Good morning backcountry travelers this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Thursday February 18th 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).
-The winds have increased again at all ridgetop weather stations and are currently strong to extreme with averages of 50mph, 23mph, and 30mph. Extreme gusts have made it up to 86mph.
-The current radars show heavy precip over PWS moving north on the Middleton radar, and moderate to heavy precip on the Kenai radar. *Note, It’s rare to see that much precip on the Kenai radar.
-In the last 24 hours (4am-4am), the snotel sites recorded 1.0 inches of water at Turnagain Pass, 0.6 inches at Grandview, and 0.3 at Summit Creek. It looks like the rain line dropped back down overnight bringing 8” of new snow at 1800′.
-Temps are about the same plus or minus a few degrees at all wx stations this morning compared to yesterday. Temps range from 36 degrees F at sea-level and 23 degrees F at 3800′.
Today’s avalanche danger will increase to EXTREME due to the heavy precipitation and wind that is being forecasted by the National Weather Service. CNFAIC Staff reasons for the increase in avalanche danger are due to recent natural avalanches, and current observations at remote mountain weather stations. The avalanche danger has been at HIGH for the past 3 days and the weather has continued to put more stress on the snowpack. AVOID ALL AVALANCHE TERRAIN.
Yesterday, we saw a medium sized natural avalanche at about 700′ near Kern Creek that failed to the ground (see photo gallery). This is an indication that the snowpack reached its maximum tolerance at that particular location. This avalanche was most likely due rain-on-snow because it was at a lower elevation less than 2000′ which is where the rain line has been reaching recently. We also observed some small to medium sized natural avalanches at Turnagain Pass that we did not see the day before.
We are in the middle of a very dangerous and widespread natural avalanche cycle. It has gotten to the point where we are not trying to forecast IF avalanches are going to occur. We know that natural and human-triggered avalanches are certain today. We are starting to try to forecast how big these avalanches will get. Recently, we have mostly been seeing small to medium sized natural avalanches, but it is likely that we will start to see larger avalanches as today’s storm hits us, and it looks like this storm will pack a big punch.
For whatever it is worth, in the six years that I have worked at the Chugach National Forest Avalanche Information Center, this is the longest time I have seen the avalanche danger at HIGH or above since I have worked here.
-Many of the recent avalanches we have observed have been happening on steep slopes between 2500′-3000′
-A very dangerous concern exists today from sea-level to 2000′ due to rain-on-snow. These lower elevation slopes might fail to the ground like we have already seen with the avalanche near Kern Creek.
Large to very large human-triggered avalanches are a deadly concern today because we have 2 dangerous weak layers in our snowpack that have been reactive to natural, human, and/or explosive triggers every day since last Friday 2/12/2010.
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. This weak layer is generally buried 4-6 feet deep. Surface hoar has been observed on top of this layer in several pits, as recently as Sunday 2/15/2010. This surface hoar has not been seen in all pits, and these feathery crystals get smaller as you gain elevation, but you have to give a weak layer like this lots of respect. This combination of surface hoar on top of a crust is historically responsible for the majority of avalanche surprises and fatalities.
2.The Jan 7 rain crust is generally buried 6-8 feet deep. This particular weak layer has been reactive to explosive triggers and most likely has been naturally triggered during our current avalanche cycle. This persistent weak layer has been showing significant signs of instability since its formation last month. This weak layer is widespread on all aspects up to 3000 feet.
WEATHER FORECAST (National Weather Service)
WESTERN PRINCE WILLIAM SOUND-
500 AM AKST THU FEB 18 2010
…URBAN AND SMALL STREAM FLOOD ADVISORY REMAINS IN EFFECT THROUGH
1000 AM AKST FRIDAY…
…STRONG WIND THROUGH LATE FRIDAY MORNING THROUGH PORTAGE VALLEY AND
.TODAY…RAIN AND SNOW…HEAVY AT TIMES IN THE MORNING. SNOW
ACCUMULATION 2 TO 6 INCHES ABOVE 1000 FT ELEVATION…LITTLE OR NO
SNOW ACCUMULATION ELSEWHERE. HIGHS IN THE 30S. EAST WIND 15 TO 25 MPH
EXCEPT EAST 45 TO 65 MPH THROUGH PORTAGE VALLEY AND TURNAGAIN ARM.
.TONIGHT…A CHANCE OF SNOW AND RAIN IN THE EVENING…THEN SNOW
AND RAIN AFTER MIDNIGHT. RAIN MAY BE HEAVY AT TIMES AFTER MIDNIGHT.
SNOW ACCUMULATION UP TO 3 INCHES. LOWS IN THE UPPER 20S TO MID 30S.
EAST WIND 15 TO 30 MPH EXCEPT EAST 45 TO 60 MPH THROUGH PORTAGE
VALLEY AND TURNAGAIN ARM.
.FRIDAY…RAIN AND SNOW. NO SNOW ACCUMULATION. HIGHS IN THE MID
30S TO LOWER 40S. EAST WIND 10 TO 20 MPH. THROUGH PORTAGE VALLEY
AND TURNAGAIN ARM…EAST WIND 35 TO 50 MPH DECREASING TO 20 TO
35 MPH IN THE AFTERNOON.
Temperature / Precipitation
SEWARD 35 34 41 / 100 90 90
GIRDWOOD 39 34 40 / 100 80 80
Short Term Weather Model Forecasts (NAM, WRF, GFS) for the Kenai Mountains near Turnagain Pass
Sea-level: temps are forecasted between 34-39 with between 1.0”-2.0” of water today!! (*Note:2” is a lot of forecasted water for Turnagain Pass for 24 hours. It’s more than usual)
3000′: temps are forecasted in the range of 32-41 degrees F with winds 25-30 mph
6000′: temps are forecasted in the range of 23-32 degrees F with winds 35-40 mph
*Note.These models shows warmer temps than what the NWS is forecasting. It’s more accurate to use the NWS weather forecast; so far, their forecast jives with the real time weather station data.
WEATHER STATION SUMMARY for last 24 hours at TURNAGAIN PASS
3800′-Sunburst Wx Station
Current Temp: 23 (one degree colder than yesterday)
Winds: Averaged moderate to extreme 24-51mph with extreme gust of 86mph
RH: 96 (one less than yesterday)
2600′-Seattle Ridge Wx Station
Broken. We will fix as soon as possible.
1800′-Center Ridge Wx Station
Current Temp: 30 (1 degree colder than yesterday)
Precip: 1.0” of water and 8” of new snow for a total snowpack of 111” (4am-4am)
RH: 100 (same as yesterday)
Thanks for checking today’s avalanche advisory. The next one will be posted tomorrow Friday February 19th.