The National Weather Service Forecasts-

WINTER STORM WATCH IN EFFECT FROM LATE TONIGHT THROUGH FRIDAY
MORNING

TODAY...SNOW LIKELY. SNOW ACCUMULATION 1 TO 4 INCHES. HIGHS
AROUND 26. SOUTHEAST WIND 5 MPH INCREASING TO 10 TO 20 MPH IN THE AFTERNOON.

TONIGHT...SNOW. SNOW ACCUMULATION 4 TO 7 INCHES POSSIBLE. LOWS 22 TO 28. SOUTHEAST WIND 5 TO 15 MPH SHIFTING TO THE NORTHEAST 10 TO 20 MPH LATE.

FRIDAY...SNOW...POSSIBLY MIXED WITH RAIN IN THE AFTERNOON. SNOW ACCUMULATION 3 TO 6 INCHES POSSIBLE. HIGHS AROUND 33. EAST WIND 5 TO 15 MPH SHIFTING TO THE SOUTH IN THE AFTERNOON.

Last month temperatures went above freezing at the Mt Roberts Tram Summit on the 12th. Since that time we have seen 9+\" of rain at the tram summit weather station.

Temperatures fell consistently below freezing again around the first of February. Temperatures started near freezing and then after the 3rd of the month things got quite a bit colder with no precip.

Be aware in wind sheltered areas there is a bit of surface faceting which will likely create a weak layer as this new snow comes in and starts to accumulate.

Since the snow started up last night we have seen winds from 5-15 at the Mt Roberts Tram Summit and gusting higher on Douglas Island with winds from 10-25.

When the snow started falling it was quite light. This light snow is easily wind transportable and will increase avalanche danger in leeward starting zones rapidly.

Winds will be rotating from Northeast to the Southeast a few times over the next two days so recognize that just because the wind isn't loading your line right now doesn't mean it didn't in the last 24 hours.

Danger is still low now with exception of small slabs at higher elevations in windloaded zones. This activity would be isolated and limited in size.

As this storm continues to build over the next few days danger will continue to build. This new snow is not going to bond very well to the cold old snow surface.

So as you start to poke around in this new snow over the next few days there are a few things to consider.

Fluff on crust with poor bonding.

Windloading of light density snow.

Winds shifting 180 degrees during the two days.

Increasing temperature during this storm cycle bringing us an upside down snowpack.

The forecast is calling for as much as a foot of snow from tonight into tomorrow night. This is another red flag of building avalanche conditions.

Be aware that although the danger rating is listed as moderate things will change rapidly over the next 48 hours depending on the rate of accumulation.

Its time to start thinking about snow safety again.

Remember right now you have super light snow poorly bonded to super hard ice that in steep areas can still create slide for life conditions.

Give this new snow time to bond to the old snow surface, settle out a bit, and the skiing will become much better over time with patience...

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