Storm-based warning · Threat-shaped area
A polygon follows the hazard—not county lines
The National Weather Service uses storm-based warning polygons to show the specific meteorological or hydrological threat area. The outline can cross part of one county, cut across several counties, or extend over water without matching a political boundary.
The polygon is drawn for where forecasters expect the urgent threat during the warning period. It can be updated, replaced, expanded, trimmed, or canceled as radar, observations, reports, and storm motion change. A map screenshot from an earlier warning is therefore not enough: open the newest official message and verify the issue and expiration times.
- Inside the polygon: follow the warning's protective action immediately.
- Near the edge: keep monitoring; boundaries can change and hazardous weather does not stop at a drawn line.
- Outside the polygon: you may still face lightning, heavy rain, strong wind, poor visibility, or later warnings even when the warned hazard does not currently include your point.
County or parish name · Geographic reference
A county listed in the text may be only partly warned
A storm-based warning may name counties or parishes so people can orient themselves, but the polygon can cover only part of each one. Hearing your county named should make you check your exact location; it does not prove that every address in the county is inside the warned area.
Read the full “affected area” wording, named towns or roads, storm location and motion, and the official polygon together. When traveling, check where you are now—not only a saved home ZIP code. A route can enter or leave the warned area even when its start and destination are outside it.
County or forecast zone · Area-based product
Not every advisory, watch, or warning uses a storm polygon
Many longer-duration hazards are issued for counties, parishes, marine areas, or NWS forecast zones. A forecast zone can follow a county boundary, split a county by elevation or geography, or separate land, coast, and water because the expected conditions differ.
Do not force a polygon rule onto a zone-based product. Read the alert's official affected-area description and map, then compare your point with that exact boundary. Mountain passes, valleys, beaches, bays, offshore waters, and urban areas may be in different zones even when they share a county name.
The product type matters more than the shape. A winter warning, coastal flood advisory, fire-weather watch, and severe-thunderstorm warning can use different geographic units because they describe different hazards and timelines.
Different product · Different decision
An outlook, forecast, watch, and warning are not interchangeable
- Outlook
- Signals that hazardous weather may develop over a broad area or future period. Use it to review plans and keep checking; it is not a local warning polygon.
- Forecast
- Describes expected conditions, timing, and uncertainty for a location or area. It supports planning but does not by itself mean an urgent alert is active.
- Watch
- Means the named hazard is possible across the watch area. Know the safe place, charge devices, review routes, and be ready for warnings.
- Warning
- Means the named hazard is occurring, imminent, or expected in the warned area. Act on the official protective instruction; the warning may be storm-based or area-based.
An advisory identifies hazardous conditions that may not meet warning criteria but can still change travel or outdoor plans. A watch can cover a much larger area than a later warning. Do not assume that leaving one boundary makes every hazard disappear, and do not wait for a warning if public officials order evacuation or another protective action.
Delivery area · A separate question
A phone alert is a prompt to read the warning—not the boundary itself
Wireless Emergency Alerts are broadcast by participating carriers from cell towers serving the threat area. Delivery depends on the alert type, device, carrier, tower coverage, and location technology. It is not a complete list of every NWS product.
A compatible phone can receive a still-active alert after entering an area, and tower coverage or older location capabilities can deliver a message outside the precise warning boundary. Conversely, not receiving a WEA does not prove there is no watch, advisory, or warning. When an alert arrives, read the hazard, location, time, and action, then use the current official map if doing so does not delay shelter.
A fast alert-reading order
Read hazard, point, time, motion, action, then updates
- Name the product. Tornado warning, flash-flood warning, winter advisory, hurricane watch, and other products require different actions.
- Plot your present point. Check your building, route, trail, beach, or venue—not only the county or home ZIP code.
- Confirm the boundary type. Is it a storm polygon, county or parish, forecast zone, marine area, or another defined area?
- Read the timing and motion. Note when the alert expires, where the hazard is now, and where it is moving when the product provides that information.
- Follow the protective action. Do not spend urgent warning time comparing apps or waiting for a second notification.
- Keep monitoring. A warning can be updated or canceled, and a new hazard can follow the first.
Official sources reviewed September 30, 2026
Source notes
MyUSA.us is an independent weather-information publisher and is not affiliated with NOAA or the National Weather Service. Official weather data and safety guidance remain available directly from the U.S. government sources linked above.
Page reviewed: September 30, 2026