To access the NWS summertime decision support newsletter, see http://www.crh.noaa.gov/images/sgf/em/Summer2013_decision_support.pdf
And here are the basic criteria for NWS heat related products:
• Heat advisory: Heat index around 105 degrees or greater, or 4 days or more of a heat index greater than 100.
• Excessive Heat Warning: Heat index of 105 for 4 days OR a heat index of 110 degrees or greater.
While these criteria are not strict, they do provide the foundation for NWS heat products though there may be some variation with surrounding NWS offices.