How does PowCall forecast snow?

PowCall uses all 31 members of the NOAA GEFS ensemble model, applies per-resort PRISM-calibrated orographic precipitation multipliers based on wind direction, converts liquid precipitation to snow using the Kuchera temperature-dependent snow-to-liquid ratio, and validates forecasts daily against SNOTEL ground observation stations.

How accurate is PowCall?

PowCall provides transparent accuracy tracking. Every forecast is verified against SNOTEL observations and accuracy is published for 1-day, 5-day, 10-day, and 15-day lead times. We publish an 80th-percentile upper estimate alongside the median rather than a single-point forecast, so you can see how much more snow is realistically possible.

What is a GEFS ensemble forecast?

The Global Ensemble Forecast System (GEFS) runs 31 slightly different simulations of the atmosphere from NOAA. By looking at how these 31 members agree or disagree, we can quantify forecast uncertainty and provide probabilistic ranges rather than single-point forecasts.

What is orographic correction?

Mountains force air upward, increasing precipitation on windward slopes. Global weather models smooth terrain and underestimate this effect. PowCall applies per-resort, wind-direction-dependent multipliers calibrated against PRISM climatology data to correct for each resort's unique terrain and wind exposure.

How PowCall Forecasts Work

PowCall generates probabilistic 15-day snow forecasts for 120 ski resorts using ensemble weather models, terrain-aware corrections, and daily ground-truth verification. Every forecast includes uncertainty ranges so you can make better powder day decisions.

Forecast Pipeline

Each forecast update runs through a 5-step pipeline, executed 4 times daily when new GEFS model runs become available.

GEFS Ensemble Ingestion

We download all 31 members of the NOAA Global Ensemble Forecast System (GEFS) at 0.25-degree resolution. Each member represents a slightly different plausible evolution of the atmosphere out to 16 days. By processing all 31 members (not just the control run), we capture the full range of forecast uncertainty.

Orographic Precipitation Correction

Global models smooth terrain to ~25 km grid cells, dramatically underestimating precipitation in complex mountain terrain. We apply per-resort, wind-direction-dependent multipliers calibrated against 30 years of PRISM (Parameter-elevation Regressions on Independent Slopes Model) climatology data. For example, Alta receives a larger precipitation boost from NW flow than from SW flow, matching its Little Cottonwood Canyon geography.

Snow-to-Liquid Ratio (SLR)

Corrected precipitation (in liquid equivalent) is converted to snowfall using the Kuchera method, which calculates SLR based on the temperature profile of the lower atmosphere. Colder storms produce higher SLR (fluffier snow), while warmer storms produce denser, wetter snow. Typical values range from 8:1 (heavy/wet) to 20:1+ (ultra-light powder).

Probabilistic Aggregation

The 31 ensemble members produce 31 different snowfall estimates for each resort and time step. We extract the 50th percentile (median/expected) and the 80th percentile (a realistic upper estimate) to form a one-sided probability estimate: low is a fixed 0", since snowfall can't be negative, and high is the 80th percentile. Ensemble agreement percentage measures how many members predict meaningful snowfall.

Verification & Accuracy Tracking

Every forecast is verified against SNOTEL (SNOpack TELemetry) ground observation stations operated by the USDA Natural Resources Conservation Service. We compute accuracy metrics at 1-day, 5-day, 10-day, and 15-day lead times and publish them transparently on each resort's accuracy page.

Data Sources

Source	Resolution	Update Frequency	Purpose
NOAA GEFS	0.25° (~25 km)	4x daily (00/06/12/18Z)	31-member ensemble for probabilistic forecasts
NOAA HRRR	3 km	Hourly	High-resolution short-range detail
PRISM	800m	Climatology	Orographic correction calibration
USDA SNOTEL	Point stations	Daily	Ground-truth verification

Accuracy Metrics

We track forecast performance using several complementary metrics:

Metric	What It Measures	Ideal Value
Accuracy %	How close the median forecast was to observed snow	100%
MAE	Mean Absolute Error in inches	0
Bias	Systematic over- or under-prediction tendency	0
RMSE	Root Mean Square Error (penalizes large misses)	0
Range Containment	% of actuals not exceeding our high (80th pct.) estimate	80%
Skill vs Climatology	Improvement over historical average baseline	> 0

Why a probabilistic estimate matters: Single-point forecasts give a false sense of precision. Our high (80th pct.) estimate tells you how much more snow is realistically possible beyond the median. When it's close to the median, the forecast is high-confidence. When it's much higher, there's genuine uncertainty in the atmosphere, and you should plan accordingly.

