Before the Sirens: How a Tornado Warning Actually Gets Made

A watch means the ingredients are there. A warning means something is moving.

The Storm Prediction Center in Norman, Oklahoma issues tornado watches, typically four to eight hours ahead of a storm. It calls one whenever the atmosphere has assembled what tornadoes need: instability, wind shear, enough moisture to fuel a supercell. A watch can cover an entire state or several states at once, and it is not a claim that a tornado is happening anywhere yet. A warning is narrower and more local. It comes from your area's National Weather Service forecast office the moment a specific storm shows rotation on Doppler radar or gets reported by a trained spotter on the ground.

Since October 2007, the warning is a shape, not a county line

Older readers may remember warnings that covered an entire county, border to border, even when the actual storm was forty miles from their house. That changed on October 1, 2007, when the Weather Service switched to what it calls storm-based warnings. A polygon gets drawn around the specific threat area, then redrawn as the storm moves. So a warning tagged to Waukesha County no longer means every zip code inside it is in danger. That polygon is what shows up on a phone map now, not a county outline, and it is the reason two neighboring towns can get very different instructions during the same storm.

Most warnings are followed by nothing

Here is the part that surprised me: most tornado warnings never see an actual tornado. Between 2016 and 2020, roughly seven out of ten tornado warnings issued nationwide turned out to be false alarms. That figure comes from Greg Schoor, severe weather program manager for the Weather Service in Norman, in a 2021 interview with weather.com. Average lead time over that same span ran eight to ten minutes for all tornadoes, stretching to as much as eighteen minutes for the most violent ones by 2020. I do not think that is the system failing. Being wrong in the cautious direction costs a delayed dinner and a few minutes in a basement. Being wrong the other way costs a lot more.

How the warning finds you without an app

Once a warning is issued, the Weather Service does not wait for anyone to check the news. Wireless Emergency Alerts go out automatically to every compatible phone physically inside the warning polygon. They broadcast from nearby cell towers rather than route to a phone number, which is why the alert still reaches someone just driving through town. FEMA, which runs the underlying system, is blunt about the design.

No registration, no setting to remember to flip on in advance. The alert carries a distinct tone and a double vibration of its own. Current geo-targeting is supposed to hold the overshoot beyond the polygon's edge to under a tenth of a mile.

Searches for tornado warnings picked back up again this month for an unrelated reason. On September 18, 2026, the Weather Service confirmed a tornado had touched down along a ridgeline near Francis Peak in Utah's Davis County, at roughly 7,500 feet, an elevation unusual enough that forecasters pieced the event together mostly from a stranger's cell phone video and a topographic map. How rare that actually is, and where it ranks against the handful of tornadoes ever recorded higher up a mountain, turns out to be its own story.

None of that makes the warning tone any less unpleasant at nine on a Tuesday night. But it is a narrow, radar-triggered decision, not a general sense of dread someone typed up. Knowing that makes it easier to do the two things that actually matter once it sounds. Get to the lowest, most interior room available. Stay there until the polygon has moved past you, not until the rain outside looks like it is letting up.