A Warning Polygon is the geographic shape that defines the area covered by a storm-based National Weather Service warning, drawn to enclose only the ground a specific storm is expected to affect during the warning's lifetime. Since 10/01/07, NWS has used polygons rather than whole counties as the primary spatial unit for tornado, severe thunderstorm, flash flood, and marine warnings.
Why NWS switched to polygons
County-based warnings had a coarse geographic mismatch with storms. A single supercell threatening a corner of a county would warn the entire county, over-warning most of the population and diluting the meaning of the alert. Polygons narrow the warning to the actual threat corridor, which improves message precision and, over time, reduces alert fatigue.
The tradeoff is a heavier drafting burden on the warning meteorologist, who now specifies latitude and longitude vertices rather than picking a county from a list. Modern warning software has largely absorbed that burden.
How polygons are drawn and updated
The warning meteorologist positions the polygon based on the storm's current location, its motion vector, radar-indicated hazards, and the office's expectation for how the storm will evolve during the warning period. The shape typically extends downstream along the storm's motion vector to cover ground the storm has not yet reached but is expected to.
Polygons can shrink during a warning as the threat clears portions of the original area, but by convention they never expand. If a storm turns and threatens ground outside the original polygon, the office issues a new warning for the additional area rather than growing the existing one.
Where polygons can mislead
A polygon is a forecast, not a boundary. Damaging wind gusts, hail, and even tornado tracks can extend just outside a warning polygon, particularly if a storm shifts direction or intensifies faster than the warning meteorologist anticipated. Being outside the polygon by a small margin is not a reliable safe zone.
Polygons also apply only to the specific storm they were drawn for. When a warned storm is followed by a second one, a separate warning and separate polygon are issued. Do not assume an expired polygon means the threat is over if new storms are approaching.