What does the hail swath on the storm map show?

The hail swath is a radar estimate of the largest hail that fell at each spot during a storm day, drawn in the colors the legend explains: yellow at 0.75 inch, orange at 1 inch, red at 1.5 inches, and dark red from 2 inches up.

It comes from the national radar network, which estimates hail size every two minutes across the country in cells about a kilometre wide. We keep the largest estimate for each cell over the storm day, so the swath shows the worst of the storm, not one moment of it.

Why it is not the same as the eyewitness circles. A circle is a report from a person who saw hail where they stood. The swath covers everywhere the radar saw hail, including the streets nobody reported from. The two usually agree on where the storm went, and the swath often shows larger hail a mile or two from the nearest report.

Reading it. Click anywhere on the swath to see the estimated size at that spot. Pick a day from the Storms list on the right to draw its swath, use the arrows in the day's panel to step to the storm before or after, or turn on Season view to see the largest hail over the whole date range at once. Set a minimum hail size and the swath shows only the hail at or above it.

It is an estimate. Radar cannot measure hail on the ground. The swath is the best available estimate and the one the industry relies on, but treat it as "probably about this size", not a measurement.

Once a storm passes, the same estimate is what triggers your morning-after hail alerts, and it is what populates the hail history on a project or address.