Heat Health Watch-Warning Systems (HHWWS) have been developed for at least 19 locations within the United States and 12 locations internationally. All of these locations are in large metropolitan areas. According to the U.S. Census Bureau, only 13% of the United States population lives in a metropolitan city of 500,000 or more people. That leaves 87% of the population that is in smaller cities or rural locations.

In contrast to the previously developed HHWWS, mortality data sets for small-population cities are difficult to work with because of low mortality numbers and high standard deviations in these numbers due to non-meteorological factors; therefore, statistical comparisons between the mortality and the synoptic scale pattern are difficult. However, work by Watts, Sheridan and Kalkstein (2004) showed the possibility of clustering the human response to heat in large metropolitan cities based on a climatological stress index for humans. Areas that are within the same climate region show similar summer patterns in the heat stress index (HSI). Thus, small population areas may be clustered with larger cities to have similar HSI patterns, and it may be concluded that all cities within the region respond similarly with respect to heat-related mortality.

Using the spatial synoptic classification system, it was shown that regions exhibit similar frequencies of air mass types. Over 10 regions were identified for the United States and southern Canada, and it is clear that, from an air-mass standpoint, these are cohesive regions. This implies that mortality response is similar across regions and HHWWS may be developed using larger, regional, mortality data sets.