Labov et al. (2006) discuss a taxonomy of configurations of short-a in American English, including the nasal system (in which the prenasal allophone of short-a is relatively high and discretely different from the low non-prenasal allophone), the continuous system (in which short-a is spread out over a continuous area of phonetic space, from higher prenasal tokens to lower non-prenasal tokens), and the raised system associated with the Northern Cities Shift (NCS), in which all tokens of short-a are high. This paper uses this taxonomy as a starting point for an analysis of the status of short-a in the different dialect regions of Upstate New York (Dinkin 2009). The data show that a fourth pattern needs to be added to the three listed above: a raised nasal short-a system, in which there is a sharp phonetic difference between prenasal and non-prenasal allophones, but even the non-prenasal allophone is located quite high in the vowel space. The raised nasal system is most frequent in the Inland North Fringe, the dialect region where some but not most speakers exhibit advanced NCS. In the Hudson Valley region, where nasal short-a patterns are extremely prevalent, NCS features are present at high degrees of advancement except the raising of short-a. An analysis based on the “life cycle of phonological patterns” (Bermudez-Otero 2007) suggests that the nasal system itself may be responsible for blocking the general raising of the non-prenasal allophone here.
Dinkin, Aaron J.
"Nasal Short-a Systems vs. the Northern Cities Shift,"
University of Pennsylvania Working Papers in Linguistics: Vol. 17
, Article 9.
Available at: http://repository.upenn.edu/pwpl/vol17/iss2/9