According to the Microsoft® Encarta® 2009,
Southern U.S. yes: used to show ready agreement --Backinstadiums (talk) 15:10, 19 July 2019 (UTC)
no deontic use
editThe Cambridge Grammar of the English Language , page 182, reads
The future + epistemic combination is more likely in conjunction with harmonic surely (which is not used deontically): It must surely rain soon --Backinstadiums (talk) 15:30, 18 October 2019 (UTC)