A severe heatwave is expected to intensify across the central and eastern United States from Tuesday through Thursday, with temperatures climbing above 38 degrees Celsius (100 degrees Fahrenheit) from Boston to Washington, DC. Humidity could push the heat index as high as 46°C (114°F), and overnight temperatures will offer little relief. The extreme weather will persist through the Fourth of July weekend, the 250th anniversary of US independence, and coincides with the FIFA World Cup knockout stage in several host cities.