Dangerous and Potentially Record-Setting Heat Wave to Scorch Millions Across the Eastern U.S. During July Fourth Weekend

Heat dome builds before the holiday weekend

Millions of Americans across the eastern half of the United States are facing a prolonged stretch of dangerous and potentially record-breaking heat as the country prepares for its 250th Independence Day.

The oppressive heat and humidity will first settle over the Midwest early this week, then shift into the Mid-Atlantic and Northeast by Wednesday. A broad heat dome will linger over the East, with the most persistent impacts expected along the I-95 corridor.

The heat is forecast to peak by Friday, just as the long July Fourth weekend begins.

This will not be a single hot afternoon. Many areas will endure consecutive days with temperatures near or above 100 degrees Fahrenheit. Humidity will push heat indices, or “feels like" temperatures, as high as 110 degrees Fahrenheit or more.

The heat will not end at sunset

Forecasters are especially concerned that the heat will continue through the night. The danger is not only in the afternoon highs, but also in the lack of overnight relief.

More than 100 daily record highs could be tied or broken. Even more striking, over 250 record warm overnight lows could be challenged this week alone.

Warm nights matter. When temperatures stay high after sunset, the body gets less time to cool down before the next day’s heat builds again.

The heat holds in streets and buildings. Asphalt and concrete absorb solar energy during the day, then release it slowly overnight. In urban areas, temperatures may struggle to fall much below 80 degrees Fahrenheit.

National Weather Service warns this heat wave is different

The message from the National Weather Service has been consistent: this heat wave is different.

In eastern Virginia, the weather service says this could be the region’s most significant heat wave since July 2012. That event resulted in a dozen deaths in Virginia, among more than 30 heat-related fatalities across four states.

Other forecast offices from the Mid-Atlantic to New England are warning that several days of dangerous heat and unusually warm nights will create a heightened risk of heat-related illness.

More than 100 million people are in Level 3 of 4 “major" or Level 4 of 4 “extreme" heat risk on at least one day this week, according to the National Weather Service. The risk zone stretches from the Deep South to parts of Minnesota, WI, Michigan and Maine, and eastward toward New York and Boston, MA.

Richmond, WA and New York face record heat

Several cities could challenge daily or local heat records as the hot air intensifies.

Richmond, VA, is forecast to top 100 degrees Fahrenheit for three consecutive days. The city could break daily records each day while approaching its hottest July temperatures on record.

In Washington, WA, forecast highs of 102 degrees Fahrenheit on Thursday and 103 degrees Fahrenheit on Friday would break daily records if they verify. Overnight temperatures could remain above 80 degrees Fahrenheit, threatening record warm lows.

New York, NY, could tie both its daily high temperature record Thursday and its record warm overnight low Thursday night.

The heat will press through day and night. In large cities, the air will stay heavy long after sunset.

Raleigh may challenge a warm-night record

In Raleigh, NC, the forecast is not expected to challenge the city’s all-time high temperature record of 106 degrees Fahrenheit.

The nights are the concern.

Overnight lows late this week could approach Raleigh’s all-time warm overnight low record of 80 degrees Fahrenheit. That kind of night-time heat can worsen heat stress, especially after several days of high temperatures.

Low temperatures this week are not expected to fall below the lower-to-middle 70s in the Midwest and Great Lakes. In the most built-up urban areas, the nights may stay much hotter.

Humidity raises the danger

Humidity will make the heat feel more intense. High moisture in the air slows the evaporation of sweat, making it harder for the body to cool itself.

That is why a temperature near 100 degrees can feel closer to 110 degrees or more when humidity is high.

The National Weather Service warns that heat this severe can affect anyone without proper cooling or hydration. Little relief is expected at night, and that persistence increases the risk over time.

Health risks rise during major and extreme heat

Prolonged exposure to extreme heat can cause heat-related illnesses, including heat exhaustion, heat cramps and heat stroke, according to the U.S. Centers for Disease Control and Prevention.

Heat stroke is the most serious form.

Emergency room visits for heat-related illness surge on “major" and “extreme" heat risk days, according to the National Weather Service. With more than 100 million people under the two highest heat-risk levels on at least one day this week, the geographic reach of the threat is unusually large.

Heat is the deadliest weather hazard in the U.S.

Heat is the deadliest type of weather in the United States. Weather service statistics show it has a higher annual average death toll than tornadoes, hurricanes and lightning combined.

This week’s setup combines several dangerous elements:

• Consecutive days near or above 100 degrees Fahrenheit
• Heat indices as high as 110 degrees Fahrenheit or more
• Exceptionally warm nights with limited recovery time
• High humidity that slows the body’s cooling process
• A broad heat dome affecting more than 100 million people at major or extreme heat risk

Climate change makes heat waves more harsh and frequent

Heat waves worldwide are becoming more harsh and more frequent because of human-caused warming from fossil fuel pollution.

Recent examples include the record-shattering heat waves in Europe last week and in the western United States in March. This July Fourth heat wave is now set to add another major episode of dangerous heat, this time over the eastern half of the country during one of the busiest outdoor holiday periods of the year.

Holiday weekend begins under dangerous conditions

The sprawling heat dome will remain over the East through the end of the week, with the peak expected by Friday. That timing places the most intense heat directly over the start of the long Fourth of July weekend.

For millions of people heading outdoors, the risk will be both widespread and persistent. Afternoon temperatures will be high, humidity will raise the heat index, and many nights will stay too warm to provide meaningful relief.