http://www.npr.org/templates/player/mediaPlay er.html?action=1&t=1&islist=false&id=5220090& m=5220091 Professor William Labov on NCS (on NPR)
Northern Cities Shift The Northern Cities Shift affects short vowels inside the dialect area called the Inland North.
Northern Cities Shift The Inland North Dialect encompasses the areas around Buffalo, Cleveland, Toledo, Detroit, Chicago and southern Wisconsin (but somehow misses Ashtabula, Ohio and Erie, Pa). Five changes charaterize the NCS. Here are three of them.
Is your boss a fish or an insect? 1. flat a > i a not only in words like man and Amtrak, i.e. before /m/ and /n/—which occurs throughout the North and West—but in all words, e.g. cat and bad and happen. 2. boss > bass 3. bus > boss “I’d like you to meet my bass.” “In Chicago, the bosses have antennas.”
head desk bosses busses block socks mat The Northern Cities Shift affects all five short vowels
Merger of Don and Dawn Outside the Northern Cities Shift area, from New England to the South Midlands (Athens) and throughout most of the Midwest and West, Don and Dawn have merged as Don. “This is my brother Don and this is my girl friend Dawn.” Both names are pronounced the same. Open /oh/ =  has virtually disappeared in the affected areas, e.g. caught/cot, bought/bot are homophones. However,  does occur in my speech in tall, dog and broad, which are pronounced with .
However, Don and Dawn do not merge in the Inland North Dialect That’s because the whole system of short vowels shifts, making “room” for all the vowels. Matt > M i att Don > Dan Dawn > Don
NCS started in the 1950s in upstate New York, and has spread rapidly within the Inland North dialect area, first in the cities, but recently creeping into the countryside. This dialect has at least 34 million speakers.
The Northern Cities Shift defines the most profound dialect boundary in the United States. --William Labov