The computation used for the heat index is a refinement of a result obtained by multiple regression analysis carried out by Lans P. Rothfusz and described in a 1990 National Weather Service (NWS) Technical Attachment (SR 90-23).  The regression equation of Rothfusz to tabulated results of Steadman (Journal of Applied Meteorology, 1979) is

                         HI = -42.379 + 2.04901523*T + 10.14333127*RH - .22475541*T*RH - .00683783*T*T
                      - .05481717*RH*RH +  .00122874*T*T*RH +  .00085282*T*RH*RH - .00000199*T*T*RH*RH

where T is temperature in degrees F and RH is relative humidity in percent.  HI is the heat index expressed as an apparent temperature in degrees F.  If the RH is less than 13% and the temperature is between 80 and 112 degrees F, then the following adjustment is subtracted from HI:

                            ADJUSTMENT = [(13-RH)/4]*SQRT{[17-ABS(T-95.)]/17}

where ABS and SQRT are the absolute value and square root functions, respectively.  On the other hand, if the RH is greater than 85% and the temperature is between 80 and 87 degrees F, then the following adjustment is added to HI:

                            ADJUSTMENT = [(RH-85)/10] * [(87-T)/5]

The Rothfusz regression is not appropriate when conditions of temperature and humidity warrant a heat index value below about 80 degrees F. In those cases, a simpler formula (not shown) is applied to calculate values consistent with Steadman's results.

The equation for HI above with the appropriate adjustment is used to compute a maximum heat index using the forecast maximum temperature and an estimated 00 UTC dew point temperature at each forecast point location for each forecast projection day.  Similarly, a minimum heat index is computed using the forecast minimum temperature along with an estimated 12 UTC dew point temperature.  The forecast daily mean heat index for the projection day is the average of these two values, the maximum heat index and the minimum heat index. Estimated forecast dew point temperatures are obtained using the method described in the documentation of the WPC 5-km resolution grid data products.