I keep hearing about this, but I cannot find any information on it. What is it?
Northern Interior Accent
If you mean "Northern Inland" by "Northern Interior", the Northern Inland American dialect group is simply that in the eastern half north of the US away from the Atlantic coast and which ranges around the southern side of the Great Lakes from upper New York state to Wisconsin, Minnesota, and possibly North Dakota. Note though that today some split it into a "Northern Inland" and a "Nnorthern Central" group, contrasting the two based on whether they have the Northern Cities Shift or not, with usually the non-included dialects being those of most of Wisconsin outside of southeastern/southern Wisconsin (Milwaukee and Madison being non-excluded), and of Minnesota and North Dakota. However, from what it looks to me, such a distinction is an artificial one, since it seems that the Northern Cities Shift has definitely spread into Minnesota and North Dakota today, so I would be likely to simply call them all Northern Inland American together.
However, I have heard some (well, really just Brennus, who I don't consider to be a reliable source with respect to linguistic information) use the term to just refer to all those dialects in the US itself which area away from the eastern seaboard which contain Canadian-like features, in particular some degree or another of Canadian Raising (which may apply to both /aI/ and /aU/, or just /aI/). Such itself would apply to basically a section of the North American English dialect continuum basically stretching from western New York state to Washington state, along the Canadian border. However, the meaning does not really have any more clear meaning besides referring to "Canadian-like" English dialects spoken in the US.
However, I have heard some (well, really just Brennus, who I don't consider to be a reliable source with respect to linguistic information) use the term to just refer to all those dialects in the US itself which area away from the eastern seaboard which contain Canadian-like features, in particular some degree or another of Canadian Raising (which may apply to both /aI/ and /aU/, or just /aI/). Such itself would apply to basically a section of the North American English dialect continuum basically stretching from western New York state to Washington state, along the Canadian border. However, the meaning does not really have any more clear meaning besides referring to "Canadian-like" English dialects spoken in the US.