Good morning backcountry travelers this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Thursday April 8th, 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 are calm to light this morning averaging 0-11 mph with some moderate gusts up to 23mph
-The 24 hour snotel sites are reporting:
-0.0 inches of water, 0 inches of new snow, and 3” of melting/settlement at 1800′ at Turnagain Pass
-0.0 inches of water, 0 inches of new snow, and 4” of melting/settlement 1100′ at Grandview
-0.0 inches of water, 0 inches of new snow, and 0 inches of melting/settlement at 1400′ at Summit Creek.
-This morning’s (5am) radars are mostly clear over PWS and Kenai Peninsula
-Temps are the same or warmer at most weather stations by 1-11 degrees compared to yesterday. Above freezing temps made it up to 33 degrees at 1800′ yesterday. This morning’s temperature range is 20 degrees F at sea-level and 13 degrees F at 3800′.
Today’s avalanche danger will start out as MODERATE, but will rapidly increase to HIGH and become VERY DANGEROUS on any sunny slopes shortly after the sun comes out. The weather models are showing a warming trend over the next couple days, and the skies are already clear over Girdwood this morning. Temps at most weather stations are already the same or slightly warmer than they started yesterday by 1-11 degrees. AVOID traveling on or under any sunny slopes especially after noon. Get out of avalanche terrain immediately if you start to see point release avalanches, and especially if you see slab avalanches. Small avalanches might be an indicator of larger avalanches to come. CAUTION!! Today could be the first full day of intense sun on a fresh load of loose snow.
Last week, natural slab avalanches were triggered by the sun as early as 11am on SE aspects. SE aspects include terrain like the entire Sunnyside of Seattle Ridge including Repeat Offender above the snowmachine up-track. Throughout the day, the sun will wrap around to southern aspects then to south west aspects. Since it is almost the middle of April, our sun is high enough in the sky to affect multiple aspects, including some northern aspects, but SE, S, SW are the most likely areas for natural avalanches today.
The primary snowpack concern today is: intense sun and/or warm temps triggering the top 1-3 feet of new snow on top of a buried sun crust on southern aspects. 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 buried 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 from the Sunday-Monday storm 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. Areas where there is only 1 foot of new snow on top of the sun crust might avalanche earlier than the deeper areas.
Yesterday (Wednesday 4/7/2010), a small glide crack avalanched on the southern aspect of Tincan. Plus, the surface snow was still light and powdery which confirms that the surface snow has not been pre-baked. Point release avalanches will be reactive today shortly after the sun hits the loose snow on top.
Tuesday, the sun came out for about an hour 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 over the past two days, it has not run a full avalanche cycle on the new storm snow on the surface. Today might be the day for this avalanche cycle.
We have been taking a close look at the storm snow and sun crust interface over the past two days on Tincan and Sunburst. We have been finding hard failures with relatively clean but not completely smooth shear planes (for example-CTH29Q2/Q3@120cm on Tincan) on top of the most recent sun crust. There is anCNFAIC Staff shear plane about a foot deeper down with a smoCNFAIC Staff surface. CNFAIC Staff than a small collapse on a 31 degree slope when the third person stepped on to that slope, we have not been observing any obvious signs of instability. However, there were numerous point release and slab avalanches within minutes of the sun coming out on Tuesday. Ski cuts on steep rollovers, and stomping around on steep test slopes have produced no results CNFAIC Staff than some shooting cracks up to 30 feet long in the top 1-3 inches on of wind skin on the surface.
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.
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 on warm sunny days. The most recent one occurred yesterday (Wednesday 4/7/2010).
WEATHER FORECAST (National Weather Service)
WESTERN PRINCE WILLIAM SOUND-
500 AM AKDT THU APR 8 2010
.TODAY…PARTLY CLOUDY…EXCEPT MOSTLY CLOUDY ALONG TURNAGAIN ARM.
HIGHS IN THE MID 30S TO LOWER 40S. VARIABLE WIND 10 MPH…EXCEPT
NORTH 10 TO 25 MPH NEAR SEWARD AND WEST 20 TO 35 MPH NEAR WHITTIER.
.TONIGHT…MOSTLY CLEAR. LOWS IN THE TEENS TO UPPER 20S…COLDEST
INLAND. VARIABLE WIND TO 10 MPH…EXCEPT NORTH 10 TO 20 MPH NEAR
SEWARD AND WEST 20 TO 35 MPH NEAR WHITTIER.
.FRIDAY…MOSTLY SUNNY. HIGHS IN THE UPPER 30S TO MID 40S.
LIGHT WINDS…EXCEPT NORTH 10 TO 20 MPH NEAR SEWARD AND WEST
15 TO 30 MPH NEAR WHITTIER.
TEMPERATURE / PRECIPITATION
SEWARD 37 24 42 / 0 0 0
GIRDWOOD 36 18 39 / 0 0 0
Short Term Weather Model Forecasts (NAM, WRF, GFS) for the Kenai Mountains near Turnagain Pass
Sea-level: temps are forecasted between 18-36 and between 0.0” of water forecasted
3000′: temps are forecasted in the range of 14-23 degrees F with winds 10 mph
6000′: temps are forecasted in the range of 14-23 degrees F with winds 10 mph
WEATHER STATION SUMMARY for last 24 hours at TURNAGAIN PASS
3800′-Sunburst Wx Station
Temp (5am): 13 (1 degree warmer than yesterday morning)
Winds: In last 24 hours winds have been calm-light averaging 0-3mph with a light max gust of 9mph
HIGH TEMP of 18 between @ 4pm
2600′-Seattle Ridge Wx Station
Temp (5am): 14 (same as yesterday morning)
Winds: BROKEN, We will fix ASAP
HIGH TEMP of 26 @ 4pm
1800′-Center Ridge Wx Station
Temp (5am): 18 (2 degrees warmer than yesterday morning)
Precip: 0.0 inches of water, 0 inches of new snow, and 3 inches of melting/settlement
HIGH TEMP 33 at 4pm
Thanks for checking today’s avalanche advisory. The next one will be posted tomorrow Friday April 10th, 2010.