The National Operational Hydrologic Remote Sensing Center (NOHRSC) is the National Weather Service center of expertise for snow information and remote sensing. Based in Chanhassen, Minnesota, it gathers snow observations from the ground, aircraft and satellites and turns them into national maps of snowfall, snow depth and snow water equivalent.
Where it fits
The NOHRSC website lists the center under the NWS Office of Water Prediction, which has elements in Maryland, Minnesota and Alabama.
Hydrologists at the NWS River Forecast Centers use its remotely sensed snow data to support spring snowmelt flood outlooks and water supply forecasts.
The National Gridded Snowfall Analysis
This analysis estimates how much snow fell in the recent past across the contiguous United States. It starts with a background estimate built from Stage IV precipitation estimates, High-Resolution Rapid Refresh (HRRR) precipitation forecasts, a gridded snow-to-liquid ratio climatology and temperature grids.
It then adjusts that background toward reported snowfall from cooperative observers, Community Collaborative Rain, Hail and Snow Network (CoCoRaHS) observers and NWS spotter reports, using two passes of a statistical interpolation method called kriging. Totals cover 6, 24, 48 and 72 hours plus a seasonal total that resets at the start of each water year.
Snow depth and water content
The National Snow Analyses map snow water equivalent, snow depth, snowpack temperature, snowmelt and sublimation. They come from a snow model run at 1 square kilometer and hourly resolution, with ground-based and remotely sensed snow observations assimilated daily.
One of those observation sources is the airborne snow survey. Aircraft measure natural gamma radiation from the upper 8 inches (20 centimeters) of soil, and because water in the snowpack blocks part of that signal, the loss gives a measure of snow water equivalent.
What it does not tell you
The analyses are only as good as their inputs. Where reports are sparse, the background estimate carries more of the result.
The snowfall analysis is regenerated several times a day for five days after each analysis time, so early totals can change as late reports arrive. Snowfall and snow depth also differ, because settling and melt reduce depth after the snow falls.