We're going to write a blog post that goes into the technical details soon.

Basically, we rely on the fact that while weather is chaotic and turbulent in the long run (by "long run" I mean hours to days), it's fairly linear on the timescale of minutes.

So we're able to extract velocity data from NOAA radar images, and use that to project the storms into the future. Some storms are more coherent than others so we're constantly monitoring how accurate our predictions are and adjusting the projections accordingly.

Sometimes this technique breaks down, but it's surprisingly rare.