Sunday, December 11, 2016

Summit Creek Glades - December 11, 2016

The best we can say about today is - It was a REALLY good Day. Nice cool tranquil weather, light winds, and fantastic soft snow conditions. Saturday denser snow surface was capped by 15-20 cms of low-density snow that made riding fast and FUN!
Grateful for having this wonderful resource so close to Boise.
Today felt like a yo-yo ski day. Instead of going for a grand tour we headed directly into the Summit Creek Glades. The two most popular glades were busy with other skiers. Besides, tracks from Saturday were still visible on those two ski runs as well. But Pilot Peak is full of great lines that are less obvious. Our plan was to ride on a pure north aspect, with steep wide corridors between trees.
Somewhere in Summit Creek Glades - Dead North slopes with steep pitches.
Saturday was relatively warm, with temps slightly above zero. The snowpack was loaded with an additional 87 mm of SWE (snow water equivalent). In effect, the snowpack more than doubled the total SWE for December 8, which was 76 mm. When a snowpack receives such a load in less than 48 hours, it should be expected the avalanche danger to augment. Multiple observers noticed on Saturday natural releases at steep slopes. Other folks experienced energetic whumpfs. When we arrived Sunday morning it was easy to find evidence of Saturday's instability cycle with a 24 hours old slab released across our parking spot. Overnight 15 to 20 cm of new light density snow was not sufficient to cover the instability cycle evidence.
Slab release at I-21, north of Mores Creek Summit. 
The coverage at Pilot Peak is improving rapidly, and today we were able to easily access backcountry terrain from parking pullouts and avoid using the Pilot Peak road. Elevations between 6000 and 7000 feet are now skiable with depths at the meter mark.
Skinning up Pilot Peak East Ridge -6400 feet of elevation
The forest fire at Summit Creek glades had minimal impact on the landscape. There are burnt trees, but the fire was spotty and it did not create new open areas for backcountry skiers with one exception, a glade below 7000 feet where the fire burn hot enough to leave match stick looking trees without limbs. To a great extent, the most visible impact is that the tree foliage is much reduced and visibility through the forest has greatly increased. It did not make sense that it seems that evergreen trees burn, but not  the ever-present brush. Weird!
Today's Playground - Summit Creek Glades
 As we skied in the glades, we noticed a number of slab release from the previous day on the same terrain and slope angles that we were skiing. The difference was that by Sunday the instability had decreased. The temps Saturday night and Sunday dropped to -6 Degrees C with light precipitation and calm winds. The liquid water resulting from above zero temperatures on Saturday had also percolated to the base of the snowpack and was retained in the basal facets (they were acting as a capillary barrier). And more importantly, the snowpack had adjusted to a large loading event.
Soft slab release at Summit Creek Glades
Quick pits are particularly important early in the season. It gives an insight into the development of the snowpack. Snow-pits are invaluable as they are repeated early the season since they allow us to identify changes the snowpack is experiencing in the most dynamic part of the winter.

The quick snow-pit we did, did not take time at all, and it told us as many things; water presence at the basal layer, 1-meter depth has been exceeded, the majority of the snowpack does not consist of facet crystals anymore, and layers are bonding. Quick pits are not only about stability testing, it is a LOT more important than that! It is about detecting the temporal changes the snowpack is going through! These observations improve your situation awareness. You are aware of what is happening under your boards.
Today's snow pit.
Snowpits are also valuable to test your snowpack development hypothesis. Your snowpack hypothesis is formulated earlier, perhaps during the planning of your outing. Our hypothesis was that the near surface weak layer identified last Monday had sintered, and the basal weak layer was still reactive. Very quickly we learned that indeed last Monday precipitation crystal weak layer had sintered and bonded, and that the basal layer was rendered ineffective by free water. Test results were unreactive at the basal layer.

We also gained new insights, from the snow-pit.  At a depth of 30-cm, a weak layer consisting of precipitation particles was responsible for moderate compression test results (CTM-Q2-Sudden Planar). This weak layer was also incapable of propagating a fracture at the location we inspected the snowpack because the slab lacked sufficient cohesion. The fracture propagation was arrested by a slab fracture. This information was key for selecting our terrain. We avoided areas where the snowpack is slabby, and rode on areas with cohesionless snow. EASY!

It is likely that the 30 cm instability will continue to decrease, the weak layer consisting of precipitation particles will bond to other layers rapidly, in contrast to persistent facet crystals that take longer to bond to other snowpack layers.

When the day was over, we skied out into the road along to the bottom of Summit Creek, and we were greeted by several snowpacks collapses (whumpfs) we triggered at the base of south aspect slopes. No surprise here, the snowpack topmost layer was dense and cohesive. A slab was present, and it was capable of sustaining propagation. It needed a steep slope to slide after the failure. But we were in low angle safe terrain.


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