CAIC

CAIC: Colorado Avalanche Information Center

BC Zone Observation Report

Tuesday, December 18, 2012 at 12:00 AM
Front Range

Details

Date: 2012/12/18

Observer: Spencer Logan
Organization: Forecaster, CAIC

Location

BC Zone: Front Range

Weather

Ridgeline Wind Speed: 10-20 mph
Ridgeline Wind Direction: SW
Wind Loading: Moderate
Temperature: 12 F
Sky Cover: Overcast
Depth of New Snow: 8 cm
Depth of Total Snow: 115 cm
Most Significant Precip Rate: S1 - 1 cm/hr

Snowpack

Snowpack Obsvervation #1

Cracking: Minor
Collapsing: Moderate
Persistent Weak Layer: Yes
Comments: Variable early season snowpack. Lots of deceptive shallow areas now covered by new snow. Thin, soft wind slab developing on the snow surface.
Facet and depth hoar layer at the ground is thin, for CO. Restricted to specific areas: shady NW-N-NE-E aspects. Very weak where it exists.
The discontinuous nature of the weak layer and slabs are limiting the size of avalanches, for now.

Avalanches

Date

Location/Path

#

Elev

Asp

Type

Trig

SizeR

SizeD

View

2012/12/18

Berthoud Pass west side

3

<TL

N

SS

AS

R1

D1

View

2012/12/18

Berthoud Pass west side

2

TL

E

SS

AS / r

R1

D1

View

2012/12/18

Berthoud Pass west side

4

TL

NE

SS

AS

R2

D1.5

View

2012/12/18

Berthoud Pass east side

1

<TL

NW

SS

AR

R2

D2

Media

Images

<b>Figure 1:</b> Crown and bed surface of upper avalanche on Big Roll. Trigger point was probably rocks and shallow snow (red arrow). The dotted line approximates the lower margin of the old hard slab (early Dec?). Avalanche failed in faceted snow at the ground, then in a layer of small facets above the old hard slab. Adjacent to the crown, test results were moderate (CT12Q2) breaking at the top of the hard slab. (<a href=javascript:void(0); onClick=win=window.open('https://caic-production.imgix.net/nion3rfm1xqigjhrgzg9htlwvw57?ixlib=php-3.1.0&s=0ca146f0ba755ff46f0ef70bb6891f79','caic_media','resizable=1,height=820,width=840,scrollbars=yes');win.focus();return false;>see full sized image</a>)

<b>Figure 2:</b> Triggered avalanches (D1.5) on Big Roll. Upper avalanche was triggered first. Another party triggered lower avalanche after descending the upper bed surface. Trigger point of the second avalanche was similar, a shallow rocky spot below and looker's left of the trees. (<a href=javascript:void(0); onClick=win=window.open('https://caic-production.imgix.net/m6uxwxnnjg4p8xttn5wh0ljfxerc?ixlib=php-3.1.0&s=0cf97811d6ecb139eab58c39979789f4','caic_media','resizable=1,height=820,width=840,scrollbars=yes');win.focus();return false;>see full sized image</a>)

<b>Figure 3:</b> Looking down larger of the R1D1s below treeline. Note crown zippering along small trees. Debris pile up 3-4 feet deep (<a href=javascript:void(0); onClick=win=window.open('https://caic-production.imgix.net/eexo1tr2h7y981oq91mb94inoutc?ixlib=php-3.1.0&s=69bc4069e148257ccd2cd6daa0e3294b','caic_media','resizable=1,height=820,width=840,scrollbars=yes');win.focus();return false;>see full sized image</a>)

<b>Figure 4:</b> Crown of R1D1 on steep roll below treeline. (<a href=javascript:void(0); onClick=win=window.open('https://caic-production.imgix.net/m5s2b51frdj6wqokfpc1wajeif5j?ixlib=php-3.1.0&s=ad659832150329e5beaac47886dac9c7','caic_media','resizable=1,height=820,width=840,scrollbars=yes');win.focus();return false;>see full sized image</a>)