The National Weather Service has warned Americans to limit outdoor activity, stay hydrated and ensure access to air-conditioning and other cooling areas during the extreme heat (Getty Images)Millions across the central and eastern United States are bracing for a prolonged and dangerous heatwave this week, with temperatures set to soar ahead of the Fourth of July holiday and high humidity making conditions feel even more oppressive.More than 130 million Americans in southern and Great Plains states were under moderate to severe heat risk over the weekend, a figure expected to grow as the week progresses and temperatures intensify across regions like the lower Great Lakes, mid-Atlantic, and the Mississippi and Ohio River valleys.Forecasters predict several consecutive days of temperatures exceeding 100 degrees Fahrenheit, with heat indexes potentially reaching 100 to 115 degrees Fahrenheit, impacting major cities including New York, Chicago, Washington, and Dallas.The National Weather Service warns that this heat is impactful to everyone, not just vulnerable groups, and the lack of relief from high nighttime lows in the 70s or 80s Fahrenheit increases the risk of heat-related illnesses.Concurrently, severe wildfire conditions are developing across the Western U.S., fueled by hot, dry, and windy weather in states like Utah, Colorado, and Arizona, leading to the deaths of three firefighters and injuries to two others near the Colorado-Utah border.In fullAmerica faces record-breaking heatwave with dangerous temperatures and humidity ahead of Fourth of July