Can someone explain this? Snowpack question

[yesIsaidyes]

This is the Nov. 16 report for the Southern Chugach:

How ever much new snow is present, the importance lies in the warming temperature and very weak underlying snowpack that has formed over the last month. This weather system will bring warm weather and the chance for large amounts of precip. The freeze line will determine whether it falls as rain or snow. One model I looked at this morning has the rain line rising to 3000 feet. This could be good for our overall winter snowpack. It wouldn't hurt to put some warmth and moisture into our very dry and weak snowpack. Natural and human triggered avalanches will be likely were large amounts of snow or rain are precipitating. If the rain line does rise in elevation, remember that rain on new snow will increase the chance for avalanche activity quickly.

I don't get it - is it better that some rain is going to put warmth and moisture into the snowpack thereby making it stable (it was full of surface hoar and depth hoar before) or will extra rain increase the slide potential? This report seems to contradict itself...

[bcrider]

yeah, not the clearest forecast.

I think you‘re on the right track though. Sounds like the current snowpack is weak due to being unusually dry (maybe due to cold, dry weather following the last new snow?).

The rain will add some needed moisture to the snowpack which could help stabilize and bond it but if too much rain falls then it could make the conditions really bad. Above the snowline (he's not exactly sure where it will be) the conditions could also be bad based on how much new snow does or doesn’t fall on the weak snowpack and how much the rain and warm weather help or doesn’t help with bonding.

I think that‘s what he’s saying but I‘m not an avalanche professional.

[Trackhead]

^^^agree.

Rain will make things unstable in the short term. But, with cooling, it will likely add stability over the long term to a shallow, depth hoar ridden snowpack, if it percolates into the snowpack and doesn't just make an ice crust. In which case you will potentially have faceting below the crust in the future, as it acts as a vapor barrier.

[Toby]

Yep. I'd say:

(a) warm weather will help stabilize the faceted snow.
(b) significant rain (like the report says is possible) will either:
(i) help stabilize the facets or
(ii) cause slides on dangerous terrain, hopefully wiping out the facets.

The dodgy area would be above the rain line. Heavy, wet snow on top of facets :eek:.

[Agent 00X]

Interesting situation. Good responses posted. The recent previous advisories were a good read too. It is good they accumulate and archive them. You might email (or phone in) questions to the advisory author, email at bottom, in addition. I hope they would like to hear from you, it may help them post even better advisories too.

[Avmapper]

I was out last weekend at both Hatcher and Turnagain Pass. Conditons are funky and dangerous. In the Talkeetna's 2 ft of heavy wet wind loading snow deposited on top of the 1 to 2 feet of old snow with surface hoar and rounded grains at the botton. Avalanches were wide spread and April Bowl went Wall to Wall and to the ground. In Chugach the snow got a bunch a rain on a it and then it froze. The skiing is pretty rough. There is a layer of old snow at 35cm that had surfacehoar on it as well and that will linger until things really shake out. Generally high risk avy conditions. Not good for any high angle skiing or riding. Be careful if you are out there.

[yesIsaidyes]

Hey Doug,

Yeah, I was at Hatcher on Sunday - watched the first big slab kick off at April Bowl. There were a ton of people up there skiing after the slope ripped but I skipped it for some lower angle stuff. Observed loud whumping all over the place...its a shame, the snow was so light.

[MSulkers]

yesIsaidyes wrote:


I don't get it - is it better that some rain is going to put warmth and moisture into the snowpack thereby making it stable (it was full of surface hoar and depth hoar before) or will extra rain increase the slide potential? This report seems to contradict itself...

I suspect what is causing trouble with this report is that the forecaster is presuming that the reader will already know why rain would eventually stabilize the snowpack and understand the process by which this would occur.

Warmth and moisture into the snowpack will eventually increase its likelihood of stabilizing, as it will cause the crystals to metamorphosize and promote rounding or "connections" between the crystals. While this is happening, the snowpack will actually become less stable initially because of the added weight added to the top of the snowpack. So, in the short term, the slide potential will be increased, but in the long term, this process should result in a more stable snowpack.

The last line of the report notes that freezing levels will jump up and down during the storm event. When rain begins to fall on top of fresh snow, the snowpack immediately trends towards greater instability. Take a look at this data from the CAA. Notice the frequency of slide events when temperatures have changed just a little from the previous day.

http://static.flickr.com/34/66905844_b38dc31d01.jpg

[powderdave]

MSulkers, thanks for the good info. The CAA data is interesting, but I'm wondering if the reason most accidents occur at small changes in temperature is due to a much higher frequency of small changes versus large changes.

My logic: if small changes predominate (say, 90%), it would follow that most accidents would occur on those days.

What do you think?