Urban Avalanche Advisory
Current Advisory as of
January 23, 2019
Expires 7:00 AM the following morning.
Issued By Tom Mattice
Primary Avalanche Problem
With the 7" of new that came in over the weekend with winds to 30mph from the SE we have shallow windslabs built on lee slopes.
This new snow came to rest on a hard icy bed surface. Look to see how that bonding has increased over time. This will give you a good stability indicator.
With light precipitation today and warming this weakness will remain present a little longer.
I am sure stability is far increased over Mondays conditions yet possibility of avalanches remain in these areas.
These are forecast to be small in nature and not a hazard for the urban environment.
As loading increases tonight into tomorrow with both precipitation and light winds this danger level will remain and slightly increase as temperatures go above freezing tomorrow afternoon.Learn more about Wind Slab.
Secondary Avalanche Problem
Today I would say the danger of a persistent slab event is almost non-existent. You could say todays secondary problem will be wet loose avalanches at lower elevations during the warming and precipitation today. But these will be so small almost to not be of concern.
My test results on Douglas Island over the weekend and into the week were quite mixed.
After the big foot of snow two weeks back we had an inversion with a period of rains to summit levels. This created tremendous settlement and in most places I found drainage channels built into the snowpack which greatly helps to build strength over time.
After this first big snow we had a period of cold dry weather for several days during which I said he weakness from the storm had passed and the slab in place had bonded and healed well.
That slab had previously been sitting on multiple weak layers consisting of surface and near surface faceted crystals that were quite weak. After the rain and warming the upper slab was so stiff it had isolated the weak layer for the most part and these drainage channels drive vertical colums in the snow (so to speak) and create great strength.
When we went into last weekend snowfall event I predicted that we would see mostly surface activity if any at all. Shallow wind slabs. But I said we had a secondary concern of this deeper weak layer becoming reactive again. The snowfall amounts were light but once again we went above freezing during a period of loading.
Over last weekend I dug 6 snowpits along the Ridge on Douglas Island. In 4 of my 6 pits I saw very stable snowpacks. The drainage channels had been established and my weak layers appeared to be healing and gaining strength. The temperature regime was consistent with no real temperature gradient. But in 2 of my 6 pits at mid mountain elevations I dug where the snowpack was perhaps the deepest. Around 150-160cm in each site. In neither of these locations did I see deep drainage channels established where I thought they would be. Test scores were very weak with CT 12 and ECTP16-18.
My only thought about why I saw drainage channels and stronger snowpacks on the higher reaches of our mountains remains in the inversion. I wonder if the inversions warming allowed for more rains on summit and deeper drainages to be established. Cooler mid mountain temps and lighter precips may have limited establishing drainage channels especially in the deeper snowpack.
That was my head scratcher for the week.
I assume with these light loading rates and slow warming today that this deeper weak layer is very much stable in place. As temps increase tomorrow (to above freezing on summit again, and precip rates also increase) it will be interesting to see if I can again find any weakness in the snowpack deeper layers again.
I am not overly concerned with lots of avalanches on these deeper weak layers. As I said I found most of our mountain regions had healed significantly. But there appears to be weakness present in the snowpack in places my concern will be increased loading, warming, or stress.
Let me know what your seeing out there if your digging at Tom.Mattice@juneau.orgLearn more about Persistent Slab.
Today’s Avalanche Tip
Digging a snowpit:
Dig your pit quickly in a representative area for your test. Don’t waste time, but also keep your pit wall where you will do your test vertical and smooth.
How deep to dig a snowpit:
Since it’s difficult for humans to trigger avalanches more than about 1.5 meters (5 feet) thick, (unless they are triggered from a shallower spot) you seldom need to dig snowpits deeper unless you specifically know there’s a deeper weak-layer that may cause problems. If you already know that the deep layers have no worries, then just concentrate on the shallow snow. Each situation is a little different and in time you will get a feel for it. But in general, keep your snowpits less than 1.5 to 2 meters deep unless you know of a good reason to go deeper.
Where to dig a snowpit:
Where to dig a snowpit is probably more important than how to dig one. Choosing a representative location is an art, and art is difficult to describe.
Dig it on a slope most representative of the slope you are interested in but without putting yourself in danger. Often you can find a small representative test-slope–one that won’t kill you if it does slide. Never dive into the middle of a dangerous avalanche path without first gathering lots of additional data about the stability of the slope.
Don’t dig it along ridgelines where the wind has affected the snow–a common mistake. Although sometimes the crown face of an avalanche may break right up to the ridge, the place where we most often trigger avalanches is 100 or more feet (30 meters) down off the ridge. Avoid thick trees because conditions are often quite different than on open slopes. Avoid places where people have compacted the snow.
LOOK FOR NEUTRAL, OPEN AREAS AT MID SLOPE WITHOUT WIND EFFECTS.
Use an avalanche probe to find a representative place with average depth. Poking around with a probe can save a lot of time digging in stupid places, like on top of a rock or tree or where a previous party had their lunch. Most important, dig lots of snowpits in lots of different areas because the snow can vary quite a bit from place to place. Look for the pattern of instability.
Extended column test:
Extended column tests are becoming the standard stability test for folks in the backcountry. You isolate a block 90 cm wide by 30 cm deep and tap on one side using the same loading taps as the compression test (see below). Look for how many taps it takes to fracture the block. More importantly, note whether the fracture propagated across the entire block or not. Any fractures that propagate across the entire block are a red flag, no matter how hard you have to tap. If you don’t have other information that strongly suggests the snow is stable, avoid slopes with conditions where ECTs are propagating.
Isolate a small column (30 x 30 cm). Then take the blade of the shovel and lay it flat on top. Finally start tapping progressively harder on the shovel blade until the column fails. Start with ten taps by articulating from your wrist, then ten more taps by articulating from your elbow, then ten more from your shoulder using the full weight of your arm. Don’t push your arm into the snow, but let it fall with its own weight. Easy taps are bad and hard taps are good. However, even with hard taps we strongly urge you to also do an ECT to see if it fully propagates.