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Location Name: Forecaster Observation - Marion Lake Observation date: Thursday, December 20, 2018 - 17:15	Is this an Avalanche Observation: No

Observation made by: Forecaster

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Quick Observation

Calm winds until mid-day then increasing winds from SW 5-10 mph with occasional gusts to 20mph.
New soft, shallow wind slabs forming at ridgelines on northerly aspects. Not reactive. WInds not sustained enough to build widespread or large slabs.
On southerly slopes, found a widespread weak structure. 1F slab over F hard facets above November crust. The slab overlying facets was brittle and did not propagate in an ECT.
The 12/9 interface did not propagate in an ECT

Snowpack, Avalanche, Weather Images:

Travel Details

Region:

Flathead Range - Middle Fork Corridor

Route Description:

Start from Essex, up Marion Lake summer trail, then gained ridgeline to 7100ft on S-SE aspects. Looked into N siide but didn't travel there.

Activity:

Skiing

Snowpack Details

Snowpack and Weather Details:

Terrain

Elevation of observation:

3500-5000 ft

5000-6500 ft

Above 6500 ft

Aspect(s) of observation:

Red Flags:

Blowing snow

Persistent Weak Layers:

Facets or Faceted Crust

Buried

New Snow in the past 24 hours:

0.10in.

Total Snow Depth:

70 cm

More comments about the snowpack and weather:

Broken skies midday but otherwise overcast with temps below freezing throughout day. Winds calm until mid-day then SW winds increased to 5-10mph with G 20 mph. Not sustained at or below 7100 feet. S-SE aspects (6,200-7,000 ft) weak structure to ground. Brittle 1-finger slab above Fist-hard, fine-grained facet layer and distinct crust. This structure could be troublesome if loaded with a stiff slab. The one old crown we checked out looked to have broken on these facets. ECTN 22 at top of facets. The 12/9 interface was not reactive (ECTN 13) North aspects showed soft shallow winds slabs forming along the ridgeline that were not reactive.

Blowing Snow:

Moderate

Wind Speed:

Moderate (Small trees sway)

Wind Direction:

Southwest

Air temperature:

Below Freezing

Sky Cover:

Mostly Cloudy (BRK)

Avalanche Details

Avalanche Details:

Date and Time of Avalanche:

Thursday, December 20, 2018 - 17:15

Avalanche Type:

Unknown

Failure Plane/Weak Layer:

Old snow

More information or comments about the avalanche:

Old crown - possibly from 12/11 cycle. ~15 cm new/ drifted snow on bed. Appeared to fail in soft facets above crust. Be ~15-20 cm above ground.

Trigger

Trigger:

natural

Terrain

Start Zone Slope Angle:

Aspect:

Southeast

Starting Elevation:

7150 feet

Size

Destructive Size:

D1.5

Relative Size:

R2 Small

Crown Height:

1 ft

Avalanche Width (Average width):

50ft.

Avalanche Location:

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