How PowCall Forecasts Work
PowCall generates probabilistic 15-day snow forecasts for 62 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 10th percentile (conservative/low), 50th percentile (median/expected), and 90th percentile (optimistic/high) to form probabilistic ranges. 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 falling within 10th-90th range | 80% |
| Skill vs Climatology | Improvement over historical average baseline | > 0 |
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 62 Ikon Pass ski resorts across 19 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 | Jay Peak, Killington, Loon Mountain, Okemo, Stowe, Stratton, Sugarbush, Sugarloaf, Sunday River, Whiteface |
| Sierra Nevada | Heavenly, Kirkwood, Mammoth Mountain, Northstar, Palisades Tahoe, 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 | Crystal Mountain, Crystal Mountain, Mt. Bachelor, Mt. Hood Meadows, Stevens Pass |
| colorado | Beaver Creek, Breckenridge, Crested Butte, Keystone, Loveland, Monarch, Vail |
| queenstown | Coronet Peak, The Remarkables |
| snowy_mountains | Perisher, Thredbo |
| tetons | Bridger Bowl |
| 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