Up to 80mm of rain is expected to fall during the warning period between 3am and 6pm, particularly in the south west, with snow possible north of Perth.
The yellow warning area also includes Glasgow and Edinburgh, and the western Borders, with flooding and travel disruption possible.
A Met Office spokesperson said: "Heavy rain will develop in the early hours of Friday morning, lasting into the afternoon before clearing from the west. 15-30mm will fall widely within a 12 hour period, with 60-80mm possible in a few areas, mainly across high ground in the south west.
"There is quite large uncertainty over the north-south position of the rain, which will likely require further updates to the rain warning.
"There is also an additional risk of rain turning to snow along its northern edge. This is currently most likely over high ground across the central Highlands and may require further warnings."
The warning follows BBC weather presenter Tomasz Schafernaker reporting a “quite nasty” area of low pressure heading for the UK on Friday with 50-60-mph winds in places.