Updated: Jul 12
Do you notice anything unusual about the XM-delivered satellite weather radar image below? This image was taken from the XM-delivered satellite weather broadcast and shows a grid of five recent lightning symbols and a single Storm Cell Identification & Tracking (SCIT) marker just to the west of Charlotte, North Carolina. If this were truly a thunderstorm, where are the radar returns? Is it possible to have lightning without any precipitation?
The radar feature is definitely selected and the radar portion of the broadcast was indeed received. This can be verified by looking at a wider view of the Southeast on the image below This view clearly shows that the radar broadcast was received since there are valid returns shown in eastern Alabama. In addition to the missing returns west of Charlotte, notice that there is also a similar issue in northern Georgia. So what could cause such an anomaly?
First, let's take a look at the source of this data, namely, the NWS WSR-88D NEXRAD Doppler radar out of Greer, South Carolina. If the data is missing from the source, then that may explain why the broadcast was incomplete. The radar image shown below is valid at the same time and clearly shows a cell to the west of Charlotte in the same location as the lightning symbols and SCIT marker. The complete image also clearly shows precipitation over northern Georgia this is also missing from the image above. Consequently, the cell west of Charlotte is a real thunderstorm in progress. But where's the XM composite radar data associated with this cell?
This issue has nothing to do with the XM weather receiver or its display. The broadcast was received successfully. The display was updated with the latest and greatest composite radar mosaic. But somehow, the radar data containing real precipitation over a fairly large region was obviously not included in this radar mosaic. This anomaly is caused by a delay in removing a manual gross filter. These gross filters or masks are applied by meteorologists at The Weather Company (formally WSI) to regions in the coverage area that have a very low probability of precipitation, normally in areas with a totally clear sky. A filter such as this helps to completely remove annoying ground clutter and other returns caused by anomalous propagation (you can see the speckled blue returns around the GPS radar site in the image above). In other words, it produces a high quality image while masking out unwanted returns. However, if that mask is not removed once convection starts to develop, it will not only remove the clutter, but it does a great job removing any real precipitation in the areas where the mask was applied.
In this case, the mask was finally removed before the next broadcast at 1815Z (1415 EDT). As you can see from the image below, the composite radar data magically appears out of nowhere to show a very healthy thunderstorm in progress over 10 miles wide.
How long was this data missing? Looking back at the NWS NEXRAD data, this cell first appeared at 1743Z or nearly 30 minutes prior to the broadcast shown above.
Could echo tops have also provided some clues that a thunderstorm was in the making? Unfortunately not. Echo tops are also generated by the NWS WSR-88D radars. As a result, they were conveniently filtered as well. While these tools provide us with much needed guidance in the cockpit, they also cannot be used solely as primary guidance. If your subscription includes lightning data, make sure it is always displayed along with the Storm Cell Identification Tracking markers (significant storm cell directional arrows). Also try to remain in visual meteorological conditions when traversing through a convective environment. This allows you to integrate what you see outside of the cockpit with the technology inside the cockpit.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS research meteorologist