We've all experienced it - gathering around the TV as the broadcast meteorologist goes through a list of cities and towns in the path of a tornado-warned storm. But how do meteorologists know when to warn a storm? What are meteorologists looking for?

If you've ever driven past a National Weather Service office, there's likely one thing that stood out - a NEXRAD (Next-Generation Radar) doppler radar.

A Brief History of Weather Radar

Prior to the NEXRAD, the national radar network consisted of WSR-57 (developed in 1957) and WSR-74 (developed in 1974). While helpful at the time, the United States knew it needed an upgrade. Why? Because neither radar allowed meteorologists to see wind speed and wind direction, which is crucial for detecting rotation within storms.

What Makes NEXRAD Different?

NEXRAD came with a number of improvements, including Base Velocity (BV), which shows wind direction. Not only that but it also improved resolution and sensitivity and most importantly, an increased range.

Scientific American's graphic below shows exactly how doppler weather radar works:

So, back to our scenario - sitting in the living room, around the TV, watching the tornado-warned storm heading towards you. How do meteorologists know that there's rotation?

On a velocity radar scan (bottom left), colors represent wind motion:

• Green shades = wind moving toward the radar

• Red shades = wind moving away from the radar

When these two strong signals appear right next to each other, it indicates rotation within the storm. This is what meteorologists refer to as a velocity couplet, and it can be a strong sign that a tornado is forming—or already occurring.

Another critical radar product is Correlation Coefficient (CC) - the image on the right.

This tool helps meteorologists determine what types of objects are being picked up by the radar.

• High values (reds/oranges/yellows): uniform precipitation like rain or hail

• Lower values (blues/greens): mixed or non-uniform objects

  
  

When a distinct “drop” or pocket of low correlation appears inside a storm, it can indicate debris being lofted into the air—such as:

• roofing material

• siding

• trees

• or other ground-level debris

  
  

This is often referred to as a debris signature, and it can be strong evidence that a tornado is on the ground.

So do we always know when a tornado is on the ground? Not always. Radar gives meteorologists powerful clues, but it is not perfect. Some tornadoes are short-lived, rain-wrapped, or occur in areas where radar resolution is limited. However, when strong rotation and a debris signature appear together, confidence increases significantly that a tornado is ongoing.

The bottom line: if a Tornado Warning is issued, take cover immediately.