"Thumper crusts" and wet slabs

The type of crust and upper snowpack structure out there is one that I like to call a "Thumper Crust", a surface crust common in springtime that breaks down with a thump and noticeable collapse. It is distinguished from rain crusts because there are usually no ice lenses and accordant drainage channels in the wet snow grains that underlie the crusts. They are also different from most mid-winter sun and freezing rain crusts because of the wet grains as opposed to dry snow grains beneath the crust.

How it works (I think): Recent surface snows that fall without much wind become wet before a thin and gradually thickening surface crust forms from cooler air and/or radiational cooling. In some cases the snow may(?) also become wet from long wave radiation gain beneath a radiationally cooled already existent surface crust. In any case the key factor is that the slowly thickening surface crust that forms in cooler periods lessens the settlement of the underlying wet snow during those same cool periods. At moderate elevations the thickness of the crust is minimized by the limited cooling and diurnal warm periods that prevail during the spring. The limiting of settlement doesn't seem important while the surface crust is in it's cold and frozen state but becomes a key factor during warmer periods as the crust begins to weaken. (Some faceting may also be taking place in the wet grains beneath the crust.) But, in any case, what is going on is that the upper crust and some transient grain to grain bonding of the wet grains is causing the underlying wet grains to settle less than they ordinarily would. This results in the storage of potential energy (that turns into kinetic energy when released) in the wet grains during cold periods.

When the surface crust begins to weaken it eventually may reach the point at which it may fail locally and cause the underlying wet grains to collapse suddenly. This can initiate slab failure particularly if there is a bad bed surface of a smooth ice lens or hard crust and more so if water from melting lubricates this substrate. (The wet grains in this case would be very wet). The slab could be initiated naturally or may fail when disturbed by a skier, causing the crust to fail locally and collapse.

This is kind of an interesting situation because ordinarily one would think of MF crusts as both providing some strength and also indicating that during the wet phase before crustal formation that significant settlement is likely to have occurred; but, in this case the key is that the crustal strength becomes minimized before failure and that the underlying structure is storing some potential energy. The crust and/or the very weak bonds in the underlying wet grains I believe are critical for providing the ability of the slab to propagate.



Anyway, I think it is interesting and that is what I think could be happening out there right now.

Looks like it thumped, and went big on Granite Mountain a day or two ago. Lots of debris filling the main gully to below 3000'.
There were fracture lines 1 foot high near the top of the bowl,(about 5500'). Other less steep areas closer to the south ridge haven't gone yet. I was surprised to see fracture lines, not point releases, but now it is explained.

This condition was widespread on the South facing slopes of East Peak today.
We toured up the summer trail to the bottom of Bullion Basin. There was evidence of a large fracture avalanche (not deep, but wide) maybe a day or two old. We had a discussion about the conditions and potentials. Evidence of the deeper (3-5") ski Pen made me thing there would be a good chance of wet hisser slides. We followed a set of possibly day old or early morning skin tracks that took extra care to avoid excessive exposure wrapping in an out of the trees, very impressive, but steep. I broke off the trail off near the ridge where I felt that we were in the clear. But there were signs of this Thumper Crust layer where whole sections when weighted would audibly make a SSSSHHH sound and I could feel the slightest drop in the snowpack. Some areas were smaller, 15x15 foot sections and others were the whole slope. Interesting enough that there was a crease, not a shooting fracture at the uppermost reach of the settled areas. In some cases the crease was zig zagging over 150 feet. Freaky!
Needless to say we stuck to the trees on our safe ski escape. Glad to be out of that noisy mess.

A couple years we were on a group tour where all of us are very conditioned to various snowpack conditions. We ran into similar collapsing snowpack conditions, yet the snow was older corn that had a MF supportable crust layer of 1-2 inches. This had the same effect, collapsing in large sections. We dug pits and analyzed the snow pack and there was a Crust layer that bridged the wet corn below leaving a 1-2" AIR gap between the Crust layer and the Wet Corn. We surmised that radiation cooling combined with a deep freeze in the low 20's caused the upper layer to set up and the underlain corn continued to settle leaving the air gap. No new snow was present, Maybe this happened over a couple MF cycles since the days before had similar radiational overnight cooling cycles. Maybe there was a cold fog that made a very short appearance that glued the upper layer?
Really left a unsettling feeling-
No pun...

I found similar above about 7000 feet near Cutthroat Pass today: the sound was less the whumphing I'm accustomed to in winter, more like the "fffffft" a dog makes when you blow on its ears (how would I know this?). For the first few times I wasn't sure what had happened, though my heart rate sped up a bit. Then I heard one which had a bit of a whistling sound to it, and I felt the slope settle. Gave me the creeps, it did.

