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NEXRAD Anomaly: ANOMALOUS PROPAGATION (AP)
Anomalous Propagation (AP) anomalies may be observed under highly stable atmospheric conditions,
such as those found on a calm, clear night. In these cases, the radar beam can actually be
bent almost directly into the ground at some location far from the site. This results in a
strong, intense area of returns on the image. When a NEXRAD site is at low elevation on a
coastline, a "sea return" may be produced as the radar beam's energy is reflected off of the
waves of the ocean. This, too, is a form of anomalous propagation.
AP may also occur in the wake of a passing strong thunderstorm. As the storm passes, the surrounding
atmosphere is quickly cooled and creates superrefractive conditions temporarily behind the
thunderstorm. This can cause an AP anomaly which can often look like another storm cell trailing
the original storm even though it's really just an AP anomaly. Even professional meteorologists can
conclude that the AP is actually a new storm when, in fact, there is nothing but clear, stable air.
In the image above, the large mass of returns on the right half of the image is a real storm that
is moving to the east. The smaller grouping of returns on the left half of the image is Anomalous
Propagation caused in the wake of the passing storm. While it looks like additional storm activity,
it doesn't actually exist.
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