This is usually called FM Ducting, and is more likely in summer on very
hot days, and temperature inversions also help out to make it
possible...

Just like the shortwave bands, different freqs are affected first...I
was once sitting in my car back in the early 1980's, eating my lunch in
Novato Calif (Marin Co.) on a hot August day, when I was lead tech for
Harris DTS, and had my scanner going listening to the local California
Highway Patrol channel, and I started to hear skip on 42.08 Mhz, the
local CHP "silver net" frequency, and it came in real strong for over
and hour.

AND it became 2-way skip (ducting) because I could hear a dispatcher
ask her units to repeat their radio traffic because they were being
covered by skip from California...and much to my boss's consternation,
I was late in returning to work that day beacuse this was a very good
day to keep listening to my scanner...

I listened long enough to get a call sign, and I called up a gal I knew
back then at the San Francisco FCC office, had her run the call sign,
and it came back as the Florida Highway Patrol...

I can't remember if I made a QSL request or not...