Is That Storm Rotating? How to Find Out.

July 10, 2017 by David Moran
Topics:   RadarScope | Tornado |

A velocity couplet seen on a RadarScope weather radar image may indicate the potential for a tornado, leading to tornado warnings being issued. While some cases clearly show the tornado potential, other cases may not be as clear cut.

To start off, let’s explain what a velocity couplet is. It is simply a signature in which winds blowing away from the radar are next to winds blowing toward the radar. Green pixels show wind moving towards the radar while red pixels show wind moving away from the radar. This signature can clue meteorologists in to areas that may need further investigation. Below is an example of a velocity couplet from a storm near Minneapolis Sunday evening.

RadarScope Velocity Couplet- No Tornado Warning

In the case of this particular storm, there was not a tornado warning at the time of this image. The reason being that the velocity couplet does not line up with any reflectivity structures where a potential tornado would be located. Looking at the top image in the graphic, you see a notch to the northeast of New Ulm. The notch indicates inflow into the storm and is one area where tornadoes could develop. Comparing the reflectivity in the top image to the velocity image on the bottom, the couplet does not line up very well with the notch (the couplet is south of the notch in the reflectivity) and therefore, a tornado warning is not warranted.

About an hour and a half later, a velocity couplet was indicated just north of Lake Crystal, MN.

RadarScope Velocity Couplet- Tornado Warning

In the image above, the velocity couplet is very well defined and it lines up with the location of the notch in the reflectivity data. This suggests that there is the potential for a tornado. As such, a radar indicated Tornado Warning was issued with this storm.

Velocity couplets can provide clues to areas that may need further investigation. When examining these couplets, knowledge of the storm’s structure can help assess the tornado potential. RadarScope’s dual pane feature is especially useful in identifying these areas in that it allows you to assess the couplet, as well as the storm structure, in the same view.

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David Moran

David is a WeatherOps forecaster at WDT. He began working at WDT after receiving Bachelor’s Degrees in Meteorology and Mathematics from the University of Oklahoma. He prepares extended forecasts on potential weather hazards, as well as forecasts for utility clients. David also prepares and maintains the daily forecast blog. He became an AMS Certified Consulting Meteorologist in January 2017.