As someone else had reported a day or two ago, there have been scattered wet slab releases on all sorts of interesting aspects, including in cross-loaded gullies. I also spotted some large new cornices which appeared to face south by southeast, which is not where I'm accustomed to seeing them. I couldn't discern a pattern, however: no cornices at all in some spots, giant overhangs in others nearby.

Sadly, skiing below 7000 feet featured bottomless, unconsolidated slop. I am not unskilled with this stuff, but today it seemed far more difficult than usual. It also felt like it should have slid easily, but as far as I could tell it did not. I didn't spend enough time actually upright and turning to do any real testing or slopecutting.

Just to add some geographic data points, this was very much in evidence on west facing slopes near Hurricane Ridge (Mt. Angeles at about 6,000 ft) on Sunday May 10. As noted above, anywhere from 10x10 foot areas to at least one several hundred square feet remotely triggered above us. We didn't notice any cracks, fissures, or propagation lines, but we could feel the settlement, and agree with the weird sound. Probing the snow found the top 4-6 inches considerably harder than the next several feet, but the crystal size was all rather large.

We also noted several large (500 vf or nearly full runout) wet slides on northeast facing slopes that may have went while we were there as well as one with a 20 ft wide x 2 feet deep crown that had slid probably Friday or Saturday.

When it's collapsing that way it's really hard to know what it wants to do but slabs are certainly a possibility. It may be in some cases that all that will happen is that the crust itself will propagate energy but in other cases it could provide the energy for slab release. It's a very interesting phenomena. It represents one of the few occasions when you have a surface crust that slabs are at all likely. I chose not to ski this past weekend but it sounds like so far only the topmost layer is going as a slab but there are several of these layers now buried from the various snowstorms and intervening warmer periods. As the crusts get buried more deeply it is progressively harder for the warming to get down to the deeper depths on a diurnal basis but there is a lot of unconsolidated snow in the upper part of the snowpack and on certain warmer aspects, especially at moderate elevations it seems unlikely these deeper crust layers could be very strong and so deeper failure has to be considered. Also, as Stimbuck observed at Chinook Pass shallower surface slabs are likely to entrain these weaker underlying wet snow layers so the volume can get big in a hurry.

Check out the current forecast:

WINTER WEATHER ADVISORY IN EFFECT UNTIL 5 PM PDT THIS AFTERNOON...

.TODAY...SHOWERS AND A CHANCE OF THUNDERSTORMS. SNOW LEVEL 2500
FEET. SNOW ACCUMULATION OF 3 TO 8 INCHES. AFTERNOON PASS
TEMPERATURES IN THE MID AND UPPER 30S. WEST WIND IN THE PASSES 10 TO
15 MPH.
.TONIGHT...MOSTLY CLOUDY. A CHANCE OF SHOWERS. SNOW LEVEL 3500 FEET.
WEST WIND IN THE PASSES 10 TO 15 MPH.
.WEDNESDAY...MOSTLY CLOUDY. A CHANCE OF RAIN AND SNOW IN THE
AFTERNOON. SNOW LEVEL 4000 FEET. AFTERNOON PASS TEMPERATURES IN THE
LOWER TO MID 40S. LIGHT WIND.
.WEDNESDAY NIGHT...RAIN AND SNOW. SNOW LEVEL 4500 FEET. EAST WIND
IN THE PASSES NEAR 10 MPH.
.THURSDAY...SHOWERS. SNOW LEVEL 4000 FEET. AFTERNOON PASS
TEMPERATURES IN THE MID 40S. WEST WIND IN THE PASSES NEAR 15 MPH.
.THURSDAY NIGHT...MOSTLY CLOUDY WITH A CHANCE OF SHOWERS. SNOW LEVEL
4000 FEET.
.FRIDAY...PARTLY SUNNY. FREEZING LEVEL 5500 FEET.
.FRIDAY NIGHT AND SATURDAY...PARTLY CLOUDY. FREEZING LEVEL
7500 FEET.
.SATURDAY NIGHT THROUGH MONDAY...MOSTLY CLEAR. FREEZING LEVEL 11000
TO 12000 FEET.

This is the first time that freezing levels will have gotten this high since early February so the possibility (likelihood) of deeper slabs has to be considered especially beginning Sunday and more so day by day after that until it cools once again. Maybe we can finally get a more typical period of consolidation and hopefully better skiing conditions some time thereafter.