Extreme heat wave in its final stretch and could fuel storms during July 4 celebrations

By Meteorologist Mary Gilbert, CNN

(CNN) — America’s 250th birthday also marks the final stretch of one of the most intense eastern US heat waves in years — one made worse by climate change — and it could fuel an added storm threat as the nation gathers to celebrate.

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Dangerous heat is on the guest list at every Fourth of July celebration in the region, with high temperatures in the 90s and low 100s forecast from New England into the Southeast. Saturday will likely be the third day in a row of triple-digit temperatures for major metros like Philadelphia and Washington, DC. At least 22 locations broke or tied daily temperature records Thursday and 17 more fell Friday.

Here are some headlines from the heat wave:

• Heat-related illness and death: A 68-year old man died after trimming bushes on July 2 in Bethel Township, Pennsylvania, where high temperatures passed 100 degrees, the Berks County Coroner’s Office told CNN. The cause of death was a heart attack from strain “due to heat exhaustion,” the office said. Meanwhile, the CDC reported “extremely high rates of heat-related illness” in regions across the Northeast on Thursday.

• Events canceled or delayed: The July Fourth parade set for Saturday morning in Washington, DC, was canceled due to extreme heat in the nation’s capital. President Donald Trump’s Great American State Fair is opening two hours late at 12 p.m.

• NYC power outages: Thousands of customers in the New York metro area were without power Friday and Saturday morning, according to local utility Con Edison. Some of the outages were shutdowns to prevent extended outages. Voltage reductions were also happening earlier in the day in parts of Staten Island, Queens, the Bronx, Brooklyn and Westchester, Con Edison spokesperson Jamie McShane told CNN.

• Air quality alert: With an oppressive heat dome lingering over the eastern US, millions could face a double dose of unhealthy air. Fireworks from formal rocket shows to impromptu street celebrations can create extremely unhealthy air quality, sometimes rivaling what comes from wildfires. In some cities, air quality often doesn’t return to normal until around noon the next day.

Thunderstorms could be another uninvited guest this Independence Day. Extreme heat and humidity will fuel a storm threat for parts of the Mid-Atlantic and Northeast beginning Saturday afternoon.

Some of these storms could be severe and deliver damaging wind gusts, but any thunderstorm will contain lightning — a major danger for any outdoor activities, especially for anyone trying to escape the heat in or on the water. Stormy conditions could also disrupt fireworks displays.

The greatest chance for any storms with damaging wind gusts of 60 mph or more will be in the Mid-Atlantic and the Central Plains. There is a heightened risk area for the DC metro and surrounding area, noted in the map below.

There is one silver lining: Any thunderstorms in the East Saturday are expected to be more hit-or-miss in nature rather than a widespread, organized line. That means any area impacted by a thunderstorm may eventually clear out enough to resume the festivities.

A strong heat dome — a stagnant area of high pressure that traps and enhances hot, humid air — is behind the heat wave. But human-caused climate change driven by fossil fuel pollution is making this familiar summer weather pattern far more dangerous.

A steamy 250th

Saturday’s high temperatures are expected to only be slightly lower than what unfolded Thursday and Friday as the intense heat dome persists.

When the Founding Fathers signed the Declaration of Independence in 1776, the high temperature in Philadelphia was at least 76 degrees — Thomas Jefferson himself recorded the measurement. That was a mild day for the time, but 250 years later, the city is absolutely roasting.

A high of 100 degrees is expected Saturday in Philadelphia and the heat index — how the air actually feels to the body, given humidity — will climb several degrees above the actual air temperature.

Washington, DC, is staring down yet another triple-digit high and may see its hottest Fourth of July ever recorded.

New York City is forecast to top out in the upper 90s after triple-digit temperatures on Thursday and Friday, but the heat index will still keep the city feeling like those triple-digits never left.

Heat will start to ease Sunday for much of the East, with highs next week sinking back into the more seasonable 80s. However, parts of the Southeast will remain stuck in the 90s for much of next week.

Historic heat

Since Thursday, heat in the East has broken at least three dozen daily high temperature records, many warmest-low-temperature records as heat reached levels that some cities haven’t experienced in more than a decade.

New York City’s Central Park reached 100 degrees on Thursday afternoon for the first time since 2012. It also tied the high temperature record for the day.

When temperatures soared to 102 degrees in Washington, DC, on Thursday and Friday, they broke daily records that stood for over 120 years. Philadelphia hit 103 degrees Thursday and tied a record last set in 1901.

The intensity of the heat and humidity combined this week would have been “virtually impossible” without the effects of fossil fuel pollution. This finding was published early Friday from World Weather Attribution, a scientific network which analyzes the role of climate change in driving extreme weather events.

“When a historic 4th of July celebration is disrupted, and World Cup matches are played in conditions that are unsafe for players and fans, it shouldn’t take another scientific study to wake people up,” said Friederike Otto, a professor of climate science at Imperial College London.

“Climate change is here, it’s already impacting the things we enjoy in our everyday lives, and it will continue to get worse the longer we drag out the inevitable transition to net zero emissions.”

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CNN’s Angela Fritz contributed to this report.