Don’t Let Satellite Weather Get You Down

The radar depictions you see from the SiriusXM broadcast are highly filtered to provide only real precipitation areas. Ground clutter, anomalous propagation, birds, insects and such are carefully removed to provide the cleanest and most representative image. But like any process, there will be times where non-precipitation returns do not get filtered out. More importantly, you may see real areas of precipitation filtered out as well.

While rare, the latter usually occurs in regions where WSI (the weather provider for SiriusXM) implements what is called a manual gross filter. This kind of filter is the most efficient way to eliminate any clutter in large areas that are not expected to see precipitation. But when that filter is left on too long, it’ll be just as efficient at removing real precipitation from the broadcast.

zoomed-in-gross-filter

Lightning and a single hail storm attribute marker with no radar depicted.

Here’s one such example depicted above. While connected to the SXAR1 I panned the map over Texas and I saw some lightning and a lone hail attribute marker showing echo tops at 45,000 feet in north-central Texas, but no radar returns. Hmmm?

I verified that I had the Radar Composite turned on (I did) and zoomed the display out as shown below to see that there are plenty of other precipitation areas shown to the northeast and southeast of this area. Given that the area wasn’t cross-hatched with “Radar not available” why wasn’t there any precipitation shown?

texas-gross-filter

Zoomed out to show the presence of other precipitation on the radar composite.

About 15 minutes later I came back to the map to see if there was any change. Notice below that plenty of lightning and storm attributes are being depicted here in north-central Texas; however, there are still no radar returns being rendered. Given this activity, you’d expect there to be some precipitation shown when both lightning and storm tracks are present. This is a classic indication that the real precipitation in this region was being erroneously filtered.

gross-filter

This is a classic signature for a gross filter being left on too long. With the radar composite on, no precipitation is being shown despite the presence of lightning and storm tracks.

Just five minutes later, the gross filter was removed by WSI and the returns suddenly popped into existence as you can see below.

gross-filter-pulled

Once the gross filter was removed, the NEXRAD returns associated with these thunderstorms were rendered.

I took a look at the NEXRAD archives and discovered that the first precipitation developed in this region around 12:05 p.m. CDT. The gross filter wasn’t removed until 12:50 p.m. CDT. That’s 45 minutes with no radar for this area of rapidly developing and potentially severe thunderstorms. Moral of the story is to always have lightning ON and be sure the SiriusXM Storm Markers are also set to ON in the Maps Settings menu (the gear button on the Maps view). Having both of these layers on will likely expose these kinds of uncommon events.