The National Weather Service Forecasts-

TODAY...SUNNY...LOCALLY BREEZY. HIGHS AROUND 25. SOUTHEAST WIND 15 TO 25 MPH BECOMING NORTHEAST 10 MPH IN THE AFTERNOON.
STRONGEST WINDS OUT OF INTERIOR PASSES SOUTH OF JUNEAU. WIND
CHILL TO 20 BELOW IN THE MORNING.

TONIGHT...MOSTLY CLEAR. LOWS 17 TO 23...EXCEPT 3 TO 9 IN WIND
SHELTERED AREAS AND IN THE MENDENHALL VALLEY. NORTHEAST WIND 10 MPH SHIFTING TO THE SOUTHEAST LATE.

FRIDAY...PARTLY CLOUDY. NOT AS COLD. HIGHS AROUND 36. LIGHT
WINDS.

FRIDAY NIGHT...RAIN AND SNOW LIKELY. LOWS AROUND 27. SOUTHEAST
WIND 10 MPH.

We are still seeing some active windloading. This windloading is creating shallow slabs in areas and continuing to build deeper slabs in others.

The shallow slabs are still weak and may easily be triggered by loose snow sluffs off of rocks, trees, and even simply by more windloading. Most of this slide activity is limited to steeper pitches and open areas.

The deeper weak layers of instability appear to be gaining strenth.

As todays winds die off danger levels will decrease.

Please understand this is not a backcountry forecast. In the backcountry with the presence of human triggers the potential for avalanches is greater. We have seen a great deal of slab instability within the region in the last week.

Surface Hoar:

Surface hoar is a fancy name for frost. When you have to scrape your windshield in the morning, surface hoar grows on the surface of snow?hence its name. It grows during clear, humid and calm conditions and once buried, it is a particularly thin, fragile and persistent weak layer in the snowpack, which accounts for a number of avalanche deaths each season. Especially in maritime climates, surface hoar accounts for most avalanche accidents.
Surface Hoar:

Surface hoar is a fancy name for frost. When you have to scrape your windshield in the morning, surface hoar grows on the surface of snow?hence its name. It grows during clear, humid and calm conditions and once buried, it is a particularly thin, fragile and persistent weak layer in the snowpack, which accounts for a number of avalanche deaths each season. Especially in maritime climates, surface hoar accounts for most avalanche accidents.

Surface hoar is an especially tricky weak layer because it can form very quickly. One calm, clear night?sometimes just a few hours?is enough time to deposit a thin layer on the snow surface. And once buried, it is very thin and difficult to detect, yet very weak. Also, it tends to form in a complex, hard-to-predict distribution pattern on the terrain. For instance it might form only above a certain elevation where the mountain rises above the clouds. It might form below a certain elevation where cold, humid air pools. It might form in a distinct elevation band where thin clouds form a ?bathtub ring? in a confined mountain valley. It tends to form on open slopes as opposed to in trees. Also, when deposited on the snow surface, since it is so fragile, any small disturbance?especially wind?can easily destroy the layer making it very ?pockety? i.e. you find it in one spot but not another. No wonder Canadian research indicated that surface hoar accounts for most unintentional human triggered avalanches triggered by professionals

Because surface hoar is so thin, it is also difficult to detect. Often you can?t see it in a snow pit wall and it only reveals itself when you get a clean shear and you look at the bottom of the block and see the flat, feathery, sparkly crystals glittering back at you. The best way to detect surface hoar is to carefully pay attention to the snow surface each day. Before the storm arrives, carefully make a mental map of where surface hoar remains intact. You can typically find surface hoar in basin bottoms and near creeks or lakes.

How it forms:

During a clear sky, the snow in the shade or at night radiates a tremendous amount of heat away and the snow surface becomes very cold. Since we know from earlier in this chapter that warm air holds more water vapor than cold air, the vapor from the warmer air above the snow will condense onto the surface of the snow, and voila, we have surface hoar. Surface hoar (frost) is simply the winter equivalent of dew.

Note: in arctic latitudes, the mid-winter sun is so weak that surface hoar grows all day long, even in the sun. You can grow HUGE surface hoar in the north-country, especially in basin bottoms and near streams.

Next, let's take a short lesson in the second ingredient for surface hoar--humid air. Humidity, or relative humidity, is the amount of water air can hold compared to the amount it actually does hold. For instance, air at 50 percent relative humidity contains only half the amount of water vapor it could if there was an infinite supply of water around. How much water can air hold? It depends on the temperature. Remember, warm air holds much more water vapor than cold air. In other words, we can change relative humidity two ways, first, by adding or taking away water (humid air left over after a storm or humid air near streams), and second by raising or lowering the temperature. This second method, as it turns out, creates much, if not most, of the humidity that forms surface hoar. As air cools down during a clear, calm night, it becomes more humid. Often, this cold, humid air pools up into the bottoms of mountain valleys and basins, exactly where we find surface hoar.

Finally, we need the last ingredient, calm air. Too much wind will destroy the fragile surface hoar crystals, plus, too much wind doesn't allow the cold, air to pool and become humid. Actually about 3 mph is best for surface hoar production because it's just fast enough to bring a continuous supply of humid air to the snow surface but not too fast to destroy it.

In summary, surface hoar forms in the following conditions:

? Clear sky
? No direct sunshine, or very weak sun
? Calm or light winds (about 3 mph is best)
? Open slope exposed to a clear sky (trees or clouds can radiate their own heat and disrupt the process)
? Humid air