The Reds have been trailing McKenna for some time and had seen three approaches knocked back by the Lilywhites before finally reaching an agreement.
An offer lodged on Wednesday has been deemed acceptable by North End and they have granted McKenna permission to discuss personal terms.
The 31-year-old has spent his entire professional career to date at Deepdale, and is club captain at the Lancashire club.
However, he could now be set to take on a new challenge at the City Ground under former Preston boss Billy Davies.
A statement from Preston said: "Preston North End can confirm they have reluctantly reached an agreement with Nottingham Forest over the terms of a possible transfer of club captain Paul McKenna from Deepdale to the City Ground.
"The 31-year-old has been coveted by Billy Davies for much of the summer but the Lilywhites have turned down three approaches for the midfielder, however, an agreeable offer was finally made late on Wednesday and permission has now been granted for talks to take place between the player and Forest.
"The club believes McKenna will travel to the East Midlands on Thursday to have talks with his former manager and no further comment will be made by PNE until after these discussions have taken place."