My own dialect derives from the Cincinnati-Dayton region of Ohio which is one of a small number of places where the "General American" dialect with no distinctive, Northern, Eastern, Western or Southern features that is favored on radio and television broadcasts with national audiences is favored. This dialect is also spoken in Southern Illinois, Southern Florida, and the part of Northern Ohio where I went to college. It is a variant of the "Midland" dialect, although I tend towards the "Lowest Common Denominator" English fourteen vowel system of the American West, North Central dialect members, and Canadians, probably as a result of having lived in Colorado for fourteen years.
New York City and New Orleans have the most complex sets of dialects, with segregation of dialects by social class in New York City, and by neighborhood in New Orleans.
The distinctions generally track cultural, political and historical trends in U.S. history. The main eight dialects of white Anglo local native born people identified in North America are:
2. Northern New England
3. The North - mostly Michigan, Milwaukee, Chicago and Upstate New York
4. Greater New York City
5. The Midland
6. The South -- in turn divided into Inland South (including but not limited to "Hillbilly" and Lowland South in a divide tracking slavery's extent, particularly in the subset of the Lowland South dialect called "Classical Southern."
7. North Central -- the dialect of the Upper Penninsula of Michigan and Northern Wisconsin and Minnesota
8. The West
The confluence of the most dialects turns out to be in Nebraska, a bit removed from the confluence of most of the major American river basins, which is in Colorado.
African American Dialects
African American Vernacular English (AAVE), the dialect of most African Americans in the United States, is derived from Classical Southern, and shares its main features and many other features. However, it also has a number of distinctive features. I have not included AAVE in this study, since its geographical distribution tends to be independent of “white” dialects.
The Wikipedia article linked on AAVE states that: "There is little regional variation among speakers of AAVE. Several creolists, including William Stewart, John Dillard, and John Rickford, argue that AAVE shares so many characteristics with creole dialects spoken by black people in much of the world that AAVE itself is a creole, while others maintain that there are no significant parallels. . . . AAVE has pronunciation, grammatical structures, and vocabulary in common with various West African languages." But, some scholars argue that the West African contribution is minor. The lack of regional variation is attributed "in part to relatively recent migrations of African Americans out of the South (see Great Migration and Second Great Migration) as well as to long-term racial segregation."
My own intuition is that there are emerging regional variations between AAVE in Northern and Midwestern cities, the American South, and California (as well are areas influenced by it). In Northern and Midwestern cities, the pace at which both whites and blacks speak is faster, the diction is more crisp, and there has been some borrowing from local dialects in usage and vocabulary. In California, there has been influence from Spanish and from other immigrant languages, in addition to the influences seen in Northern and Midwestern cities and considerable innovation of new grammatical forms in the hip-hop/R&B/rap scene.
It isn't clear how much staying power new waves of immigration from Spanish speaking and Asian areas will have on existing English dialects. Second and third generation immigrants sound very much like other native English speakers of the area the areas where they grow up.
There are a couple of dialects, spoken predominantly by neither whites nor blacks, but stable, regional American English dialects notwithstanding, that the map does not seem to capture. One is the dialects of Hawaii, and another is the dialect spoken in Southern Colorado by people like Ken Salazar and John Salazar (which sounds superficially similar to a Hawaiian accent with its open wide sounds, at first hearing).
The other dialects of English that I hear increasingly in the United States, which are not strongly regional are Australian English (recently heard in the local grocery store), British and Scotish English, South Asian English, and the somewhat generic "universal British colony second language English" that has a more refined name that escapes me at the moment.