From NWAC - This is worth noting

It looks as though there are big rains in the Cascades near future.
I have always been curious when I read that the weight of the rains will load the snowpack and create natural avalanche cycles. Is there a time estimate that it takes for water to percolate through a dense fresher snowpack? I notice that it takes a great deal of rain water for the snow on my garage to become completely percolated before it wets the metal roof before it creeps off the roof. The fresh or even long standing snow slows the percolation process.
What is the significant weight difference if it is just snow compared to rain? It all weighs the same when distributed on the snow doesn't it?
I do know that the chance of saturation on cornices causing them to break and suddenly load a slope can cause a slide and wet rollers that fall off trees and build as they roll can stress a slope, but just the weight of rain that has not percolated to a PWL is that possible too?

Is there a time estimate that it takes for water to percolate through a dense fresher snowpack?

Not that I know of, but percolation would be more rapid the lower the density of the snow layers. Water often tends to flow as an aquifer either along hard layers that can't be easily penetrated or through weak layers that have a lot of porosity. What I have observed in snowpits (from  meltwater colums and their locations) is that water can enter the snowpack through weaknesses, such as along the trunks of trees or along rocks. Penetration of meltwater along rock bands was likely a factor in differential crustal faceting that developed at the base of the snowpack in the starting zone of the tragic Strathcona Tweedsmuir accident off Mt. Cheops in Glacier National Park in 2003.

What is the significant weight difference if it is just snow compared to rain? It all weighs the same when distributed on the snow doesn't it?

That's right, Joe. The Avalanche Handbook has quite a bit on this. It is all loading.

I do know that the chance of saturation on cornices causing them to break and suddenly load a slope can cause a slide and wet rollers that fall off trees and build as they roll can stress a slope, but just the weight of rain that has not percolated to a PWL is that possible too?

Yes, either melting or just solar warming can cause a redistribution of stress by causing near surface layers to become more plastic (to flow or creep).

But to start avalanches when snow changes to rain, it may happen very quickly, perhaps because of this plasticity, but maybe just because a snowball rolls down a slope or wet snow is released from a tree or rockband. This then can initiate an avalanche which could step down to a deeper slab if there is weakness at depth. Twit Conway, working with the Department of highways, said in a presentation at an ISSW that avalanches often initiate just as soon as snow changes to rain. So percolation isn't the only factor.

Testing....1....2...3...4....

Here comes that load of rain/snow:

From Garth at NWAC: THE 12 UTC 29 MARCH GFS40 KM SHOWS OVER 9 INCHES OF RAIN IN THE NORTH TO CENTRAL CASCADES FOR THE 48 HOURS ENDING THURSDAY NIGHT. WARNING ISSUED FOR EXTREME AVALANCHE DANGER

That's 90" of new snow at 6500' to 7000'! If the Cascades were only higher....

The glaciers are going to be very happy.

Thanks for the response Gary. It gives me more to chew on. I find it interesting the evolution of a snowpack from a layered Winter pile to a homogeneous spring pack of rounded grains.  Your comments keep me thinking about water being a persistent aggressor and will penetrate into any easement it is given. Essentially, while the snowpack will absorb some of the rains, until the snow pack can absorb water, a lot is channeled on the surface of the colder pack to trees, rocks. Then it keeps making it's gravitational paths down.

What I find intriguing is the physical nature of the snow to have plasticity and tension.
Is information on that also in the Avalanche Handbook?

I have been on Spring melt-freeze snowpacks that have had a whoophing or settling effect when weighted (for me it was unsettling). Large areas would suddenly collapse and settle. I investigated the snow and the upper supportable melt freeze layer was suspended 2-3 inches above what appeared to be a settled well rounded snow pack. Is it possible for the snow to settle that much overnight under a melt freeze layer?

Maybe it's just me, but see anything wrong with this picture?? Taken this past saturday.

What I find intriguing is the physical nature of the snow to have plasticity and tension.
Is information on that also in the Avalanche Handbook?

You may find this link pretty interesting: www.ucalgary.ca/asarc/

Jamieson's group is pretty much the most preeminent research group regarding snowpacks and avalanches. Look for information on "fracture toughness".
 

I have been on Spring melt-freeze snowpacks that have had a whoophing or settling effect when weighted (for me it was unsettling). Large areas would suddenly collapse and settle. I investigated the snow and the upper supportable melt freeze layer was suspended 2-3 inches above what appeared to be a settled well rounded snow pack. Is it possible for the snow to settle that much overnight under a melt freeze layer?

My obsevations tell me pretty much the same thing. I usually find that situation when the snow underneath is pretty sloppy. The crusts are strong when formed and are stiff, resisting settling. But then with warming the snowpack continues to settle. The crusts resist that settling because of remaining stiffness. Those crusts scare me when the snow underneath is weak and sloppy. It seems to me that the collapse of the crust may be all it takes for the sloppy, wet snow to begin to slide as a slab. I call those thumper crusts because they kind of thump and collapse in stages.

I'll bet Stimbuck could add some information about this as he works with highways in the spring as they open the passes.