It seems that every big AM station covers it's metro and then some, around here. I really did not consider the possibility of the AM station having problems covering the metro area.
Much of California is endowed with relatively good ground conductivity and many of the markets have several good, full coverage facilities. However, several of the largest CA markets don't have a complete day and night coverage station. San Diego does not, and the Riverside / San Bernardino market does not. Even some smaller markets like Oxnard / Ventura, Santa Barbara, Monterrey / Salinas and Santa María / Lompoc don't have a single really good AM signal covering the whole Nielsen defined market.
In the rest of the country, this is even more dramatic. Of 1680 AMs that are home to the top 100 markets, only about 160 have signals that cover at least 80% of the market both day and night. That's just over a station and a half in each major market.
Even some of the 50 kw AM clear channel stations don't cut it... WSB in Atlanta is an example of a station with a great facility but poor coverage due to terrible ground conductivity.
Some markets, like DC, Miami / Ft Lauderdale, Phoenix, and Houston have perhaps one station that comes close to full coverage but most are not competitive. In these same markets, there are a dozen or more FMs with total market coverage.