I doubt this information will help, but we lived in Boulder City, NV. for 6 yrs.(just south of Las Vegas) The flies were horrible. Dry, hot, wet, or cool. We now live in the Dallas area. Mostly hot and humid. We live across the street from a horse pasture. The fly problem is minute compared to NV.
Thank you for that post, JRabbit. That is just what I have been trying to say. Flies are at their worst in DRY even VERY DRY DESERT areas, for they need almost no water at all, just what was in the carrion or feces they inhabited as larvae, they never drink.
BAR flies do at Dad's, but that is another species.
Flies proliferate especially in dry areas because of fewer competing insects or predators to eat them, less mold and fungus on their larvae, etc. Very wet areas have almost no flies at all.