takes 48 hours for the new layer "bond".
I was up snow shoeing yesterday and notice a layer of hoar frost on the snow. Beautiful to behold but it makes for a weak bond to new snow. The weak layer from hoar frost can persist to weeks or more. So yes, stay away from avalanche terrain. A warm spell and a lot of rain (Pineapple Express) would wipe out that weak layer.

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Water
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Re: Lots of Snow/Rain in Forecast, bad idea to snowshoe right afterwards?

Post by Water » December 20th, 2019, 10:50 am

"48hrs for new layer to bond" -- I have not heard this as a hard rule but generally it can take 'some' time for a new layer of snow to bond to the old one, aka, for the snow pack to settle after a storm. But this hard rule is kind of...arbitrary. Depends on more factors than could be communicated here. You can get out the day after a big storm and be okay, or could find avy issues 5 days after a small snowfall, depending on the multitude of factors. That said generally speaking, all things equal, that our snowpack risk stabilizes quickly after a 'big storm' within a few days.


I am sure the warm temps and wind has absolutely knocked down any hoar frost. We've got a pineapple express right now with freezing levels going up to 6500~ so any hoar is going to be taken care of by the rain. Fortunately it is rare for us to get persistent weak layers in the NW, especially in Oregon, though it does happen from time to time.

The reason avy danger is high (in part) is because this latest storm put the snow down upside-down, starting cold and ending wet. This makes finer sharper snow at the bottom with bigger wetter, heavier snow on top of it. The storm will end right-side up, as the freezing levels are expected to fall as the precipitation tapers off at the end of the weekend. Most storms however start warm, end cold, this means the first flakes that fall are wet, big, and heavy which helps bond to whatever the existing layer is. If it's a fine/sharp hoar layer, those big wet flakes help knock it down.

Even with that detail it's a simplification and the snow experts could go for pages about snow crystal shapes, sizes, temperature gradients, bonding, etc.
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