Heat dome to push temperatures above 100°F across Chicago, NY, DC and Philadelphia through 4th of July

A POWERFUL heat dome is expected to expand across much of the central and eastern United States this week, bringing days of dangerous heat, extreme humidity and unusually warm nights to more than two dozen states through the Independence Day weekend.

Forecasters warn that temperatures in many cities will climb into the 90s Fahrenheit, with some locations topping 100 F (38 C). Combined with high humidity, it will feel even hotter, increasing the risk of heat-related illnesses.

What is a heat dome?

A heat dome is a weather pattern in which a strong area of high pressure traps hot air over a region for several days or even weeks.

The sinking air beneath the high-pressure system suppresses cloud formation and rainfall, allowing the sun to continuously heat the ground. The trapped heat builds each day, making temperatures progressively hotter.

Where will the heat hit hardest?

The most intense heat is expected across the Mississippi Valley, Ohio Valley and parts of the eastern United States.

According to AccuWeather, cities expected to experience prolonged extreme heat include:

Chicago: Four to five consecutive days at or above 90 F

St. Louis: At least eight straight days in the 90s, with temperatures nearing 100 F

Philadelphia: Up to three days around 100 F

Washington, D.C.: As many as four days at or above 100 F

New York City: Temperatures could reach 100 F for a couple of days

Newark, New Jersey: More likely to exceed 100 F on at least two days

Many of these temperatures are well above normal for late June and early July.
Why humidity makes the heat more dangerous

Meteorologists say the combination of high temperatures and tropical humidity will significantly increase health risks.

"The setup will deliver widespread highs in the 90s F with high humidity over a broad area of the Mississippi and Ohio valleys throughout this week," said AccuWeather Meteorologist Alex Duffus.

"People who mind the heat will be very uncomfortable if they don't have air conditioning."

Duffus added that dew point temperatures will remain well into the 70s across many areas.

A dew point above 70 F indicates very humid conditions, making it much harder for sweat to evaporate and cool the body. This raises the likelihood of heat exhaustion and heatstroke, particularly during prolonged outdoor activity.

Warm nights offer little relief

Forecasters say one of the most dangerous aspects of this heat wave will be the unusually warm nights.

"While there will be a handful of locations that set record highs daily, there may be many more where record warmth occurs at night," said AccuWeather Meteorologist Brandon Buckingham.

In several large cities, nighttime temperatures may remain above 80 F, preventing homes and buildings from cooling down and increasing health risks, especially for vulnerable populations.

Urban areas are particularly susceptible because concrete, asphalt and buildings absorb heat during the day and slowly release it overnight—a phenomenon known as the urban heat island effect.

It will feel much hotter than the thermometer shows

Even where air temperatures stay in the low 90s, RealFeel temperatures could exceed 100 F and, in some locations, rise above 110 F because of humidity, sunshine and light winds.
Wildfire smoke may worsen conditions

Smoke drifting eastward from wildfires in the western United States could reduce sunshine slightly, limiting daytime temperatures by a few degrees in some areas.

However, forecasters warn the smoke could also combine with stagnant, humid air to create poor air quality, posing additional risks for people with asthma and other respiratory conditions.
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When will relief arrive?

Meteorologists expect a shift in the jet stream during the Independence Day weekend, pushing the heat dome back toward the Plains.

That change should allow cooler air and more frequent thunderstorms to spread into parts of the East.

Cities such as Boston may begin cooling before July 4, while Washington, D.C. may not see significant relief until late on Independence Day or the following day.
Safety advice

Health experts recommend:

Stay hydrated by drinking plenty of water.

Limit strenuous outdoor activities during the hottest hours.

Spend time in air-conditioned buildings or cooling centers.

Take frequent breaks if working outdoors.

Check on elderly relatives, young children and people with chronic illnesses.

Keep pets indoors during peak heat, provide fresh water and avoid walking them on hot pavement.

Forecasters warn that prolonged exposure to extreme heat and humidity can quickly become life-threatening, particularly during multi-day heat waves when overnight temperatures remain unusually high.