Saturday-Tuesday

Issued: Sat, Dec 31, 2016 at 8AM

Expires: Tue, Jan 03, 2017

 

With strong post-storm, NE winds yesterday afternoon creating fuzzy ridgelines due to dramatic snow transport, mind the formation of thick and touchy wind slabs in exposed terrain.

The unpredictable nature of wind slabs can catch you off guard and fail larger than expected if you aren’t strategic in your terrain selection.

Also consider how the new loose snow is moving in the steeps and the possibilities of it to trigger deeper snow instabilities to ground.

 

Above 2,500ft Considerable

1,800 to 2,500ft Considerable

Below 1,800ft Moderate

Degrees of Avalanche Danger ?

1. Low
2. Moderate
3. Considerable
4. High
5. Extreme

Avalanche Danger Rose ?

Avalanche Problems ?

Problem Details

SATURDAY SUNDAY MONDAY & TUESDAY

DANGER SCALE

WIND SLAB:
Elevation: Above 1800′
Aspect: Various due to SE onshore and NE offshore winds
Terrain: Near ridgelines, gullies and rollovers
Sensitivity: Touchy
Distribution: Specific
Likelihood (Human Triggered): Likely
Size: Small to Medium
Danger Trend: Increasing w NE winds
Forecaster Confidence: Good

LOOSE SNOW:
Elevation: All
Aspect: All
Terrain: Greater than 30* and in runouts of wind loaded slopes above
Sensitivity: Responsive
Distribution: Widespread
Likelihood (Human Triggered): Possible
Size: Small to Medium
Danger Trend: Increasing w wind loading
Forecaster Confidence: Good

PERSISTENT SLAB:
Elevation: Mostly above 2500′
Aspect: All
Terrain: Steep rocky slopes where facets exist under old windslab, especially interior of Thompson Pass
Sensitivity: Stubborn
Distribution: Specific
Likelihood (Human Triggered): Possible
Size: Medium
Danger Trend: Increasing w/ snow transport onto slopes
Forecaster Confidence: Poor

AVALANCHE PROBLEM SCALE DESCRIPTORS:
Sensitivity: Non-reactive, Stubborn, Responsive, Touchy
Distribution: Isolated, Specific, Widespread
Likelihood: Unlikely, Possible, Likely, Nearly Certain
Size: Small, Medium, Large, Very Large (size scale <here>)
Danger Trend: Increasing, Steady, Decreasing
Forecaster Confidence: Good, Fair, Poor

LIST OF AVALANCHE PROBLEMS <here>

SNOWPACK DISCUSSION: 

Over the last 72 hours, two concentrated storms left us with 16 inches of new snow on Thompson Pass (12 in town). In the higher elevations near wind exposed ridges and passes, expect cleanly scoured faces in areas and deeply loaded lee aspects with slabs to 3 feet. Remember that the winds have been reversing direction dramatically from the SE with the storm to NE post. This directional switch leads to a variety of newly loaded slopes that can be suspect.

coastal-zone-iconMaritime (Coastal):
The maritime zone has built a more uniform snowpack than the other two areas within our forecast region due to more snow and warmer temperatures. Most recent avalanche activity has involved the upper layers of the snowpack and occurred in steep, rocky terrain. This doesn’t mean that you can completely write off the chance for deeper snow instabilities. We had a long period early season that created a weak, faceted snowpack that is now buried. Colder, higher elevations should be investigated. Below ~3000, it is capped by a surface crust.

intermountain-zone-iconInter-mountain (Transitional):
Above 3000′ and just interior of the Pass has stayed cooler; preserving early season facets at the ground and between layers of windslab. The weak interface between these deep facets and firmer layers above have been failing and propagating very easily in areas (see observation from 12/28). It is not out of the picture for heavy loads such as a snowmachine or multiple machines to trigger a steep, wind loaded north facing slope and have it step down to persistent weak layers, increasing the avalanche size and destructive potential.

Very easy failure at top of weak facets when isolating column up Nicks @ 4200'

Very easy failure at top of weak facets when isolating column up Nicks @ 4200′

Propagation supported up Nicks Valley @ 4200': PST 40/100 (end)

Propagation supported up Nicks Valley @ 4200′: PST 40/100 (end)

interior-zone-iconContinental (Interior):
More interior beyond Mile Post 46 and Billy Mitchell, the snowpack is very thin and weak. We are seeing avalanches often step down to ground similar to the Inter-mountain zone a month or more ago. This zone has not been able to establish a firm, bridging mid-pack slab similar to Thompson Pass, but will soon be on its way.

Find more photos and observations at the bottom of the page. Sharing your observations helps others make informed decisions.

