It is understood Fornah, 22, will seal a loan deal at Montgomery Waters Meadow for the remainder of the season this week.
London-born Fornah is a powerful and athletic central midfielder and will provide the competition in that department boss Steve Cotterill is looking for.
The former Brighton youngster, who joined Forest in 2018, will spend a second season on loan in League One after he impressed for Plymouth Argyle under Ryan Lowe last season.
Fornah played 44 games in all competitions for the Pilgrims, including 39 of 46 league fixtures.
The all-action midfield man previously made five appearances on loan at Casa Pia in Portugal’s second tier.
He has been a regular for Forest’s under-23s this season and played once in the Championship, as well as two Carabao Cup appearances.
The Forest prospect will provide competition for David Davis, Luke Leahy and Josh Vela in Town’s three midfield spots.
Fornah becomes Cotterill’s second January signing, following the loan capture of Bristol City striker Saikou Janneh.