Improving the NWS Heat Index
Lance Wood, NWS Houston, League City, TX
Mark Keehn, NWS Houston
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Abstract
Excessive heat was the biggest contributor to weather related deaths in the United States during the period between 2006 and 2015.Because of the impact that heat has on the population, the National Weather Service (NWS) issues heat advisories and warnings and provides heat related decision support services to emergency managers.The NWS Heat Index (NWSHI) is the primary tool used by NWS forecasters to assess the excessive heat threat to the population. The NWSHI equation was developed in 1990 to fit a multiple regression analysis of the Apparent Temperature (AT) model developed by Robert Steadman. The benefit of the NWSHI equation is that it simplifies the complex formulas that comprise the AT model and makes it possible to calculate the NWSHI using only temperature (T) and relative humidity (RH) as input variables.

The NWSHI formula was modified slightly since its inception to adapt to extremely high and extremely low RH conditions; however, there has been no modification to include Steadmanís most current AT formula (1993).Steadmanís research indicates that surface winds along the Western Gulf Coast may lower the AT by as much as 2į C in the summer while solar radiation can raise the AT by up to 6į C.The NWS gridded forecast database contains the necessary parameters to calculate the AT using temperature, RH, wind, and solar radiation as variables. This presentation will show the differences of using Steadmanís 1993 AT formula as compared to the current NWSHI; and therefore, the potential improvements that could be made to the NWSHI, especially along the Western Gulf coast, where wind speeds increase in the wake of the seabreeze boundary.