The National Weather Service Forecasts-

TODAY...MOSTLY CLOUDY. LOCALLY WINDY. PATCHY FOG IN WIND
SHELTERED AREAS. SLIGHT CHANCE OF SNOW IN THE AFTERNOON. HIGHS 32 TO 39. EAST WIND 10 TO 20 MPH. NEAR DOWNTOWN JUNEAU AND DOUGLAS...NORTHEAST WIND 20 TO 30 MPH WITH WIND GUSTS UP TO 50
MPH.

TONIGHT...MOSTLY CLOUDY. LOWS 17 TO 25. NORTH WIND 10 TO
15 MPH...EXCEPT NORTHEAST 25 MPH DIMINISHING IN THE LATE EVENING NEAR DOWNTOWN JUNEAU AND DOUGLAS. NEAR DOWNTOWN JUNEAU AND DOUGLAS...WIND GUSTS UP TO 40 MPH IN THE EVENING.

MONDAY...DECREASING CLOUDS. HIGHS 27 TO 33. SOUTHEAST WIND TO
15 MPH.

MONDAY NIGHT...PARTLY CLOUDY. LOWS 16 TO 26. SOUTHEAST WIND
TO 15 MPH.

Temperatures are below freezing still this am. We are expecting a several degree rise in temperatures this am during a period of strong winds.

Yesterday am we saw almost 4\" of new snow at upper elevations.

This new snow will start to build weak small slabs during this wind event and the diurinal fluctuation will add stress to this wind slab formation.

Natural avalanches are not likely yet human triggered avalanches will be possible in isolated areas.

Most of the lower snowpack is quite solid in place. We do not expect to see slides of any size.

Be cautious of wind loaded pockets near treeline, ridgeline, and summit.

Enjoy the Superbowl! Be Safe and have a great day.

Avalanche educators like to say, ?weather is the architect of all avalanches.? After all, it?s the weather that builds the snowpack and causes changes to occur within it. And it?s the weather that can eventually overburden a snowpack to the point of failure, resulting in an avalanche.

Since slab avalanches are the most dangerous type of avalanche, we?ll focus on them.

A slab avalanche has three basic components: a slab, weak layer, and sliding surface. An avalanche can occur without all three, but the presence of each one increases its potential.

?The slab is the consolidated mass of snow that?s put into motion as a unit when the avalanche releases
?The weak layer is made up of unconsolidated or poorly bonded snow that can easily collapse under stress; most avalanches occur when a weak layer fails, putting the slab above it in motion down the slope
?A sliding surface under a weak layer provides a relatively smooth surface upon which the avalanche can readily and rapidly move downhill
Let?s look at the weather conditions that promote the development of each one.

Slab Formation

Slabs usually form when wind transports new snow from the windward side of a mountain onto a leeward slope. This most commonly occurs through turbulent suspension in the air above the snow slope but also through rolling or saltation along the snow surface. (Saltation is a process in which snow crystals move forward through a series of jumps or skips, like a game of leap-frog.)

Wind fractures the new snowflakes in the air or the snow crystals that it picks up from the ground. It breaks them into smaller pieces and deposits them on the leeward slope as a dense, cohesive layer of snow.

Although slabs usually form during storms, they can also develop between storms if the wind is strong enough to move the snow on the ground from a windward to leeward slope. The lower the density of the snow, the lighter the wind required to move it.

Snow transported by wind blowing perpendicular to a ridge crest sometimes forms a cornice. Cornices are good visual indicators of the prevailing wind direction, pointing towards wind-loaded leeward slopes.

Key points

?New snow combined with wind forms slabs
?It doesn?t need to be snowing to form a new slab, you just need wind
Weak Layer Formation

Weak layers can form through several processes that generally occur between storm periods when clear skies and cold temperatures are present. The longer the clear, cold weather, the weaker a weak layer can become. Temperature gradients drive these processes, both within the snowpack and at the surface. There are three main types of weak layers: surface hoar, near-surface facets, and depth hoar.

1. Surface hoar

?The frozen winter equivalent of dew
?Isn?t a problem until it becomes buried by the next snowfall
?Forms through ?deposition? where water vapor (a gas) is directly transformed into an ice crystal (a solid) without first becoming a liquid
?Develops when relatively warm, moist air very close to a cold snow surface becomes oversaturated, reaching the frost point, and deposits onto the colder snow surface, forming feathery ice crystals on the surface; these crystals can range in size from a baby?s fingernail to an oak leaf, with the size dependent on how long the weather remains clear, cold, and calm
?Can only form with near-calm winds (less than 10 mph)
?Can form in just a few minutes although it typically forms overnight
?Can continue forming during the day on north-facing, shaded slopes
?Will sublimate (go from a solid back to a vapor) when contacted by warm air or direct sunlight
2. Near-surface facets

?Commonly referred to as ?facets? or ?faceted snow?
?Not a problem until the layer is buried by the next snowfall
?Develop with a large temperature gradient (over 1?C per 10 cm) in the upper 20 cm of the pack; the gradient drives vapor transport within snowpack from warmer towards colder temperatures, transforming new snow crystals near top of the pack into very small, angular, snow grains, usually less than 2 mm in size
?Can occur day or night and will continue developing as long as air temperatures remain cold
?Develops regardless of wind speed
3. Depth hoar

?Highly faceted snow that creates a weak basal layer (a weak layer at the base or bottom of the pack)
?Develops near the bottom of a snowpack whenever there?s a large temperature gradient (greater than 1?C per 10 cm) between the ground and pack above; note that even in mid-winter, ground temperatures remain at or just below freezing due to Earth?s heat and the insulating properties of snow
?Requires relatively shallow snowpack (less than about three feet or 1 m deep) for the metamorphic process to continue; any more snow will stop the process, usually because the temperature gradient will be too small
?Snow crystals near the ground are sometimes metamorphosed into very large, angular, cup-shaped grains that can exceed 5 mm in diameter
Key points

?Weak layers form when air temperatures are cold and skies are clear
?Surface hoar, near-surface facets, and depth hoar are known as ?persistent? weak layers; they can remain in the snowpack and cause delayed-action avalanches weeks or even months after forming