Weather forecasters expect persistent rain to turn heavy on Wednesday night and for the conditions to continue on into Friday .
The Met Office said driving conditions could "deteriorate" and expects localised flooding to occur.
It has also warned that the bad weather could affect outdoor events and holiday camping activities.
The worst of the conditions are expected from between 00:05 Thursday to 18:00 Friday.
The Met Office said: "A strong and very moist westerly airstream is expected to cover Scotland from Wednesday night until sometime during Friday.
"As this encounters the mountains of northwest Scotland, large amounts of rainfall are expected.
"Through the whole period, rainfall totals of 50-100 mm are expected quite widely, perhaps with 120-150 mm locally over the mountains."
It added: "There remains uncertainty regarding how long the heavy rain lasts into Friday and this warning will be kept under review."
The Met Office has issued a yellow "be aware" warning for Skye, the west Highlands, Mull and parts of Argyll.