Could be atmospheric conditions creating ducting from those locations. Happens in the summer when conditions are right. Not unheard of and is often specifically used in the amateur radio community to send VHF and UHF traffic over longer distances. Since it's an atmospheric phenomenon, it sometimes works, sometimes doesn't.
If you are using the scanner inside your home, then yes, a metal roof will likely impact performance. Same with metal siding, foil backed/vapor barrier insulation in the walls and ceiling, metal lath behind plaster, or even lots of copper pipe and wiring in the walls.
A properly installed outdoor antenna will absolutely improve performance. Since VHF and UHF are -usually- line of sight, the higher the antenna is, the more you'll be able to hear.
The type of coax depends on how long the cable run is. All conductors have a certain amount of losses inherent to them. The longer the cable is, the more pronounced the losses are. Also, the higher the frequencies, the more loss there is. So, longer cable runs and higher frequencies will require higher grade coaxial cable for better performance.
For hobby use, however, 75Ω RG-6 TV/CableTV/Satellite type coaxial cable can work quite well. It's cheap, easy to install connectors on, and available at most hardware stores.
Digital/analog, doesn't matter, antennas and coax only care about the received frequency, not the modulation type.
