Storm analysis / Turning wind near the ground

Rotation, live radar analysis

Over Pearl River Navigation Canal Lock and Dam No. 2, Louisiana, where the map opens. The whole country

How sharply the wind turns near the ground in each storm, in mph across a mile: where storms are rotating now. How to read it.

Rotation on the map

The rings are the storms the radars detect, the larger ring a severe one; each line runs to where its storm will be in an hour if it keeps its present speed and heading. Rotation and hail are what the radars detect and the analysis measures, not warnings: the official watches and warnings in force are drawn on the map in their own colours and set out in full under Alerts.

What the rotation map shows

How sharply the wind turns in the lowest 6,500 ft of each storm, in mph across a mile, from the radars’ measure of the wind blowing toward and away from them. A tight counterclockwise turn is the rotation a storm needs to bring a tornado, drawn in warm colours; a clockwise turn is drawn in blue.

How to read it

Under 20 mph a mile: Little turning.
20 to 40: A storm beginning to rotate.
40 to 80: Strong rotation; a tornado possible.
Over 80: Intense rotation, the kind under a strong tornado.
Blue: The wind turning clockwise, often beside a counterclockwise turn or where the wind spreads out of a storm.

What it is for

See which storms are rotating, and how tightly.
Follow the rotation inside a line of storms, where a tornado can form quickly.
Read with the warnings: a tornado warning drawn round a strong turn.

How it is made

The storm analysis joins every radar in the national network every two minutes into one picture of the country on a grid about two thirds of a mile across, cleaned of ground clutter, birds and insects, and measures from it what no single radar shows: the largest hail, the turning of the wind, how high a storm’s heavy echo reaches above the freezing level, and what is falling, read with the temperature of the air. Storm Station keeps it every ten minutes for a day, the gauged rain and the radars’ view each hour, and draws it at every zoom from the analysis’s own grid.

A frame every 10 minutes, played as a loop of 1 to 24 hours. As data: /radar/now.json?show=azshear, the times of its frames.