Get our free mobile appWhat Is An Extreme Heat Warning? An Extreme Heat Warning, the National Weather Service explains, is issued when dangerously high temperatures are expected and create a significant risk of heat-related illness. Justin Sullivan, Getty Images Justin Sullivan, Getty ImagesIn Arizona, these warnings typically happen during prolonged periods of triple-digit temperatures. Navigating extreme heat is challenging, and if you spend too much time in the heat, your body might not be able to cool down properly. Justin Sullivan, Getty Images Justin Sullivan, Getty ImagesThe difference is dramatic.