Dangerous heat wave to engulf US ahead of July Fourth weekend

Heat index values of 100 to 110 degrees are forecast for the Philadelphia and Mount Holly area from Wednesday through Saturday as a heat wave spreads from the Midwest to the East Coast and pushes toward the July Fourth weekend. Record-breaking heat will intensify across most of the central and eastern United States, with heat indices likely above 100 degrees in many places.

The National Weather Service’s New York office issues a Heat Advisory when heat index values are forecast to reach 95 to 99 degrees for at least two consecutive days, or 100 to 104 degrees for any length of time. An Extreme Heat Warning comes when heat index values are expected to reach or exceed 105 degrees for at least two consecutive hours. NOAA’s Weather Prediction Center’s HeatRisk product flags impactful heat seven days ahead and is used alongside official watches, warnings and advisories.

Children, older adults, pregnant people, outdoor workers and anyone living with asthma or another chronic condition are at highest risk. CDC guidance also flags people without adequate cooling or hydration as especially vulnerable, and the agency says heat exposure is one of the leading causes of weather-related deaths in the United States.

Independence Day will draw more people outdoors, onto highways and into parks, beaches and stadiums, while some parts of the Midwest and Northeast could feel like 110 degrees. Overnight lows in the 70s and 80s will offer little relief, especially in dense cities where heat lingers after sunset.

Philadelphia set a new daily record high of 98 degrees on June 12, breaking the old mark of 95, and all-time record highs could be challenged again before some relief arrives Sunday and Monday.