Severe weather

How to spot a tornado on radar

The hook echo, the rotation pair on velocity and the debris signature, on the El Reno storm of 2013 and a Dallas tornado in 2019.

AleruaOctober 25, 20263 min read

Hook echo on weather radar, with dark gaps at its center where low-correlation gates were hidden — 1The hook
2Gaps where the echo was too jumbled to be rain

Radar can’t see a funnel. From 40 miles away the beam is most of a mile wide, wider than many tornadoes. It sees the storm that makes them and sometimes what the tornado picks up. Forecasters look for three signs in the same place: a hook on reflectivity, a tight pair of green and red on velocity, and a patch of low correlation coefficient where debris is in the air. One alone is a hint. All three together is as close as radar gets to confirmation.

The hook echo

The first sign is the hook echo: a curl of rain and hail wrapped around the southwest side of a supercell, a thunderstorm with a rotating updraft. The El Reno hook above was scanned eleven minutes after the National Weather Service says the tornado began.

Rotation on velocity

The second sign is on velocity, the Doppler view: green is rain moving toward the radar, red is rain moving away.

Rotation shows up as a pair. Where a patch of green sits right beside a patch of red, the air between them is turning. Looking out from the radar, green on the left and red on the right is counterclockwise rotation, the way most tornadoes in the Northern Hemisphere turn. The tighter and brighter the pair, the more worrying it is. When it lines up with the hook and a debris signature, forecasters treat it as a tornado on the ground even if no one has seen it.

On US radars, Alerua’s storm relative velocity view subtracts the storm’s own motion, which makes rotation easier to see.

The debris signature

The third sign is the gaps at the center of the hook, marker 2 in the first picture. Rain returns a clean, uniform echo. Debris does not: shingles, insulation, leaves and dirt tumble in every direction and send back a jumble. Alerua’s Filter ground clutter switch, on by default, hides any gate that jumbled, a correlation coefficient below 0.8. Switched off, the gates come back, and two dual-polarization views show what they are.

The El Reno hook echo with the ground clutter filter off, showing no gaps

Correlation coefficient radar image: mostly light blue rain with a patch of orange and red where debris lowers the value — About 0.6 in the orange and red patch at the tip of the hook, inside rain near 1 (light blue).

Differential reflectivity radar image: rain in green and yellow with a curved band of gray-blue near zero at the hook — Rain reads high (green to yellow). Gray and blue, near zero, is hail or debris tumbling at random.

A debris ball

On the night of October 20, 2019, an EF3 tornado crossed north Dallas in view of the Dallas–Fort Worth radar (KFWS). With the filter off, the gaps at the end of its hook become a red spot, a debris ball: debris lofted high enough to read as a strong echo.

A hook-shaped echo over north Dallas on radar, with dark gaps where debris scrambled the echo

The same Dallas radar scan with the filter off, showing a red debris ball inside the hook

A warning beats the radar

None of this replaces a warning. The beam can miss a tornado that is small, shallow or far from the radar, and a National Weather Service warning is the signal to act on.

Alerua radar screen with tornado warning polygons outlined in red and severe thunderstorm warnings in amber over a supercell near El Reno, Oklahoma — Tornado warnings (red) and severe thunderstorm warnings (amber) in force at 6:14 PM. In Alerua: Settings, then Warnings.

If you are inside a tornado warning, take shelter. Don’t wait to find a hook.