What do the black lines with black dots on the radar return mean?

When storm cell paths are reasonably predictable, you will see a black line with black dots spaced at 20 minute intervals extending out from a storm cell that indicate the estimated position that storm will be in 0, 20, 40, and 60 minutes. The track lines make it easier to judge a storms direction of movement and speed.

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Image 1.  Example of radar that contains storm tracks.