The Met Office has issued a weather warning for heavy and persistent rain in Tayside and Fife.
A yellow weather warning has been issued for western areas, effective all day on Saturday.
Some parts of Scotland could see more than 3in of rainfall.
Locals have been told to prepare for flooding on roads, while bus and rail services may be affected.
A Met Office statement reads: “Heavy rain will spread from the west later Friday evening (and) will last for much of Saturday, before slowly clearing from the north-west.
“30mm – 50mm (1.2in – 2in) is expected quite widely, especially on high ground with 60mm – 80mm (2.4in – 3.1in) possible in a few areas.
“Spray and flooding on roads will make journey times longer. Bus and train services affected with journey times taking longer.”