How to Read NWS Alert Polygons: Know If You're In the Warning
Understand National Weather Service alert polygons, county vs. polygon warnings, and how to verify if your location is inside a hazard area.
Winter Weather Warnings Team
8 min read
Modern NWS alerts are polygon‑based, which means only locations inside the hazard polygon are included—regardless of county lines. Knowing how to confirm whether you are inside the polygon helps you respond appropriately.
Polygon vs. County Alerts
Older systems were county‑based; polygons are more precise. Many apps and broadcasters still display county shading, which can be misleading. Always verify polygon inclusion.
Where to See the Polygon
- weather.gov: Open the alert and select the map tab.
- CAP/ATOM feed viewers: Show the polygon coordinates.
- Reliable apps: Some apps render the official polygon, not just counties.
How to Verify Your Location
- On a map with the polygon, enable your device location.
- Compare your GPS dot to the polygon boundary; if inside, you’re covered.
- Edges matter: if you live near the polygon edge, conditions can change quickly.
Key Polygon Attributes
- Event: e.g., Blizzard Warning, Ice Storm Warning.
- Onset/Expiration: Start/end times and possible extensions.
- Motion: For warnings tied to moving hazards, expect updates.
“If you are inside the polygon, the warning is for you. If not, it isn’t.” — NWS