Sawtooth & Western Smoky Mtns

Large avalanches are unlikely.  As the day warms up, the threat of wet snow avalanches will increase on very steep slopes that face the sun. Be alert to changing snow conditions and seek out colder snow or low angle slopes as the surface snow gets wet.

TRANSITION

We find ourselves in the middle of the seasonal transition from a dry winter snowpack to a consolidated spring snowpack. On upper and middle elevation slopes that don't directly face the sun, the snowpack is still cold and dry. At lower elevations and on all slopes that face the sun, the warm temperatures and direct sunshine of the past two weeks have begun to gradually push melt-water down into the snowpack. When the snowpack is freezing solidly at night it tends to behave in a predictable, diurnal pattern: the surface snow melts during the day and then refreezes at night. When this is happening, avalanches are usually restricted to surficial wet loose avalanches. However, when ambient air temperatures stay above freezing overnight and thick cloud cover prevents the snow from radiating out the energy it absorbed during the day, this re-freezing is much weaker. Last night was somewhere in the grey area in between those two ends of the spectrum. 

Periods of transition are awkward and unpredictable times for the snowpack. This is especially true when dealing with a snowpack that has buried persistent weak layers in it, as ours does. The overlap of the seasonal transition and our buried weak layers produces one of the most unpredictable types of avalanches: wet slabs. I'm writing about wet slabs, not because I think they are going to be happening everywhere today, but because they are likely to be a problem at some point in the near future and it is good for us to be thinking about them. We've received reports of a few small wet slabs and one large, destructive wet slab (information here) thus far this season. 

In many ways, wet slab avalanches are similar to other types of slab avalanches (wind slabs and persistent slabs). A cohesive mass of snow (a slab) detaches from the snow below it, producing an avalanche. However, there is a key difference with wet slabs. Instead of failing due to overloading, as is generally the case with dry slabs, they fail as a result of a decrease in strength at the failure plane of the avalanche. This decrease in strength is caused by liquid water moving through the snowpack. When overnight temperatures are cold and the snowpack is able to freeze solidly, there isn't much liquid water in the snowpack to worry about. But when warm temperatures at night prevent a solid freeze over the span of several days, significant amounts of melt-water can accumulate in the snowpack. Based on remote weather stations, the water in the snowpack probably did not completely refreeze last night. If this pattern continues over the next few days there is a good chance that we will see Avalanche Danger increasing due to wet slab concerns. If you are out in the mountains today and you are finding a snowpack that is deeply saturated with water you will be well served the think about the possibility of wet slab avalanches and give yourself a wide berth around the type of terrain where they will be most likely. What sort of terrain is that? Steep, rocky slopes that directly face the sun where the snowpack is thin.

You can catch up on what happened last week in Ethan's Weekend Update.