What It Means and How To Use This Guide
Column one (both tables): Location and altitude of snow measurements. The majority of locations are lift-serviced ski areas. However, others are either popular off-piste ski areas or weather stations used as model sites. These are included for interest and sake of comparison to ski areas in the regions they are found in. Compare the elevation figure here to the next column to tell whether snow data is from the top, middle or bottom of a ski area. For example, neighboring ski areas Alta and Snowbird measure their snow near the middle and bottom, respectively, of their areas.
Column two (both tables): Altitude range of lift-serviced skiing. Snow accumulation, quality, and preservation are all enhanced by the colder temperatures at high elevation. Rain becomes less likely as altitude increases, becoming occasional above 6,000 feet and rare above 9,000 feet. Double stations, i.e., information from two separate altitudes on the same mountain, are included in a few cases to illustrate the variable nature of this phenomenon. Most double stations show a typical pattern of slightly higher snowfall at the higher elevation, but two, Whistler, B.C. and Park City, Utah, are illustrative of highly significant differences in snowfall between elevations.
Column three (both tables): Average annual snowfall. By utilizing the extensive monthly statistics compiled for avalanche forecasting, we can project reliable seasonal averages for the period November 1 through April 30. Within each region there is at least one area with a minimum of 30 years of complete monthly data that can be used to index snowfall to any closely correlated area in that region. Discrepancies between averages presented here and those advertised by ski areas are due to several factors: 1) Ski areas don't always measure snowfall over the monthly periods required to make such broad, even comparisons; 2) Ski areas may not keep long-term snowfall data, and utilize averages over shorter periods of time--typically five years--for marketing purposes; 3) ski area snowfall figures are occasionally "white" lies--marketing directors blow smoke with anecdotal information and inflated figures. More often legitimate data is collected from a choice snowfall area and marketed as representative of the entire mountain. Altitude comparisons from columns 1 and 2 can be revealing here.
Column four (first table): Season standard deviation. Readers who have studied math or anything ending in "ology" will understand standard deviation. The rest of you should read this section closely to understand what it is and why it's important. Standard deviation measures the fluctuation in snowfall from year to year. About two thirds of all ski seasons will be within (plus or minus) one "standard deviation" of the average in Column three. One sixth of seasons will be more than one standard deviation above average while one sixth will be more than one standard deviation below average. For example, standard deviations can show how similar average snowfalls over long periods don't actually equate with reliable annual snowfalls: Grand Targhee has a 32-season average of 469 inches and a low standard deviation of 98 inches. This means that snowfall for two-thirds of all ski seasons at Targhee will be between 371 and 567 inches. (Indeed, during this period there were only three seasons of over 600 inches and only two seasons under 300.) Contrast these figures to Kirkwood, where the 34-season average is a comparable 472 inches, but the standard deviation is a whopping 175 inches, the result of four seasons of over 700 inches, five additional seasons over 600, plus three seasons of under 200 and another four under 300--a much greater range of seasonal variation than found at Targhee.
Column five (first table): December to March percentage of 6+
Inch Powder Days Daily snow records from 3 diverse resorts (Jay Peak,
Steamboat and Squaw Valley) reveal that frequency of days with 6 or more
inches of new snow can be closely modelled by monthly total snowfall.
Why doesn't standard deviation make a statistic like this more variable by
area? Usually it does. Low deviation areas like Targhee and Steamboat
will have a higher percentage of days with 1-3 inches while high deviation
areas like Squaw and Kirkwood will have a higher percentage of 12+ inch
days. It turns out that a 6-inch per day threshold is close to a balancing
point with minimal sensitivity to monthly snowfall volatility. If you're
a purist and think 12+ inches is the real minimum for a powder day, that
percentage can be approximated by half the 6-inch percentage minus 3%.
That means Alta's 22% odds of 6+ translates to 8% odds of 12+, while
Aspen's 10% chance of 6+ means a 2% chance of 12+. That half multiplier
is more like 60% for high volatility areas and 40% for low volatility areas.
What is the Probability of a Powder Day? also analyzes the range of probable powder days over a one week vacation.
Column six (first table): Percentage of all winter months (December to March for a minimum of 18 years) with 90 inches or more snowfall. Pay attention here: This is a strong indicator of the likelihood of experiencing deep powder conditions over a shorter period, such as a week. Not surprisingly, Little Cottonwood Canyon (Alta and Snowbird) and Grand Targhee are the hands-down best bets to catch a dump, outside of the much wetter Pacific states. Surprisingly, southern Colorado's Wolf Creek is next in line, although winter drought here is twice as likely as Utah.
Column seven (first table): Percentage of all winter months (December to March for a minimum of 18 years) with less than 30 inches snowfall. Pay attention again: This shows vulnerability to winter drought and/or low natural snowfall. If you're looking for a powder vacation out west, this figure could be critical. In the Northeast it would be less informative as this percentage will typically be high. The combination of high and low percentages in this and the previous column illustrate volatility of snowfall amounts, but remember the fertile middle ground: For example, Crested Butte gets over 90 inches only six percent of winter months, and under 30 inches 42 percent of winter months. But that means that it gets between 30 and 90 inches during 53 percent of winter months, and that's not bad.
Column eight (first table): Average maximum base depth. A good indicator of even coverage. With comparable snowfall amounts, this figure will be higher in the Pacific states than in the drier snow areas of the Rockies. For example, compare Boreal, California (393 inches average snowfall, 117-inch average base) to Solitude, Utah (400 inches average snowfall, 78-inch average base).
Column nine (first table): Direction of exposure. Slope exposure information was generated from on-site research, trail map information, and, ski areas themselves. Most areas have some northern exposure to preserve base depths and packed powder conditions. South-facing runs will soften and refreeze after a few sunny days even in December and January. East- and west-facing runs will often have spring conditions by mid-February. The highest and steepest north-facing runs can remain packed powder well into April. Just ask the folks who skied A-Basin and Mammoth into August in 1995.
Column ten (second table): Weather restrictions, powder potential, and other considerations. Here we mention where lifts or terrain must be closed during storms for high winds, lack of visibility, or avalanche danger. The high snow month percentage in column five is the single most important indicator of powder potential, but here we mention other factors that should also be considered, such as terrain (gladed trees are best, followed by open bowls), layout (an expansive area with traversing or short hikes to reach the best stashes is desirable), snow quality, and low skier density. Other considerations include useful information not mentioned elsewhere.
Column eleven (second table): Best time to ski. The crux of the matter. In general, areas with high snowfall and poor preservation (like Steamboat and Jackson Hole) are best in the colder early months while areas with lower snowfall and good preservation (like Aspen, Crested Butte and western Colorado in general) are best in the later months when coverage is maximized. A-Basin, Mammoth and Snowbird are the best bets for late winter conditions, i.e., packed powder, into the spring on extensive terrain, based upon superb preservation combined with above average snowfall.