How Snow Totals Are Estimated: NOAA's National Snowfall Analysis
Where the snow totals on SnowDex come from, why they are estimates for a ~4 km grid cell, when they are published and revised, and where they go wrong.
Published October 30, 2026 · SnowDex
One national grid of snowfall
The snow totals on SnowDex come from NOAA's National Snowfall Analysis, produced by the National Operational Hydrologic Remote Sensing Center (NOHRSC) (NOHRSC snowfall). It covers the contiguous United States on a grid of cells about 4 km (0.04°) on a side, and it is, in NOAA's words, the only nationwide snowfall analysis that blends observed snowfall into a background estimate (product description).
How each number is made
The analysis starts from how much water fell — radar-based precipitation estimates — and turns it into snow depth with a snow-to-liquid ratio built from model guidance and 30 years of observations. It then corrects that background with snowfall reports from cooperative observers, the CoCoRaHS volunteer network and National Weather Service spotters (product description).
So each total is an estimate for a grid cell, not a ruler reading in one yard. Near many observers it tends to be close to what they measured; far from any observer it leans more on the radar and the ratio.
Windows, delay and revisions
Three things about timing explain most surprises:
- Fixed windows. 24-hour totals end at 12:00 and 00:00 UTC (7 am and 7 pm Eastern in winter). 72-hour and season totals are built from the daily 12:00 UTC analysis. None of them is a rolling total ending right now.
- About two hours' delay. A window's first version usually appears roughly two hours after it closes.
- Revisions for about five days. NOAA re-runs each analysis as late reports arrive, so a total can still change for days. SnowDex says so next to every number that may still move.
Where it struggles
Lake-effect snow is the hardest case. Bands downwind of the Great Lakes can be only a few miles wide, dumping feet of snow in one town and inches in the next; a ~4 km grid built partly from radar can miss the peak by a lot. Mountains are another: totals change quickly with elevation, and a cell averages over that. SnowDex shows a warning on pages inside the main lake-effect belts.
Your own measurement can also differ for ordinary reasons: snow settles and compacts, drifts in the wind, and melts on warm ground. The National Weather Service measures new snow on a cleared snow board every few hours for that reason (NWS snow measurement guidelines).
Check the snow totals and forecast for any ZIP code with the free snow lookup. A SnowDex iPhone app is in the works.
Common questions
Is the SnowDex snow total a measurement?
No. It is NOAA's estimate for the ~4 km grid cell your place is in, blending radar-based precipitation, a snow-to-liquid ratio and nearby observer reports.
Why did yesterday's total change?
NOAA re-runs each snowfall analysis for about five days as late observer reports arrive, so a window's total can be revised up or down.
Why is my lake-effect snow total so different?
Lake-effect bands can be only a few miles wide. A ~4 km grid estimate can miss their peak by a lot, which is why SnowDex warns about it in lake-effect belts.