Old, stubborn, wind-deposited slabs that lie on top of weak sugary snow will be the primary avalanche concern through Wednesday. Normal caution is advised during backcountry travel but that does not mean there is no danger. Watch for unstable snow on isolated terrain features. Lingering, smooth slabs can be found on the down wind side of ridges and on steep leeward slopes at mid and upper elevations.
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Above Treeline
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Near Treeline
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Below Treeline
A weak storm system on the 21st deposited wind slabs at middle and upper elevations on the leeward sides of ridges, the sidewalls of chutes, and gullies. In some locations, especially near treeline, slabs developed atop a persistent weak layer of sugary facet snow. This layer is still present in the snowpack and remains the greatest concern. Observations have been few, so it’s important to do your own assessments and realize that stability can vary over even short distances.
Persistent high pressure and warm daytime temperatures have been edging snowline ever higher in the eastern Sierra. Coverage is thin and patchy below ~9,500’ around Mammoth and higher elsewhere. Early season conditions exist with many obstacles hiding just under the snow surface.
A weak storm system moved through the region Wednesday (12/21) with 1 to 4” of snow reported. Winds began as moderate from the Southwest and then veered to the Northeast as the system left our forecast zone. Sunday the wind increased from the Southwest and some limited snow transport was observed across ridges and in exposed areas. Test slopes near and above treeline showed recent wind slabs to be isolated and increasingly stubborn to trigger. However, temperature gradients in our shallow snowpack have been forming weak, sugary, facet snow in more sheltered locations. The combination of these old wind slabs and persistent weak layers may increase the potential for triggered release on previously wind loaded features near treeline.
As the weather has warmed across the mountains, surface conditions have become moist and sticky below ~10,000’. Treed slopes, rock outcrops, and hazy clouds will all increase surface warming at mid and lower elevations. Below ~9,500’, natural and triggered releases are unlikely due to little or no snow.
A persistent ridge of high pressure along the west coast will provide a dry northwest flow with periods of passing high clouds, light valley winds, and temperatures above seasonal averages. Warming will be more pronounced over the Sierra with only modest inversions developing. Winds may become breezy along the Sierra ridgetops. Wind gusts around 45 mph will be possible this evening through Thursday. There is some potential for weaker systems to move through the region during the first week of January.
| Tuesday | Tuesday Night | Wednesday | |
|---|---|---|---|
| Weather: | Sunny. | Clear. | Sunny. |
| Temperatures: | 44 to 50 deg. F. | 22 to 27 deg. F. | 43 to 49 deg. F. |
| Mid Slope Winds: | Light Winds. | W | W |
| Expected snowfall: | 0 in. | 0 in. | 0 in. |
| Tuesday | Tuesday Night | Wednesday | |
|---|---|---|---|
| Weather: | Partly cloudy then becoming sunny. | Clear. | Sunny. |
| Temperatures: | 37 to 43 deg. F. | 20 to 25 deg. F. | 37 to 42 deg. F. |
| Ridge Top Winds: | W | W | SW |
| Expected snowfall: | 0 in. | 0 in. | 0 in. |
