THIS SNOWPACK SUMMARY EXPIRED ON December 19, 2018 @ 8:01 am
Snowpack Summary published on December 17, 2018 @ 8:01 am
Issued by Ryan Lewthwaite -

bottom line:

A really quick blast of precipitation has given us +2" of new snow with moderate to strong SW winds. This new snow may have the ability to cause wind slab or cornice fall avalanches on aspects which face NW-N-E-SE. The surface conditions which the new snow has fallen on is firm & likely to help propigate wind slab avalanche failures.

Avalanche Character 1: Wind Slab
Wind Slab avalanches release naturally during wind events and can be triggered for up to a week after a wind event. They form in lee and cross-loaded terrain features. Avoid them by sticking to wind sheltered or wind scoured areas.

We have received +2" of new snow accumulation early this morning, with much wind. Dominating SWerly moderate to strong winds have moved old & new snow from the alpine to deposits on leeward aspects near & below treeline. The new snow sits atop a wind & sun crusty landscape & wind created slabs could be readily available for minor avalanching. Concern should be taken when riding across aspects that face (NW-N-E-SE) which are above 30 degrees in slope, as moving snow could knock you from your intended travel route, or cornices could release above you.

Avalanche Character 2: Cornice
Cornice Fall avalanches are caused by a release of overhanging, wind drifted snow. Cornices form on lee and cross-loaded ridges, sub-ridges, and sharp convexities. They are easiest to trigger during periods of rapid growth from wind drifting, rapid warming, or during rain-on-snow events. Cornices may break farther back onto flatter areas than expected.

As seen in an avalanche observation, strong ridgetop winds have developed large cornices on lee aspects which have broken off. These collections of wind-driven snow are unpredictable, unsupported & have a mass large enough to crush humans & bury their equipment. Look above you when traveling in avalanche terrain!

Snowpack Discussion

Winds have been dynamically altering the snowpack since the first arrival of snow. Sun & a duration of prolonged dryness has left us with challenging riding conditions, but there are still pockets of soft if you know where to look. Lastnights storm will leave us with less than 4" of new snow on the wind battered & sun crusted landscape. Any accumulations can be found below treeline in low-lying areas as wind slab deposits. Be cautious as wind slabs that have formed on lee aspects (NW-N-E-SE) on steep slopes could release naturally & especially with the weight of a traveler. Wind slab avalanches will usually break above the snowmobiler as a larger condensed plate of snow.

recent observations

One cornice fall avalanche was discovered on a prominent wall above Hwy 108 on the Stanislaus NF, west of Sonora Pass (R1D2) above the travel corridor. Another, difficult to view wet-loose avalanche was seen from a considerable distance away, far above Finley Meadows. This incident was likely the result of Southern sun exposure to rocks, which loosened the snow grains in a point-release style wet avalanche.

CURRENT CONDITIONS Weather observations from Sonora Pass
0600 temperature: 31 deg. F.
Max. temperature in the last 24 hours: 40 deg. F.
Average wind direction during the last 24 hours: SW
Average wind speed during the last 24 hours: 40 mph
Maximum wind gust in the last 24 hours: 65 mph
New snowfall in the last 24 hours: 2 inches
Total snow depth: 30 inches

The winds have been at work stripping the landscape of any potentially loose old snow & redepositing it in low-lying areas. The diminished hopes for proper snow accumulations is unfolding with only 2" new at 6AM as this storm blows out to the East. Southwest winds have been moderate with strong gusts to 65 mph at ridgetops. Expect this new snow to be blown off areas above treeline & found in slabs of varying thickness near & below treeline on aspects NW-N-E-SE.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8000 ft. to 10000 ft.
  Monday Monday Night Tuesday
Weather: Mostly cloudy with decreasing clouds & snow showers in the morning. Partly cloudy Partly cloudy
Temperatures: 32 to 37 deg. F. 18 to 23 deg. F. 40 to 45 deg. F.
Wind direction: Southwest Light winds Light winds
Wind speed: 10 to 15 mph; Gusts to 60 mph decreasing to 40 mph in the afternoon Light winds Light winds with gusts of 60 mph decreasing to 45 mph in the afternoon
Expected snowfall: 2 to 4 in. 0 in. 0 in.
For above 10000 ft.
  Monday Monday Night Tuesday
Weather: Mostly cloudy with decreasing clouds & snow showers in the morning. Partly cloudy Partly cloudy
Temperatures: 27 to 32 deg. F. 16 to 21 deg. F. 36 to 41 deg. F.
Wind direction: Southwest Southwest West
Wind speed: 30 to 45 mph with gusts to 75 mph; decreasing to 20 to 30 mph with gusts to 50 mph in the afternoon 10 to 15 mph; gusts to 25 mph 15 to 25 mph with gusts to 40 mph; increasing to 20 to 35 mph with gusts to 60 mph in the afternoon
Expected snowfall: 2 to 4 in. 0 in. 0 in.

This snowpack summary applies only to backcountry areas in the Bridgeport Winter Recreation Area. Click here for a map of the area. This snowpack summary describes general avalanche conditions and local variations always occur. This snowpack summary expires in 48 hours unless otherwise noted. The information in this snowpack summary is provided by the USDA Forest Service who is solely responsible for its content.