Geolocation in progress...
The map above shows 24 hours of snow accumulation, not snow depth or current snow pack conditions. So your town could be under a foot of snow from the week but if no snow has fallen in the past day, then your area will show up empty. The map may also have some difficulty finding snow for your location if you live near a body of water. You can zoom into the map and click on any shape to discover how deep it is.
How does the National Weather Service compile the data? Reports come in from a network of 50,000 weather observers, ranging from NWS Cooperative stations to volunteer spotters. About 7,000 to 9,000 sites provide enough data for a given day, said Greg Fall, Physical Scientist at the National Weather Service’s Office of Water Prediction and the lead for the National Snowfall Analysis project.
Around 7 am local time, these observers report the snow accumulation for the past 24 hours by measuring the depth of accumulated snow on a measurement board. Once the measurement is recorded, the board is swept clean for the next 24 hours.
The snowfall analysis shows an estimated average of snow accumulation in a given area around 6 square miles that includes data points from around 95 miles of that area (about the distance from New York City to Philadelphia). That’s why the snow accumulation in your backyard might not match up with what exactly the map shows for your area.
The majority of observations are reported between 7 am on the east coast and 7 am on the west coast (7 to 10 am EST). A first pass of the snow map is generated shortly after 7 am EST, but the analysis updates throughout the day as more results stream in from stations across the US.
There are some caveats, though. The analysis excludes patches of the Western US (along with Alaska and Hawaii) where there are few observed reports available. And because the analysis deals with averages, the map doesn’t typically show areas with as much snow accumulation as the highest reported measurements from individual stations.
I wrote a script to download the snow accumulation data and convert it into shapes to be placed in the interactive map above. My colleague Ryan Mark set up a server with the required software for the script to run every hour. To display the map, I extruded the shapes to mimic depth, exaggerating the height of the accumulation so that the shapes are visible even when zoomed out. Our map is updated twice an hour throughout the day as the NWS’s model updates.
Correction: A previous version of this story misstated the size of a given area in the snowfall analysis.