 An excessive heat warning. Okay, what is the what, if you will? Although the what is dangerously hot conditions with heat index values up to 110. Of course, heat-related illnesses increase significantly during extreme heat and high humidity events. Drink plenty of fluids. Stay in an air-conditioned room. Stay out of the sun and check up on relatives and neighbors. Do not leave young children or pets in unattended vehicles. Car interiors will reach lethal temperatures in a matter of minutes. And that's the thing.