The NEXRAD radar network operated by our National Weather Service is arguably our most important tool for tornado detection. It may come as a surprise to learn that those radars can’t see funnels on the ground. But the features they do detect, combined with other information sources, are strong indicators that help us determine where tornadoes are occurring and improve warning lead times. When considered together and properly interpreted, the radar data and related products in RadarScope can provide indications of tornadoes with a high degree of confidence. Let’s talk a bit about what the radar shows us and how to interpret it.
Hook Echoes
The hook echo signature in the Reflectivity product is perhaps the most recognizable feature associated with tornadic rotation. The most classic examples often appear near the southwest extent of a storm cell and clearly show it wrapping around on itself. In such cases, the hook is easy to spot. But at other times, the rotation may be rain-wrapped and harder to see.
It’s important to remember that a hook echo signature isn’t proof of a tornado on the ground. The radar beam is pointed high above the ground, often at a few thousand feet, and the hook signature itself can be a couple of miles wide. So, the radar isn’t seeing a tornado on the ground. It’s detecting a much higher, much wider rotation signature that often accompanies a tornado.
A classic hook echo signature.
Sixty miles from the radar, the beam is seeing features thousands of feet above the ground.
The hook echo signature may be well over a mile wide. It indicates the general area, but it can’t pinpoint the location of a funnel cloud on the ground.
Velocity Couplets
The next most-viewed radar product is Velocity, and the companion to a reflectivity hook echo signature is the velocity couplet. The radar measures velocities moving in two directions: toward the radar and away from it. The rotation associated with a tornado will appear as a tight area of motion in opposing directions. If you see velocities toward the extreme ends of the scale in a ball about the same size and location of the hook echo, that velocity couplet is a second key indicator of rotation associated with a tornado.
The adjacent areas of bright red/pink and blue/green represent a velocity couplet.
Debris Balls
NEXRAD’s dual-polarization capabilities give us the Correlation Coefficient product that may be useful for detecting a tornado. If the tornado is strong enough to lift debris to the height of the radar beam, it may show up as a ball of lower CC values co-located with the hook and couplet signatures.
The isolated blue area surrounded by red, orange, and yellow is a possible debris signature.
Tornado Vortex Signatures
NEXRAD’s data processing system includes an algorithm to detect rotation in the form of a tornado vortex signature. These are automated algorithms that run in real-time and are represented in RadarScope by projected storm tracks with a small, black-and-white tornado icon at the start of the track. That icon doesn’t represent an eyewitness report. As with other signatures in radar data, a TVS isn’t proof of a tornado on the ground. The computer is detecting a pattern in the radar data consistent with rotation. While that pattern can be consistent with the conditions for a tornado, it can also be prone to false positives. TVS and related storm attributes are available in Radarscope’s Storm Tracks layer.
TVS depicted by a white circle with a black tornado icon. It indicates the algorithm detected a rotation signature in the data, but that doesn’t always mean there’s a tornado on the ground.
Azimuthal Shear
Azimuthal wind shear signatures are akin to velocity couplets. They are areas of adjacent high velocities in opposite directions. RadarScope Pro Tier Two subscribers can view azimuthal shear in near-real-time via the Multi-Radar Multi-Sensor Azimuthal Shear product. A 24-hour representation of shear is available via the Azimuthal Shear contour layer. These contours are often a good indicator of the path a tornado took over a longer period.
Azimuthal shear contours in RadarScope show the area where wind shear was detected over a 24-hour period. It can be a good indicator of a tornado path.
Tornado Warnings
Local NWS Weather Forecast Offices issue tornado warnings based on radar detection or actual sightings. That information is available in RadarScope in the annotation that appears when you tap inside the warning polygon, along with a threat assessment. You can see whether the warning is based on a visual sighting of a tornado or only the radar signature, as well as the anticipated damage threat and other factors. Tapping the blue info button reveals the full warning text with additional details.
A tornado warning confirmed by observation and presenting a considerable damage threat.
Local Storm Reports
Finally, local storm reports submitted to the WFO can confirm tornado sightings. These come from reputable sources such as local law enforcement or storm spotters. They may also be reported after the fact during NWS storm surveys.
A local storm report detail view in RadarScope for a tornado sighted and reported by a local emergency manager.