I've got an application on my computer that lets me do population studies and here are some approximate guesses as to what the regional boundreies are:
Northwest-OR, WA, AK, NC
Northern California-N. CA, N. NV, NC
Southern California-S. CA, S. NV, HI, NC
Mountain West-ID, MT,UT, CO, AZ, NM, NC
Midwest-MN, WI, IA, IL, NC
Southern Plains-TX, OK, NC
Mississippi Valley-LA, AR, MS, NC
Great Lakes-IN, MI, OH, NC
New England-ME, NH, VT, MA, CT, RI, NC
Northeast-NY, NJ, PA, NC
Mid-Atlantic-MD, DE, DC, VA, NC
Eastern-MO, KY, TN, NC
Southeast-SC, GA, FL, NC
Toss-ups: WV, ND, SD, KS, NB, AL
Again, I have no inside knowledge, and I may revise it later (Eastern and Mississippi Valley seem smallish)