A new product to evaluate the impact of heat on human activities, the Heat Stress Index (HSI), has been developed and was evaluated by select National Weather Service (NWS) Weather Forecast Offices (WFOs) during the past two summers (2007 and 2008) to determine if it has operational potential.

The HISI is a numerical index customized for each locality, designed to provide guidance with respect to human comfort and potential threat to life during excessive heat events. The HSI differs from the currently utilized Heat Index (HI) in that it incorporates factors beyond just temperature and humidity. The HSI ranges from 0.0 to 10.0 and provides a relative sense of how uncomfortable or dangerous any day is with respect to “normal” for a specific location and date. A day's value of 9.9, for example, indicates that only 1 percent of days at that place during that time of year would be more stressful in terms of heat.

The developmental test ran from July through September of both summers at ten WFOs in 2007 and twenty-one WFOs in 2008. A password-protected website was established, posting the HSI twice daily, for access by each office. The values were used by each WFO as additional guidance on when to call heat watches, advisories or warnings, and were passed on to stakeholders, such as local health departments and emergency managers, to see if the product would be helpful for them as well. The results from the first test indicated that continued refinement and testing was necessary.

After two seasons of testing, the feedback from the WFOs and stakeholders will be evaluated to determine next steps on how the Heat Stress Index can best be used to alert the American Public of heat hazards, including a recommendation as to whether it should be issued to the public. This paper will present the results of the developmental test.