You may already know this, but others may not, but this is completely normal and happens in all of these events. The radar beam is increasing with height as it goes away from the radar, so it looks like it is snowing all around it but in reality the column is slowly moistening with height. That's why the ring will slowly close in as the column moistens downward with time.
This is why when you look at higher tilts, the ring of radar returns is closing in.
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