Recent Avalanche Activity

Dec. 30 (late afternoon visibility provided some insight into how the new snow was sticking to the mountainsides):

coastal-zone-iconMaritime (Coastal):

  • Several natural Loose D2 avalanches running hundreds of feet in the maritime initiating in steep and rocky terrain: Mile High, W of West Peak, S side of Loveland
img_20161230_142630

Loose slide near West Peak & Airport

intermountain-zone-iconInter-mountain (Transitional):

  • 2x D2 Storm Slabs w at least 4ft crowns (fading light) pulling out on N face of Berlin Wall: stepping down to rocky ground in places and hitting the glacier.
  • Wind loaded south face above Mile Post 37 near 3 Pigs: D2 slab pulled out of steep and rocky rollover and immediately stepped down to ground (see photo)
img_20161230_152840

South facing slab release to ground near 3 Pigs (MP37)

  • South facing steep, rocky gully at Mile Post 42 and ran to aldered apron: likely slab initially due to volume.

interior-zone-iconContinental (Interior):

  • D2 Storm Slab pulled out to ground mid south slope of Mt Tiekel over Mile Post 46

Dec. 28:

  • Natural D2 slab avalanche on N face of Berlin Wall
  • Natural D1 slab avalanche on S facing roll-over below Little Girls
  • Snowmachine Triggered Avalanche below Catchers Mitt in photo below: photo credit: Eric Christensen

 

Recent Weather

WEATHER FORECAST for NEXT 24 HRS at 3,000 ft:
Temperature Forecast (Min/Max *F):  5 / 25
Ridgetop Wind Forecast (mph): ENE / 10-20
Snowfall (in):  0
WIND & TEMPERATURE
past 24 hours
Ferry Terminal Thompson Pass
Average Wind Speed (mph) / Direction 2 / NE 21 / Var
Max Wind Gust (mph) / Direction 14/ ENE 47 / NE
Temperature Min / Max (*F) 21 / 31 16 / 26

Weather Forecast: Clear skies and northerly outflow winds have returned and backed down since yesterday afternoon’s gusty period. Look for some clouds to move in tonight into Sunday morning until it breaks again Sunday and a long high and dry period welcomes us for the first week of the new year.

Additional Info & Media

SNOW HISTORY: Valdez 12/31 AM Thompson Pass 12/30 AM
24 Hour Snow / Water Equiv. 1” /0.09” 9″ /0.8″
Storm Snow /Water Equiv. (12/30-12/31) 5” /0.68″ 16″ /1.5″
Current Snow Depth 34″ 36″
December Snow / Water Equiv. 73.5″ /4.1″ 69″ / 6″
Total Winter Snowfall / Water Equiv. 95.9″ / 8.16” 140″ / 14.3″
Snowload in Valdez 23 lbs/sq. ft.

img_20161228_130954img_20161228_130730

Photos of our new Nicks Valley Weather Station in the shadows of Python and the Berlin Wall.

SNOWFALL for LAST 24 HRS at OTHER STATIONS:
Nicks Valley at 4200 ft (in): ?”
Upper Tsaina at 1750 ft (in): 1″
Sugarloaf at 550 ft (in): 1?”
SNOW DEPTH & WATER SURVEY (12/6/2016) Depth Snow Water Equivalent
Milepost 2.5 Valdez  13.2″  2.3″
Milepost 18 15.4″ 2.1″
Milepost 29 Worthington Flats 32.2″ 6.4″
Milepost 37 Tsaina River bridge 24.1″ 4.1″
This survey is done the first week of each month.

Weather Quicklinks:

  • NWS forecast for Northeast Prince William Sound <here>
  • NOAA NWS spot forecast for Thompson Pass <here>
  • Thompson Pass RWIS weather station <here>
  • MP 30 Nicks (Happy) Valley weather station at 4200 feet <here> (scroll to Nicks Valley)
  • Valdez Glacier UAF weather station at 6600 feet <data here> <map here>
  • Further weather resources <here>

SNOW CLIMATE ZONES:

  • coastal-zone-iconMaritime (Coastal) – from the Port of Valdez to Thompson Pass, all waters flowing into Valdez Arm and everything south of Marshall Pass.
  • intermountain-zone-iconInter-mountain (Transitional) – between Thompson Pass and Rendezvous Lodge.
  • interior-zone-iconContinental (Interior) – the dry north side of the Chugach (north of 46 Mile, including the Tonsina River).

Photo of Thompson Pass

 thompson-pass-ski-runs

Interactive Map of Valdez Forecast Areas w/ Many Resource Layers (Trevor Grams)

climate-zones-topoclimate-zones-satellitegoogle-earth1

Run Map of Thompson Pass Area (Sean Wisner) (2MB download)
VAC Run Map Thompson Pass

NEWS: Our region is “one of the snowiest places on earth” – Serendipity / Rendezvous snowfall record set in 1963 <here>.

Free smart phone avalanche forecasts at: http://www.avalancheforecasts.com/

Posted in VAC Forecasts.

Forecaster: Kevin Salys