Snowpack Summary published on March 26, 2018 @ 8:38 am
Issued by Ryan Lewthwaite -

bottom line:

As we near the end of another avalanche cycle this month we will see a change in the weather pattern. Winds that dominated from the Southwest will shift in contrast & originate from the North & Northeast. This will allow the development of soft slabs on South-Southwest aspects. During this month parts of the Sierra have gotten over 200" of snow, rapidly increasing our annual snow totals to near average amounts. This week spring will infiltrate our area & give way to Loose Wet & Cornice fall avalanche problems for the afternoon.

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.

When will the winds stop (I've run out of windscreen)? All day long huge plumes & banners of snow have burdened the skyline. The transport of nearly all dry snow has allowed drifted accumulations to exceed 300cm (118") on N-E aspects. Today we will see a wind shift from SW to N & NE as our area gains high pressure. The wind slabs are most tender in shallow areas around convex features & are triggering with the weight of a skier. Shifting winds will redistribute loads & potentially create cornices on S-W facing slopes that will be effected by afternoon sun & melting.

Avalanche Character 2: Loose Wet
Loose Wet avalanches occur when water is running through the snowpack, and release at or below the trigger point. Avoid very steep slopes and terrain traps such as cliffs, gullies, or tree wells. Exit avalanche terrain when you see pinwheels, roller balls, a slushy surface, or during rain-on-snow events.

Clouds are giving way to abundant sunshine for the upcoming week. Melting & sublimating will deteriorate our snowpack & likely could result in the loosening of surface snow. If winds broadcast snow loads to opposing aspects that receive intense sun, expect this activity to rise. With this scenario look for morning icy conditions, some radiation/recrystallization & afternoonish small loose wet sluffs originating from warm objects.

Avalanche Character 3: 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.

With the possibility of redistributed snow to South aspects, cornices could form on ridges that get more intense sun. Previously we have seen this hazard & it correlates to rapid melting of the unsupported snow collections until failure. Look for linear cracks or icicles on cornices pointing towards instability.

Snowpack Discussion

It has been difficult to escape the winds lately. Moderate & strong winds from the SW have transported dry snow consistently for over three days to NE aspects. Some areas on leeward slopes near treeline have a snow depth of over 300cm, full probe length! Today the winds will transition from SW to NE as the high pressure ridge develops. The snowpack is refreezing at night, but expect significant melting during daylight hours. The Deep slab issue is lessening in reactivity but don't yet rule it out of contention. With the wind shift & the warming trend expect wind slabs & cornices to develop on S-W facing slopes with Loose wet avalanches possible.

recent observations

After an exciting March where snowpack totals were on the rise & avalanche activity was dramatic, we will swing warm & more spring-like. The wind slabs have been sensitive lately with a small skier triggered slide yesterday near Sonora Pass. After the Wed-Thu deluge it was hard to spot every avalanche that occurred, but they were numerous & quite large. A dormant Deep Slab issue came to fruition with the exponential accumulations of wind, rain, & snow. Observations report that the Deep slab issue on NE aspects created R3.5D3 avalanches up to 13 feet deep at the largest crowns on the Leavitt Cirque.

CURRENT CONDITIONS Weather observations from Sonora Pass
0600 temperature: 14 deg. F.
Max. temperature in the last 24 hours: 29 deg. F.
Average wind direction during the last 24 hours: Southwest
Average wind speed during the last 24 hours: mph
Maximum wind gust in the last 24 hours: mph
New snowfall in the last 24 hours: 0 inches
Total snow depth: 75 inches

The cold period that we've been experiencing will subside & high pressure will invade our region for the upcoming week. We will trend warmer beginning tomorrow with temperatures in the high 30's & 40's. The moderate to strong winds will continue but will shift from SW to N & NE. Sun will dominate the sky as our spring days lengthen. Cold clear nights will help refreeze the afternoon slush, keeping our winters longevity.

Two-Day Mountain Weather Forecast Produced in partnership with the Reno NWS
For 8000 ft. to 10000 ft.
  Monday Monday Night Tuesday
Weather: Early morning patchy cloudy, mostly sunny later. Clear Sunny
Temperatures: 27 to 33 deg. F. 15 to 20 deg. F. 39 to 45 deg. F.
Wind direction: North North North
Wind speed: 10 to 15 mph; gusts to 30 mph 10 to 20 mph; gusts to 40 mph 10 to 15 mph; gusts to 40 mph
Expected snowfall: 0 in. 0 in. 0 in.
For above 10000 ft.
  Monday Monday Night Tuesday
Weather: Early morning patchy cloudy, mostly sunny later. Clear Sunny
Temperatures: 20 to 25 deg. F. 12 to 17 deg. F. 33 to 38 deg. F.
Wind direction: Northeast North Northeast
Wind speed: 20 to 35 mph; gusts to 45 mph 25 to 40 mph; gusts to 60 mph 20 to 35 mph; gusts to 55 mph
Expected snowfall: 0 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.