Thunder storms

We saw lots of those signs that say tune to 1620 AM (or something) when flashing

But even when flashing other than Yellowstone park have never heard anything. Must be super low power

WHO 1040 out of Des Moines was who I grew up listening to. Mom was close friends with Ronald Reagan when he worked there

They are very low power.  Usually you won't hear them where the signs are, you'll have to listen after you see the signs, the transmitter is a little ways past the signs.