The heat index is how a combination of temperature and humidity really feels to your body (aka apparent temperature or the "feels like" temperature).
On our main weather page, under the 7-day, you will find current conditions for some cities in the area. Plotted next to the temperature is the "feels like" heat index temperature.
Here is a link to the National Weather Service's heat index chart. You can find the heat index by using temperature and either dew point or relative humidity. It is better to use dew point, since as the temperature changes, the relative humidity changes a bit over just a few hours.
After we reached dew points in the 70-75 range Thursday, they should drop to near 60 by late Friday, but will return into the low and mid 60s over the weekend.