Good morning backcountry travelers, this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Wednesday, December 24, 2008 at 7am. This will serve as a general backcountry avalanche advisory issued for the Turnagain Arm (Turnagain Pass is the core advisory area). Local variations always occur. Note: We issue advisories 5 days a week Wednesday-Sunday.
MOUNTAIN WEATHER ROUND UP
In the last 24 hours…
-The Center Ridge weather station at 1800 feet in Turnagain Pass-
No Data. This weather station is down; so, lets look at Grandview.
-The Grandview weather station at 1100 feet along the railroad tracks-
Recorded .1 inch of water and 1 inch of new snow. Current temp is 21 degrees F (3 degrees colder than yesterday)
-Sunburst weather station at 3800 feet in Turnagain Pass-
Recorded light winds averaging 3-10 mph from the northwest with a strong gust of 29 mph. Current temperature is 11 degrees F (5 degrees colder than yesterday)
-Surface Analysis Maps-
From 3 am Tuesday to 9pm last night…
Show that yesterday’s storm flew right over the top of us super fast. It is still building but its now well past us. CNFAIC Staff than that, there not really any big storms lined up for us right now.
The analysis from 9pm last night showed it flowing out of the Bering Sea right over the top of us in a SE direction. It is forecasted to keep flowing over the top of us for West to East.
As of 6:00 am this morning…shows clearing over the top of us, and a wall of moisture over the Aluetians.
The Middleton radar shows some very scattered precip in the Gulf of Alaska.
The Kenai radar shows very scattered precip over western Kenai Penninsula.
-General Weather Observations-
Compared to yesterday…
Temps are warmer at sea level by 1-5 degrees F, and colder by 3-8 degrees F at higher elevations and ridgetops. Winds have been light, but there are some strong gusts along Turnagain Arm this morning. Based on yesterday’s observations, about 1-2 inches of snow fell at Turnagain Pass.
PRIMARY AVALANCHE CONCERNS
-Buried Surface Hoar
-crust under 2500 feet
SECONDARY AVALANCHE CONCERNS
-Glide Cracks (see photo)
AVALANCHE AND SNOWPACK DISCUSSION
Lisa and I went up to Stock Run Bowl yesterday to dig some pits. We wanted to keep looking at those October facets. Plus, we wanted to look for crusts and that old buried surface hoar. We found some spatial variability, but our pits are pretty consistant with all of our CNFAIC Staff pits this winter (see photo gallery). Last week (12/16/08), I found the buried surface hoar on top a crust in Main Bowl. Yesterday, Lisa found a similar situation in Stock Run Bowl. Surface hoar on a crust is one of the worst combinations for avalanches; so, we need more information on this layer. We have found this weak layer about 3.5 feet deep. Stability tests from last week and yesterday show that it is difficult to extremely difficult to get that weak layer to fail, but when it does the entire column falls out of the pit. This crust seems to be under 2500 feet in elevation, which means it is probably widespread all the bowls along Seattle Ridge. The surface hoar on this crust is obvious to the naked eye, but it is showing signs of breaking down.
What does all this mean? Although our general snowpack is a consolidated slab on top of a weak layer of facets, our snowpit data keeps indicating good stability. There have been some human-triggered avalanches on isolated terrain features near rocks and near complex terrain like: ribs, gullies and steep rollovers. Those human-triggered avalanches did not propogate very wide and they stayed confined to those isolated terrain features.
Terrain management is the key to safety right now. You can still see the features of the mountains under a lot of slopes, indicating shallow snowpack. There are lots of steep rollovers and rocks out there that will be the most likely areas for a human-triggered avalanche.
Here are my avalanche speculations for today…
-Some unstable slabs will exist on isolated terrain features. Evaluate the snow and terrain carefully and use good travel habits.
-Shallow snow seems to be where most of the human-triggered avalanche activity is happening.
-The general snowpack gets more and more shallow the further south you drive; so, places like Cornbiscuit almost always have less snow than places like Tincan. Summit Lake is even thinner, colder, and weaker.
-There is a possibilty of triggering small to medium sized avalanches on isolated terrain features like: steep rollovers near rocks or cliffs, wind scoured ridges where the snow depth tapers from shallow to deep
It is still early season, and we have an early season intermountain snowpack. This is not the best time of year for big steep Alaskan lines. There will be opportunity for that later in the season. Avoid any sort of complex terrain, stick to simple planar slopes, and enjoy the great powder.
Future avalanche problems will be glide cracks (see photo gallery), which are very difficult to predict. Treat them like cornices and realize that they could avalanche at any time for wierd reasons; so, don’t travel on or underneath those things. The good news is that glide cracks are very easy to avoid because you can see them!
The next big problem will be the surface hoar and rimed surface snow that has been forming all over the place. It has been difficult to track this most recent layer. We have mostly seen it a mid elevations up to 2700 feet.
WATCH OUT SITUATIONS
-Rocky terrain with shallow snow connected to deeper snow (photo gallery)
-Steep rollovers (photo gallery)
-If you hear any “whumpfing” or feel any collapsing, then get to safe terrain immediately and call it a day.
WESTERN PRINCE WILLIAM SOUND-
500 AM AKST WED DEC 24 2008
.TODAY…SUNNY. HIGHS IN THE MID TEENS INLAND TO LOWER 30S ALONG
THE COAST. NORTH TO WEST WIND 10 TO 20 MPH. GUSTS TO 35 MPH NEAR
SEWARD AND WHITTIER IN THE MORNING.
.TONIGHT…INCREASING CLOUDS. LOWS ZERO TO 10 ABOVE INLAND TO
10 TO 20 ABOVE ALONG THE COAST. NORTH TO WEST WIND 15 MPH
.CHRISTMAS DAY…CLOUDY WITH A CHANCE OF SNOW IN THE MORNING…THEN
SNOW IN THE AFTERNOON. SNOW ACCUMULATION 1 TO 2 INCHES. HIGHS IN
THE 20S. LIGHT WINDS BECOMING EAST 10 TO 15 MPH IN THE AFTERNOON.
TEMPERATURE / PRECIPITATION
SEWARD 30 17 28 / 0 0 80
GIRDWOOD 26 14 26 / 0 0 60