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ARCHIVED FORECAST - All forecasts expire after 24 hours from the posting date/time.
Issued
Tue, February 16th, 2010 - 7:00AM
Expires
Wed, February 17th, 2010 - 7:00AM
Forecaster
CNFAIC Staff
The Bottom Line

Good morning backcountry travelers this is Matt Murphy with the Chugach National Forest Avalanche Information Center on Tuesday February 16th 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).

WEATHER ROUNDUP

The height of our most recent storm hit us Sunday night, but we have continued to get noteworthy weather.

-The winds have backed off slightly at all ridgetop weather stations and are currently moderate averaging 16-19mph. In the past 24 hours, however, Sunburst had moderate to extreme winds averaging up to 43 mph with an extreme max gust of 70mph.

-The current radar is full of precip over PWS moving west directly towards us.

-In the last 24 hours (4am-4am), the snotel sites recorded .7 inches of water at Turnagain Pass, 1.0 inches at Grandview, and .2 at Summit Creek. Snowfall totals range 2-7 inches of new snow. There has been about 3.5 inches of new water weight at Grandview since the snowmachine-triggered avalanche on Saturday.

-Temps are about the same at all wx stations this morning compared to yesterday. Temps range from 34 degrees F at sea-level and 25 degrees F at 3800′.

AVALANCHE DISCUSSION

Today’s avalanche danger will remain at HIGH. Right now, the driving factor with snowpack stability hinges on current weather. Since the Saturday, we have only seen additional weight and wind adding additional stress to a snowpack that has been avalanching since everyday since Friday. Just because the weather is less intense today does not mean that the avalache danger has improved significantly. At the very best, the snowpack stability has stayed the same since yesterday, and large natural avalanches were observed at Turnagain Pass yesterday.

The height of the storm cycle may have passed by Sunday night, but the avalanche cycle is still active. Plus there are a lot of variables in our snowpack right now that we don’t have the answers to. We are still within 24 hours of significant weather that will affect stability in our snowpack. A general rule with avalanches is that 90% of avalanches occur during storms and within 24 hours following a storm. We are still in that window. We need to carefully approach our snowpack on a day by day basis. We are now involved in a new weather cycle with some more strong wind and a little bit more precip forecasted by the National Weather Service today. Large to very large human-triggered avalanches are a serious concern today because we have 2 dangerous weak layers in our snowpack that have both been avalanching everyday since Friday.

Let me be clear about this….Based on data, recent observations, knowledge of historical avalanche reports, statistics and experience….it’s my perspective that you might die in an avalanche if you get on steep slopes today.

-The snowpack became reactive to natural triggers starting last Friday 2/12/2010

-The snowpack was also reactive to human-triggers last Saturday 2/13/2010 resulting in fatalities

-Sunday night 2/14/2010 was the most intense part of the recent weather, but we continue to get additional weight and stress added to our snowpack with .7 inches of new water last night and extreme average wind speeds

-New natural avalanches were observed on both sides of the highway at Turnagain Pass as of yesterday morning 2/15/2010.

Traveling in avalanche terrain is not recommended today. STAY HOME! GO TO THE RESORT! Do something CNFAIC Staff than travel in backcountry avalanche terrain today. If you go to Turnagain Pass, then stay away from avalanche run out zones and keep your slope angles less than 25 degrees. This is not the time to get first tracks.

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 3-5 feet deep. Surface hoar has been observed on top of this layer in many pits, as recently as Sunday 2/15/2010. Not all pits show the surface hoar, 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 5-7 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-

INCLUDING…WHITTIER…SEWARD…GIRDWOOD…MOOSE PASS

500 AM AKST TUE FEB 16 2010

…STRONG WIND TUESDAY EVENING THROUGH THURSDAY THROUGH PORTAGE

VALLEY AND TURNAGAIN ARM…

.TODAY…RAIN AND SNOW. NO SNOW ACCUMULATION. HIGHS AROUND 40. NORTH

AND EAST WIND 10 TO 25 MPH.

.TONIGHT…RAIN AND SNOW SHOWERS. LITTLE OR NO SNOW ACCUMULATION.

LOWS IN THE UPPER 20S TO MID 30S. EAST WIND 10 TO 25 MPH. THROUGH

PORTAGE VALLEY AND TURNAGAIN ARM…EAST WIND 15 TO 30 MPH INCREASING

TO 30 TO 45 MPH AFTER MIDNIGHT.

.WEDNESDAY…RAIN AND SNOW. NO SNOW ACCUMULATION. HIGHS IN THE

MID 30S TO LOWER 40S. EAST WIND 10 TO 15 MPH EXCEPT EAST 35 TO

50 MPH THROUGH PORTAGE VALLEY AND TURNAGAIN ARM.

Temperature / Precipitation

SEWARD 43 33 40 / 100 90 80

GIRDWOOD 40 34 42 / 60 60 40

Short Term Weather Model Forecasts (NAM, WRF, GFS) for the Kenai Mountains near Turnagain Pass

Sea-level: temps are forecasted between 34-42 with between .25-.5” of water today

3000′: temps are forecasted in the range of 23-32 degrees F with winds 20-25 mph

6000′: temps are forecasted in the range of 23-32 degrees F with winds 45 mph

WEATHER STATION SUMMARY for last 24 hours at TURNAGAIN PASS

3800′-Sunburst Wx Station

Current Temp: 25 (1 degree colder than yesterday)

Winds: Averaged light to extreme 14-43mph with extreme gust of 70mph

RH: 97 (same as yesterday)

2600′-Seattle Ridge Wx Station

Broken. We will fix as soon as possible.

1800′-Center Ridge Wx Station

Current Temp: 31 (1 degree warmer than yesterday)

Precip: 0.7” of Water and apprx 2” of wet snow for total depth of 105”

RH: 100 (same as yesterday)

Thanks for checking today’s avalanche advisory. The next one will be posted tomorrow Wednesday February 16th.

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Tue, February 16th, 2010
Alpine
Above 2,500'
High (4)
Avalanche risk
Treeline
1,000'-2,500'
High (4)
Avalanche risk
Below Treeline
Below 1,000'
High (4)
Avalanche risk
Alpine
Above 2,500'
Avalanche risk
High (4)
Treeline
1,000'-2,500'
Avalanche risk
High (4)
Below Treeline
Below 1,000'
Avalanche risk
High (4)
Danger Scale:
No Rating (0)
Low (1)
Moderate (2)
Considerable (3)
High (4)
Extreme (5)
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Status of riding areas across the Chugach NF is managed by the Glacier and Seward Ranger Districts, not avalanche center staff. Riding area information is posted as a public service to our users and updated based on snow depth and snow density to prevent resource damage at trailhead locations. Riding area questions contact: mailroom_r10_chugach@fs.fed.us

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This is 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.