Avalanche Advisory Archive 2016 – 2018
Date Issued: | 2017-01-04 07:04:27 |
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Danger: | 1 |
Primary Trend: | 2 |
Primary Probability: | 5 |
Primary Likelihood: | 1 |
Primary Size: | 2 |
Primary Description: | We have another layer from the Mid December rain crust that remains weak in areas. This is higher up in the snowpack and is not present everywhere. But snowpit tests have shown weakness on this layer as well that is also quite deep in the snowpack. This layer appears to be healing some over time and is far less spatially variable now than it was ten days ago. |
Secondary Trend: | 1 |
Secondary Probability: | 4 |
Secondary Likelihood: | 1 |
Secondary Size: | 2 |
Secondary Description: | We have another layer from the Mid December rain crust that remains weak in areas. This is higher up in the snowpack and is not present everywhere. But snowpit tests have shown weakness on this layer as well that is also quite deep in the snowpack. This layer appears to be healing some over time and is far less spatially variable now than it was ten days ago. |
Discussion: | The National Weather Service Forecasts- Today- Partly cloudy. Areas of freezing fog with visibility one quarter mile or less at times in the morning. Highs around 28. Light winds. Tonight- Partly cloudy. Areas of freezing fog. Lows 15 to 25. Light winds. Thursday- Mostly cloudy. Highs around 31. North wind 10 mph. Winds remain quite calm around the region again this morning with the Tram and Eaglecrest both only showing 5-7mph winds. Temperatures remain cool and near freezing with Eaglcrest touching 0c yesterday for a bit but mostly hovering around 29. The tram is seeing more of an inversion with warmer temps in the upper 30's to 40's. On the lower elevations temps remain lower and the snowpack remains strong. Both Natural and Human Triggered avalanches are not likely today. The forecast calls for slight cooling over the next 24 hours. Winds to remain calm throughout the day and picking up some tonight... and no precipitation. Danger will remain low. These near freezing temps continue to help the snowpack gain strength. Most of the weak layers in the snowpack are fairly strong. |
Tip: | Taken from the FS National Avalanche Center. Depth Hoar Large-grained, faceted, cup-shaped crystals near the ground. Depth hoar forms because of large temperature gradients within the snowpack. Depth Hoar--faceted snow near the ground: Depth Hoar Summary: Looks like: Feels like: Smells like: Also called: Formed: Mechanical Properties: Distribution Pattern: Forecasting considerations: Best Stability tests: Routefinding Considerations: We normally think of depth hoar as an early season phenomenon. It begins to form after the first snowfall as soon as temperatures get cold or more important, when the skies clear. As with surface hoar, radiation plays an extremely important role in snow surface temperature (Remember the discussion of radiation in the Weather chapter); in most climates, it plays a more important role than air temperature. Therefore, in mid latitudes, depth hoar grows primarily on the shady aspects, northwest, north, northeast and often east-facing slopes. Often in mid winter when the snow is deep enough that depth hoar quits growing, we find depth hoar on the slopes with thin snow such as west and south facing slope and near ridgelines where the wind has thinned the snowpack. In high latitudes such as Alaska and northern Canada, as well as in equatorial latitudes, if depth hoar grows, it tends to do so on all aspects nearly equally. More than most other weak layers, the strength of depth hoar varies quite dramatically from one location to another, depending mostly on the depth of the snowpack. Remember: thin snowpack means a weak snowpack. Thicker snowpacks insulate the cold air from the warm ground, have a small temperature gradient and thus a stronger snowpack. For this reason, you usually don't find bad depth hoar under the thick layers of wind loaded snow near the ridgetops. It's usually much weaker at mid-slope and especially near the bottom of basins, where thin snowpacks combine with cold air pooling, and around rock outcroppings. Depth Hoar and Climate: At the other end of the spectrum, in maritime climates, depth hoar usually forms only in the early season and quickly disappears after the first couple snowstorms bury it. In very warm maritime climates you can go several years without even seeing it. In between these two extremes, in intermountain climates, depth hoar forms in the early season during most years and depth hoar instabilities commonly last until December or January, and is not much of problem after that. In bad depth hoar years, large wet slabs may release in release in spring when melt water saturates the old depth hoar layers. A hard wind slab on top of depth hoar is double trouble. It's like laying a pane of glass on top of a stack of champagne glasses. It bridges a person's weight out over a larger area allowing them to walk on eggshells without breaking them until they either give it a hard thump, reach a place where the slab is thinner, or where the depth hoar is weaker, and then the whole slope shatters catastrophically. Fractures involving hard slabs commonly form above the victim, leaving very little chance for escape. Wind slabs on depth hoar exist throughout most of the season in continental climates, and when you add large populations to the equation it also means large numbers of fatalities. It's easy to see why Colorado leads the nation in avalanche fatalities.Forecasting Concerns: Large-grained depth hoar persists longer than any other kind of weak-layer. And as long as it does, you just tiptoe around and accumulate gray hairs. Usually the larger the grain size, the more persistent the instability. The time-honored adage among experienced avalanche professionals is: \"Never trust a depth hoar snowpack.\" In other words, it's always guilty until proven innocent. Carefully watch each loading event all winter--especially the big ones. Then even after you think you've seen the last of it, percolating melt water in the spring will re-activate the depth hoar layer and produce large, wet slab avalanches. Yikes! The best stability tests for depth hoar listed roughly in the order of reliability: explosive tests, cornice drops, Rutschblock tests, compression tests (do lots of them in representative places), jump on test slopes and pay attention to recent avalanche activity. Weather isn't quite as reliable unless it's really obvious weather like a heavy loading event or rapid warming of a thin slab overlying depth hoar. If you can't use active tests, use a thermometer and carefully measure the temperature gradient across the weakest layers. As soon as the temperature gradient drops below the critical level (about 1 degree centigrade per 10 centimeters) then it is gaining strength. But remember that depth hoar is quick to form but takes a long time to gain strength after the temperature gradient is removed. With no additional loading and with a weak-layer of -5 deg C or warmer, it takes several days to a week to stabilize. With cold weak layers and a lightweight overlying layer, it can take much longer.Routefinding Considerations: At mid latitudes, people especially get in trouble in the early season with the first slab that forms on top of depth hoar. The sun melts the snow away on the sunny slopes so it forces people onto the shady slopes where the depth hoar lives and they take lots of ride in avalanches. It's common to trigger depth hoar avalanches from long distances away. Many people, including a co-worker of mine, have been killed when they triggered the slope from the bottom, in this case, from a nearly quarter mile away. It's also easy to trigger a steep slope by standing on a flat slope above or to the side. Sometimes the steep slope below will pull a hard slab off a flat ridge above, like when a child yanks on the tablecloth hanging over the edge and dumps dinner onto the floor. Victims usually trigger depth hoar avalanches from shallow (thus weaker) snowpack areas, like a wind blown ridge, a rock outcrop in the middle of a slope, or a spur ridge next to a steep slope. Most of the time we think of a rock outcrop in the middle of a steep slope to be an \"island of safety\" but in a depth hoar snowpack you should think of it as a trigger point. |
Forecaster: | Tom Mattice |