American severe weather warnings were once issued for entire counties at a time. The change to drawing a shape around the actual threat altered who receives an alert and why.
County boundaries had nothing to do with storms
Counties vary enormously in size and were drawn for administrative reasons that predate radar. A storm clipping one corner triggered an alert across the whole area.
In the large counties of the western and plains states, that meant warning communities an hour's drive from any threat, repeatedly, through an active season.
The predictable result was that residents outside the real threat area learned that warnings frequently meant nothing where they lived, which weakened response everywhere.
A polygon follows the storm's projected path
Under storm-based warning, the forecaster draws a quadrilateral covering the area a specific storm is expected to affect during the warning period.
The shape is built from the storm's current position, its motion and the uncertainty in that motion, which is why polygons are longer than they are wide.
Because the polygon ignores political boundaries, an alert can cover half of one county and a corner of the next without covering either entirely.
Precision depends on the storm's behavior
Steady, fast-moving storms with a clear track allow tighter polygons, because the projected path carries less uncertainty over the warning period.
Slow, erratic or multi-cell storms force wider shapes, since the forecaster must cover the range of plausible movement rather than a single line.
Warnings are also updated and reissued as a storm evolves, so the polygon shifts through an event rather than standing for its full duration.
Delivery systems had to catch up
Broadcast and radio alerting still work largely on county-level geography, so a polygon must be translated into the counties it touches for those channels.
Phone-based alerts use location, which allows delivery closer to the actual polygon, though the accuracy depends on how a device determines where it is.
That mismatch explains why the same warning can reach a phone in a threatened neighborhood and a television in a county that will see nothing.
Smaller areas raise the cost of a miss
Warning a smaller area concentrates the message, but it also means anyone outside the shape has been implicitly told they are not threatened.
Storms that turn unexpectedly can move outside the polygon, which is why forecasters extend shapes downstream rather than cutting them to the current radar echo.
The tradeoff is deliberate. A slightly generous polygon preserves credibility better than a tight one that has to be corrected while a storm is underway.