Plugging in some sunset values into Stellarium for Yosemite Valley
Firefall (Horsetail Fall) date of Feb. 14, 2012, sunset at 17:31:47, sun sets at azimuth 253 degrees, 33 minutes 41 seconds
Backing up to Oct. 27, 2011, sunset at 18:01:29, sun sets at azimuth 253 degrees, 33 minutes, 17 seconds
So if it rains a lot or snows and then melts on Oct 27 and sun peeks out just before sunset, shouldn't there be a firefall? Or is there another factor in this equation