Updated: Jun 18
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.
If you were flying at this time in the Northeast you might have seen this weird spike show up on your MFD (or other portable device) if you use the FIS-B datalink weather. This is because the FIS-B radar mosaic is now sourced through the MRMS system as discussed here. This is a composite reflectivity mosaic rebuilt every two minutes. When it was first released, the MRMS mosaic did not include data from the Canadian Doppler weather radars. Now it does and that poses some significant challenges.
To the right is an image from the Radarscope app that shows the unfiltered data coming from the Canadian Doppler radar (CASFT) in extreme southeastern Ontario. Notice all of the anomalous spikes occurring from this radar site. This is the radar site that is producing the two anomalous spikes shown above on the MRMS radar mosaic. This is common with many of the newer Canadian Doppler radars and is not a maintenance issue with the radar. You can see this also is occurring with the King City radar in southern Ontario as shown below.
Although the U.S. WSR-88D NEXRAD Doppler weather radars can also produce similar false returns, this particular set of spikes are from the Canadian Doppler weather radars. Normally, MRMS is able to filter these out, but in some cases, it does not do a complete job. That's the issue here. Don't get freaked out, however; this is not some strong line of storms. Moreover, it is unlikely that air traffic controllers will see this on their display either since their ground-based weather radar is from the local dual fan beam ASR-9 or ASR-11 radars or perhaps from the ARTCC (center) which does not use MRMS either.
You can also see other weirdness in the datalink weather feed from ground clutter or even from wind farms. Wind farms with a large number of wind turbines also create these non-precipitation returns like you see to the right just to the northeast of Madison, Wisconsin. Those returns in the red circle are in the same region as a wind farm that you can see on the
VFR sectional below located just to the west of Juneau, Wisconsin. A good algorithm to produce the radar mosaic you view in the cockpit should filter these easily. Even so, you can look at the loop of the mosaic and you will find that these returns will not move like we see with real areas of precipitation.
The end result is that you might see a weird-looking line of returns show up along your route even when the sky is perfectly clear outside of the cockpit. Just remember that weather analysis doesn't stop just because you close the door on the cockpit and depart.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist