Msugrad: I'm a senior citizen who got out of radio management/engineering 20 years ago, and am trusting a failing memory; however, based on that memory, I submit that your use of the term "city grade" signal is in error. City grade is the signal level the FCC requires to encompass the city of license. It is defined as 3.16 mv/m, or 70 dbu. Primary, or interference-free contour, is 1.0 mv/m, or 60 dbu. In FCC applications back in my day, only the 70 and 60 dbu contours were called for. Other contours beyond that (50 and 40 dbu, or 0.5 mv/m) can be shown in promotional pieces, but anything beyond 60 dbu was subject to interference (adjacent or co-channel).
Radio-Locator maps do not show a 70 dbu contour. In its FAQ it spells out the three shown contours as 60 dbu (red), 50 dbu (purple), and 40 dbu (blue).
As a maximum power/height C-3 facility (25 kw @ 100 meters) the station in question could be expected to put a city-grade signal (3.16 mv/m) out about 14 miles. Their 1.0 mv/m contour is the one that barely touches Yazoo and Greenwood. That contour is correctly called the PRIMARY, but not the CITY GRADE.
Someone who is up to date on current engineering terminology is welcome to update me.