I would get the biggest VHF-UHF combination antenna I could, mount it as high as possible, and point it right at Mt. Wilson. 15' is not very high - there's only 1 LOS station on your report.
Pointing through a tree is a problem. Look here at the section "Trees and UHF."
http://www.hdtvprimer.com/antennas/siting.html
If you don't want an antenna on your roof, how about a tower in your yard?
What is your budget like?