Look at the publicly available and archived level 2 NEXRAD data from the individual radar sites, not a mosaic assembled by who-knows-what software using who-knows-what clutter suppression algorithms. That looks like fairly typical clear air mode (VCP 31/32/35) early morning clutter that always shows up on radar being filtered by by some automated processing system, which for some reason stops removing it for a few frames.
If whatever was assembling the mosaic wasn't actively filtering out data to clean up the image you'd see a fuzzy haze around every single radar site, which constantly varies in size and intensity based on radar scan mode, time of day, where the sun happens to be, humidity, insect activity, etc. The only time NEXRAD radar produces an empty image with no returns is when its broken.
Here's what a fairly typical evening looks like without clutter suppression, the blooming is caused by an atmospheric inversion that sets in as the ground layer cools:
Here's another: