The National Weather Service Forecasts-

TODAY...SUNNY. WINDY. HIGHS 15 TO 20. EAST WIND 15 TO 30 MPH WITH GUSTS UP TO 45 MPH...STRONGEST NEAR DOWNTOWN JUNEAU AND DOUGLAS. WIND CHILL TO 25 BELOW IN THE MORNING.

TONIGHT...MOSTLY CLEAR. VERY WINDY. LOWS ZERO TO 10 ABOVE.
NORTHEAST WIND 15 TO 25 MPH. NEAR DOWNTOWN JUNEAU AND
DOUGLAS...NORTHEAST WIND 30 TO 40 MPH WITH GUSTS TO AROUND 60 MPH. WIND CHILL TO 25 BELOW.

FRIDAY...BECOMING CLOUDY. VERY WINDY. HIGHS AROUND 17. NORTHEAST WIND TO 45 MPH WITH GUSTS TO 75 MPH...STRONGEST NEAR DOWNTOWN JUNEAU AND DOUGLAS. WIND CHILL TO 25 BELOW ZERO IN THE MORNING.

Over the last week we have seen quite a bit of clear cold weather with very low humidity. This slowly builds surface and near surface facets in the snowpack that do not bond well to the new snow or windslabs when added.

Over this same time we have seen fairly moderate outflow winds from the E to NE loading most of our south facing slopes in places that do not often see snow.

Over this period of time the amount of loose snow available for transport has deminished, yet with the very high winds in the forecast for the next 24-48 hours expect to see quite a bit of blowing snow and continued windloading.

This continued windloading places additional stress on the weak layers in place in the snowpack. So even though over time our weak layers tend to slowly heal, with the additional stress on them their stress VS strength becomes more questionable especially as we dial up the winds and add mass more rapidly.

Early in the outflow event we saw natural avalanche activity on most all Southern aspects from SE all the way around to SW. The slab sized have varied greatly but this still shows us the presence of these weak layers in place and tells us they are near failure.

With the winds picking up today and especially tonight into tomorrow avalanche danger is CONSIDERABLE at this time.

Natural avalanches possible, potentially destructive avalanches may come near or reach developed areas. Human triggered avalanches probable especially in windloaded areas.

Be extremely cautious in places like Thane Road, The Flume and Perseverance Trails. The Urban zones along the Berhands and The White Path are also at risk. Although slide sizes are not predicted to be large enough to directly affect them, recognize these dry slab avalanches travel quite fast and push a tremendous windblast in front of them. This windblast also carries snow in it with enough force to break trees, and destroy objects. Recognize that there is potentail danger through most all of our urban zones over the next 48 hours.

Backcountry danger will also remain, especially in places that are windloaded. Although these wind slabs are becoming quite hard in places, the weak layer is still present, the stress on them is increasing, and with the presence of additional triggers (you) danger still remains.

Be safe out there and enjoy another sunny day!

Faceted snow:

Faceted snow causes the lion's share of avalanche fatalities in North America with surface hoar as a close second. And no wonder. It seems like made-to-order plot device out of a very scary movie. It grows like a parasite within the snow--often out of sight--until it's too late. It becomes inexorably more and more dangerous during the seemingly most benign conditions--clear skies, cold temperatures--and it lays in waiting, sometimes for weeks, until it's brought suddenly to life by a fresh load of snow or rapid warming. Then, when its victim bumbles into the wrong place, it pulls the rug out from under them, rockets them down the mountain at a terrifying speed, ripping them limb from limb as they bounce off trees and rocks and finally entombs them under tons of icy, hard snow.

How faceted snow is formed:

Faceted snow forms from large temperature gradients within the snowpack. Big word alert!--temperature gradient. A temperature gradient is simply how fast temperature changes over a certain distance within the snowpack. Why? Because it's a fact that warm air holds more water vapor than cold air. This means that temperature gradients also create what we call \"vapor pressure gradients\"--more water vapor in one place than another. And what happens when you concentrate something--especially a gas? It wants to diffuse--move from areas of high concentration to areas of low concentration. When water vapor RAPIDLY diffuses it changes rounded crystals into faceted ones--changes strong snow into weak snow. In other words, temperature gradients create potential weak layers that can kill us. That's why we pay so much attention to them.
Here's another way to explain it. Imagine an old woman with strong perfume walking into a cocktail party. As the perfume diffuses through the room, the people standing nearby would smell the perfume the strongest and the people standing against the opposite wall would be able to smell it the least. Next, pretend that wherever the perfume RAPIDLY diffuses through the room, it changes people to frogs. Soon there would be nothing but frogs around the old woman where the perfume is diffusing rapidly and the rest of the room would stay the same since the perfume around them is diffusing more slowly. Finally, imagine 20 old women with strong perfume spread equally through the crowd. Now, there's no more strong diffusion because the perfume has the same concentration everywhere in the room. Since there's no more diffusion, all the frogs magically turn back into people again.

A stupid example, I admit, but maybe you get the idea. The point is that it's a completely reversible process. Strong gradient turns rounds to facets. Weak gradient turns facets back to rounds. The process in reverse, however, occurs much slowly because it takes so much energy to create a faceted crystal that when we take the energy source away (the strong temperature gradient) it take a lot of time for the crystal to return to its equilibrium state (rounds). In other words, it might take a week or two of a strong temperature gradient to form large faceted crystals but after you take the temperature gradient away, it can take weeks or months for them to stabilize, depending on the ambient temperature of the snow and how much compressive load is on top. In cold climates without much load on top of the faceted snow, it may never gain much strength--even without a temperature gradient. The take-home point here is that: small temperature gradients make the snow stronger; large temperature gradients make the snow weaker. Got that?

So, large temperature gradient?how large is large? For snow of an average snowpack temperature, say around -5 degrees C, the critical temperature gradient is about one degree centigrade per 10 centimeters (1 deg C. / 10 cm.). In cold snow, say colder than -10 deg. C, you need a higher temperature gradient to cause faceting and in warm snow you need slightly less.

For example, let's stick two thermometers into the snowpit wall, one 10 centimeters above the other (about 4 inches). Say we measure a difference of only 1/2 deg. C. in 10 cm., it means that equilibrium snow is growing (snow is getting stronger). If we measure a temperature difference of 2 deg. C. in 10 cm., it means that faceted snow is growing (snow is getting weaker). All you have to do is to find a faceted layer in the snowpack, measure the gradient and you know whether the layer is gaining strength of loosing strength. Cool, huh? This is actually a powerful forecasting tool.

Click on this link for a short video showing how facets are formed in the snowpack.

http://www.fsavalanche.org/Encyclopedia.aspx