One region has become the leader in reviving the reputation of Mexican wines, and, perhaps tellingly, it lies just above the 30th parallel. 90% of Mexican quality wine comes from northern Baja California, centering around the city of Ensenada.
The major winegrowing subregions – the Guadalupe, Calafia, San Vincente, and Santo Tomás Valleys – all lie close to the Pacific where they can benefit from the cooling ocean breezes and mists. Hot days and cool nights is a classic winegrowing combination throughout the world, allowing grapes to develop their sugars without a corresponding drop in acidity. All the valleys feature a mix of alluvial soils and decomposed granite.
More, here: http://mexicanwines.homestead.com/REGIONBaja.html