It sounds like your reception of Ch 33 is dependent upon a temperature inversion bending the signal back to earth. In some areas it is quite common for slight temperature inversions that enhance VHF/UHF signal propagation to form on an almost daily basis right after sunset, and last through mid morning. I think this site explains it better then I can.
http://www.dxinfocentre.com/propagation/tr-modes.htm
Scroll down to Tropospheric Enhancement (TrE).
This quite common in my area, but Tropospheric Ducting (TrD) is very rare in this part of the country.