Updated: Jun 18, 2022
A ground-based radar image you use for weather analysis prior to a flight or after you close the door on the cockpit and depart is intended to depict the location and intensity of precipitation. Well, that's the hope. It is not unusual to see an image that not only depicts real areas of precipitation, but may also depict false returns.
These false returns occur for a variety of reasons, but datalink sources such as the Flight Information System-Broadcast (FIS-B) and SiriusXM attempt to filter those false returns from the final product that gets broadcast and received in the cockpit. They do reasonably well on most days to offer a mosaic that only consists of real areas of precipitation. Nothing is perfect, however, and occasionally both sources drop the ball and allow false returns to sneak through the filters and end up on your cockpit display.
Shown above is the Multi-Radar Multi-Sensor (MRMS) system radar mosaic. Notice two anomalous spikes, one in Canada and the other in upstate New York. It is obvious that these do not appear to be normal areas of precipitation. Instead, they are false returns that did not get properly filtered out my the MRMS system.