Heat index products are issued by the medium range desk of the HPC for days 3 through 7 during the period from May 1 through September 30. These products are based on an ensemble of model and manual forecasts. The graphic plots display the cities for which the heat index forecast is prepared, as well as shaded contours indicating the probability of the daily maximum, daily minimum, and daily mean heat index reaching or exceeding a specific threshold. The three thresholds for the maximum are 95, 100, and 105 degrees F; for the minimum, they are 75, 80, and 85 degrees F; and for the mean, they are 85, 90 and 95 degrees F. Contours are drawn for 10%, 40% and 70% probabilities, with shading between the lines of all areas greater than 10%. Alphanumeric tables are also available displaying each forecast heat index parameter (maximum, minimum, and mean) value and the probabilities of that heat index parameter exceeding the 70, 75, 80, 85, 90, 95 and 100 degree thresholds, except 80, 85, 90, 95, 100, 105, 110 degrees for the maximum.

        Studies indicate the duration of high heat indices over an extended period of time presents a greater danger to the general public than a high index over a shorter period of time; therefore, the daily mean heat index value over a 24-hour period is presented along with the daily extrema of heat index values.

        The forecast heat index values are calculated using 24-hour maximum and minimum temperature forecasts issued by the HPC medium range forecasters and relative humidity obtained by averaging the corresponding 0000 and 1200 UTC dew point temperatures over an ensemble of Model Output Statistics (MOS) and the most recent operational Global Forecast System (GFS) model run. These relative humidity values and forecast temperatures are then substituted into an equation derived using multiple regression to calculate the maximum and minimum heat indices, which are averaged to get the daily mean heat index. The daily maximum, minimum, and mean heat index are also computed for all of the members of the same ensemble that is used to obtain dew point temperatures.

        The ranges of the daily maximum, minimum, and mean heat index values over the ensemble are determined. The probability of the maximum, minimum, or mean heat index exceeding a threshold is computed on the assumption of normal distribution of probability centered on the HPC heat index value using a standard deviation equal to the ensemble standard deviation about the HPC value averaged with half the ensemble range. This augmentation of the ensemble standard deviation is done because a limited ensemble will not span the full range of forecast possibilities. All calculations are done on a regularly spaced grid of points, requiring that the HPC and MOS point data be objectively analyzed using a Barnes analysis. A contouring program is then used to make color filled displays of the probabilities. The tabulated forecast values of the probability of a heat index parameter exceeding a specific threshold are then interpolated linearly from the gridded data and made available at the display locations.

For more information on the generation of these products please click here.