Orographic Correction Details

Each resort has a unique set of wind-direction-dependent precipitation multipliers. These account for the fact that mountains preferentially enhance precipitation when moisture-laden air flows from certain directions. The multipliers were calibrated by comparing GEFS raw QPF against PRISM's 30-year, terrain-aware precipitation climatology at each resort's coordinates.

For example, a resort in a west-facing canyon will see greater orographic enhancement from westerly flow (multiplier > 1.0) and less from easterly flow. These corrections are critical for mountain forecasting accuracy and are one of PowCall's key differentiators.

Covered Resorts

PowCall currently covers 120 Ikon Pass ski resorts across 31 states and provinces:

Region	Resorts
British Columbia	Revelstoke, Whistler Blackcomb
Colorado — Central	Arapahoe Basin, Aspen Snowmass, Copper Mountain, Eldora, Winter Park
Colorado — North	Steamboat
Colorado — South	Telluride
Idaho	Brundage Mountain, Sun Valley, Whitefish
Montana	Big Sky
New Mexico	Taos Ski Valley
Northeast	Alpine Valley, Attitash, Big Boulder, Blue Mountain, Blue Mountain Resort, Boston Mills, Boyne Highlands, Boyne Mountain, Brandywine, Bromont, Camelback Resort, Crotched Mountain, Granite Peak, Hidden Valley, Hunter Mountain, Jack Frost, Jay Peak, Killington, Laurel Mountain, Le Massif de Charlevoix, Liberty Mountain, Loon Mountain, Lutsen Mountains, Mad River Mountain, Mont Sainte Anne, Mount Snow, Mount Sunapee, Mt. Brighton, Okemo, Paoli Peaks, Roundtop, Seven Springs, Snowriver Mountain Resort, Snowshoe Mountain, Stoneham, Stowe, Stratton, Sugarbush, Sugarloaf, Sunday River, Tremblant, Whiteface, Whitetail, Wildcat Mountain, Wilmot Mountain
Sierra Nevada	Big Bear Mountain Resort, Heavenly, June Mountain, Kirkwood, Mammoth Mountain, Northstar, Palisades Tahoe, Sierra-at-Tahoe, Snow Valley, Squaw/Alpine (Base), Sugar Bowl
Utah — Wasatch	Alta, Brighton, Deer Valley, Park City, Powder Mountain, Snowbasin, Snowbird, Solitude
Wyoming	Jackson Hole
andes_central	Valle Nevado
canterbury	Mt Hutt
cascades	Alyeska Resort, Banff Sunshine Village, Crystal Mountain, Cypress Mountain, Fernie Alpine Resort, Kicking Horse, Kimberley Alpine, Lake Louise Ski Resort, Marmot Basin, Mt. Bachelor, Mt. Hood Meadows, Mt. Norquay, Nakiska, Panorama Mountain Resort, RED Mountain Resort, Schweitzer, SilverStar Mountain Resort, Stevens Pass, Sun Peaks Resort, The Summit at Snoqualmie
colorado	Afton Alps, Beaver Creek, Breckenridge, Crested Butte, Hidden Valley, Keystone, Loveland, Monarch, Snow Creek, Vail
queenstown	Coronet Peak, The Remarkables
snowy_mountains	Perisher, Thredbo
tetons	Bridger Bowl, Grand Targhee Resort
victorian_alps	Falls Creek, Hotham, Mt Buller

Transparency Commitment

PowCall is built on a commitment to forecast transparency. Unlike most snow forecast services, we:

Publish probabilistic ranges, not just single numbers
Verify every forecast against independent SNOTEL observations
Show accuracy by lead time so you know how much to trust each day's forecast
Provide ensemble agreement percentages so you can gauge forecast confidence
Track accuracy over 7-day, 30-day, and seasonal windows
Rank resorts by forecast accuracy on a public leaderboard