Good morning backcountry travelers this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Wednesday April 7th, 2010 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).
Skookum Valley and Skookum Glacier are closed to motorized vehicles (snowmachines, helicopters, ATVs) except for subsistence uses.. This closure is directed in the current Chugach National Forest Land and Resource Management Plan. Placer River Drainage remains open for motorized use to Spencer Glacier.
-The winds have backed off this morning and are currently light averaging 1-4mph with some light gusts up to 11mph
-The 24 hour snotel sites are reporting:
-0.2 inches of water, 2-3 inches of new snow, and 7 inches of melting/settlement at 1800′ at Turnagain Pass
-0.1 inches of water and 7 inches of melting/settlement at 1100′ at Grandview
-0.0 inches of water and 0 inches of melting/settlement at 1400′ at Summit Creek.
-This morning’s (5am) radars show scattered precip over PWS.
-Temps have cooled off at all weather stations by 8-15 degrees compared to yesterday. Above freezing temps made it up to 32 degrees at 2600′ yesterday. This morning’s temperature range is 25 degrees F at sea-level and 12 degrees F at 3800′.
Today’s avalanche danger is MODERATE, but could rapidly increase to HIGH on any sunny slopes IF the sun comes out. Traveling on sunny slopes is NOT recommended today.
The top 1-3 feet of new snow on top of the most recent sun crust is the primary concern today. By definition, we have 2 out of 3 ingredients necessary for an avalanche:
-A slab (new storm snow)
-A bed surface (slippery sun crust)
The weak layer is the missing ingredient, but that sun crust is smooth enough to act as both a bed surface and a weak layer especially if the sun heats up the slab on top.
Storm totals at the parking lots were 35 inches at Eddies and 14 inches at the Johnson Pass North parking lot; so, there is quite a bit of variability in surface snow depth on top of the sun crust from the northern end to the southern end of Turnagain Pass.
Yesterday, the sun came out for about an hour and a half and almost immediately caused dozens of point release avalanches and small surface slab avalanches. The largest of these slabs was about 150′ wide in Portage Valley. We also saw at least one point release that entrained enough snow on Sunnyside that would have been big enough to bury, injure or kill a person. The small point releases can turn into large deep slabs if the sun is intense for a longer period of time. Since the sun was only out for a limited time yesterday, it did not run a full avalanche cycle on the new storm snow on the surface. The slopes are not pre-baked yet, expect instant point release avalanches if the sun comes out.
We took a closer look at the storm snow/sun crust interface yesterday on Tincan and Sunburst. We found hard failures with relatively clean but not completely smooth shear planes CTH29Q2/Q3@120cm on top of the most recent sun crust, and smoCNFAIC Staff shear planes on top of the next deepest sun crust. We did feel a small collapse on a 31 degree slope when the third person stepped on to that slope, but ski cuts on steep rollovers, and stomping around on steep test slopes produced no results CNFAIC Staff than some shooting cracks up to 30 feet long in the top 1-3 inches on of wind skin.
The sun is the critical “watch out” situation this time of year. April sun and temps can change the snowpack much more drastically than January or February sun. Avalanche danger can increase rapidly to HIGH during periods rapidly warming temps and direct sunlight especially when the sun hits fresh snow like we have right now. AVOID traveling on any sunny slopes IF the sun comes out and get out of avalanche terrain immediately if it does.
Get out of the backcountry early today if the sun comes out. The warmest part of the day has been between 12 noon and 3 pm according the air temps on the weather stations, but the direct sun triggered natural avalanches as early as 11 am last week.
Glide cracks are secondary concern today. These crevasse like features are those cracks that go all the way to the ground. Avoid traveling underneath these cracks. Glide cracks and full depth release avalanches have been occurring recently on warm sunny days.
WEATHER FORECAST (National Weather Service)
WESTERN PRINCE WILLIAM SOUND-
500 AM AKDT WED APR 7 2010
.TODAY…MOSTLY CLOUDY WITH SCATTERED SNOW SHOWERS. HIGHS IN THE
MID 30S TO LOWER 40S. LIGHT WINDS EXCEPT NORTH 10 MPH NEAR SEWARD.
.TONIGHT…PARTLY CLOUDY. LOWS IN THE MID TEENS TO LOWER 30S.
LIGHT WINDS EXCEPT NORTH 15 TO 20 MPH NEAR SEWARD.
.THURSDAY…PARTLY CLOUDY. HIGHS IN THE UPPER 30S TO MID 40S.
LIGHT WINDS EXCEPT WEST 15 TO 30 MPH NEAR WHITTIER.
.THURSDAY NIGHT…MOSTLY CLEAR. LOWS 15 TO 25. LIGHT WINDS EXCEPT
NORTH 10 TO 25 MPH NEAR SEWARD.
TEMPERATURE / PRECIPITATION
SEWARD 41 24 46 / 40 0 0
GIRDWOOD 39 18 43 / 30 0 0
Short Term Weather Model Forecasts (NAM, WRF, GFS) for the Kenai Mountains near Turnagain Pass
Sea-level: temps are forecasted between 18-39 and between 0.0-0.05” of water forecasted
3000′: temps are forecasted in the range of 13-23 degrees F with winds 10 mph
6000′: temps are forecasted in the range of 5-14 degrees F with winds 10 mph
WEATHER STATION SUMMARY for last 24 hours at TURNAGAIN PASS
3800′-Sunburst Wx Station
Temp (5am): 12 (8 degrees colder than yesterday morning)
Winds: In last 24 hours winds have been light averaging 1-16mph with a strong max gust of 36mph
HIGH TEMP of 21 between 3-4pm
2600′-Seattle Ridge Wx Station
Temp (5am): 14 (9 degrees colder than yesterday morning)
Winds: BROKEN, We will fix ASAP
HIGH TEMP of 32 between at 2pm
1800′-Center Ridge Wx Station
Temp (5am): 16 (10 degrees colder than yesterday morning)
Precip: 0.2 inches of water, 2-3 inches of new snow, but 7 inches of melting/settlement
HIGH TEMP 41 at 2pm
Thanks for checking today’s avalanche advisory. The next one will be posted tomorrow Thursday April 8th, 2010.