Good morning backcountry travelers this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Wednesday February 10th at 7 am. This will serve as a general backcountry avalanche advisory issued for Turnagain Arm with Turnagain Pass as the core advisory area (this advisory does not apply to highways, railroads, or operating ski areas).
There is still a major storm over the Aluetians that will affect the snowpack today.
-The winds have backed off at all ridgetop weather stations and are currently mostly moderate with average wind speeds between 16-19mph. In the past 24 hours, however, Sunburst had light to extreme winds averaging 13-56mph with an extreme max gust of 86mph.
-The radar show Prince William Sound full of moderate precip currently moving northwest towards us.
-In the last 24 hours (4am-4am), the snotel sites recorded .7 inches of water at Turnagain Pass, .6 inches at Grandview, and .1 at Summit Creek. Snowfall totals range 1-5 inches of new snow, but those totals might be a bit off due to strong winds. I’d guess that 8-10” of new snow is a better estimate for Turnagain Pass. Yesterday afternoon, however, we only measured 3 inches new snow at the Eddies Parking Lot to 2 inches new snow at Johnson Pass trailhead (see snow history at bottome of page).
-Temps have increased at all wx stations by 6-10 degrees F compared to yesterday. Temps range from 36 degrees F at sea-level and 23 degrees F at 3800′.
Due to some uncertainty, Today’s avalanche danger for the Turnagain Pass area will remain at CONSIDERABLE. That means dangerous avalanche conditions still exist. If you want to stay alive in the backcountry you have to play by the rules, and the rule book says that 90% of avalanches occur during storms and within 24 hours of a storm. We are still in that window. Why not stay away from steep slopes today and put yourself back into the 10% range after the snowpack has some time to adjust?
The biggest danger for a human triggered avalanche today will be a new wind slabs on top of the thin breakable melt-freeze layer that has surface hoar on top of it on steep rollovers greater than 35 degrees. That layer was already buried a foot deep, and will now have heavy wind slabs on top of it. This weak layer has been showing up in stability tests with clean fast shears as of Monday 2/8/2010. Since it had surface hoar on top of a crust in several locations, we have to give this type of weak layer lots of respect. Yesterday’s storm did not bring very much new snow, but the wind has been transporting snow and making wind slabs since Monday afternoon. We always have multiple layers of buried surface hoar in our snowpack. A lot of times they don’t do much of anything, but this layer has my attention. This is the type of weak layer you hear old-timers talking about. It is easy to forget about this type of weak layer, especially when the snowpack was showing very good stability in spots on Monday (see “backcountry artillery” in the forecasters video or on YouTube). Sometimes these big wind events rip out all the land mines naturally during the storm, but since we don’t have any data yet, we should take it easy out there.
There are two main weak layers in our snowpack.
1.A thin breakable melt-freeze crust with surface hoar on top of it. We have seen this weak layer up to 3000′ on multiple aspects on both sides of highway at Turnagain Pass. As of Monday 2/8/2010, this weak layer was buried about 1-1 ½ feet deep. Of course it will be buried deeper in areas where wind slabs formed. We were getting moderate failures with clean fast shears on this layer along Seattle Ridge on Monday 2/8/2010, but this weak layer was not reactive to human-triggers on Monday (see forecasters video link). This layer will become more dangerous with 2-3 feet of snow on top of it. This is the same weak layer that was responsible for the skier-triggered avalanches on Tincan on Saturday 2/6/2010. Plus I suspect this was the same weak layer that failed creating a natural avalanche on Widowmaker on Saturday 2/6/2010 as well (see photo gallery). This particular weak layer will be the likely culprit for human-triggered wind slabs today.
2.The Jan 7 rain crust is generally buried 3-5 feet deep and it qualifies as a deep slab instability. As of 2/8/2010, it was still showing moderate failures with clean smooth shears. This weak layer is still showing significant signs of instability, but the question is what kind of trigger could create an avalanche on this weak layer? It appears to require a large trigger because the last storm that hit us Friday night 2/5/2010 created medium sized natural avalanches in places like Widowmaker and Lipps, and these natural avalanches did not appear to step down to this rain crust. This weak layer is widespread on all aspects between 2000-3000 feet. Maybe a bigger storm will start to trigger this deep instability???
WEATHER FORECAST (National Weather Service)
WESTERN PRINCE WILLIAM SOUND-
500 AM AKST WED FEB 10 2010
.TODAY…RAIN WITH SOME SNOW OVER HIGHER ELEVATIONS. NO SNOW
ACCUMULATION. HIGHS IN THE MID 30S TO LOWER 40S. EAST WIND TO 15 MPH.
THROUGH PORTAGE VALLEY AND TURNAGAIN ARM…EAST WIND 25 TO 40 MPH
DIMINISHING TO 10 TO 20 MPH IN THE AFTERNOON.
.TONIGHT…RAIN IN THE EVENING…THEN SCATTERED RAIN AND SNOW SHOWERS
AFTER MIDNIGHT. LOWS IN THE UPPER 20S TO MID 30S. EAST WIND TO 15
.THURSDAY…MOSTLY CLOUDY WITH ISOLATED RAIN SHOWERS IN THE
MORNING…THEN PARTLY CLOUDY IN THE AFTERNOON. HIGHS IN THE 30S.
NORTHEAST WIND TO 15 MPH.
Temperature / Precipitation
SEWARD 39 32 39 / 100 100 20
GIRDWOOD 45 28 34 / 100 100 0
Short Term Weather Model Forecasts (NAM, WRF, GFS) for the Kenai Mountains near Turnagain Pass
Sea-level: temps are forecasted between 28-45 with up to 1.0” of water today
3000′: temps are forecasted between 14-23 degrees F with winds 10-20 mph
6000′: temps are forecasted between 14-23 degrees F with winds 30 mph
Thanks for checking today’s avalanche advisory. The next one will be posted tomorrow Thursday February 11th.