A brutal heat wave across the eastern United States will keep millions of people sweltering for another day, with record-high temperatures in the forecast again Wednesday. A cold front will bring cooler weather to the Northeast Wednesday night, but temperatures remain well above average Thursday in the Southeast. The cold front will also bring rain and a risk of severe storms. Temperatures are expected to soar to 10-20 degrees above average on Wednesday, CBS News meteorologist Nikki Nolan said, and some areas will see heat index ("feels like") readings of 110 degrees Fahrenheit. In Philadelphia, the temperaturehit 100 degreeson Tuesday for the first time since July 18, 2012.New York City'sCentral Park had a heat index of 100 degrees in the early afternoon,accordingto the National Weather Service. New York (JFK Airport), Boston and Baltimore also hit 100 degrees, as did Fryeburg, Maine, The Associated Press reported. More than 125 million Americans are under heat alerts, though many of those warnings will expire Wednesday evening. This marks one of the most expansive heat alert events ever, with major population centers from the Midwest to the Northeast under days of serious heat strain. Here's what to know about the heat wave and how to stay safe. Heat wave forecast maps "Summer has taken its time arriving in the Northeast, but it's coming in HOT," the National Weather Service wrote in a social media post last week, warning of triple-digit heat index readings "with oppressive humidity." Theheat indexis what the actual temperature feels like when it's combined with relative humidity. Meteorologists say it's due toa heat dome— an area of high pressure in the upper atmosphere thattraps heat and humidity— sitting over a large portion of the country. When a heat dome becomes stationary, the air within it can become extremely hot and stagnant, leading to dangerously high temperatures and poor air quality. The first few heat waves of the season are particularly dangerous because the human body has yet to adjust to the seasonably warmer temperatures, experts say. "You're talking about some places that could be 40 degrees warmer than last week. So that's a big deal," NOAA meteorologist David Roth said. These maps show daily forecast highs across the country and their departures from average temperatures. Rain and severe storms in the forecast Along with cooler temperatures, the front moving in Wednesday will bring rain — and the chance of severe storms — to the Midwest, Eastern Seaboard and Southeast. How long will the heat wave last? The heat shifted into the Midwest and Ohio Valley and then arrived in the Northeast and mid-Atlantic on Sunday, where it will linger through the first half of this week. The most extreme temperatures are expected to peak on Tuesday in many areas, but highs in the 90s continue through Wednesday before it finally breaks. Parts of the Northeast face a "High Risk" of extreme heat through the middle of this week, according to the National Weather Service. "I cannot emphasize this enough: extreme heat can be deadly,"New YorkGov. Kathy Hochul warned. "The best way to stay safe is stay indoors, with air conditioning. This is not the time to start training for the marathon." What constitutes a heat wave? A heat wave is a period of unusually hot weather that lasts more than two days, according to the National Weather Service. It often, but not always, covers a wide area and can expose a large number of people to hazardous heat, which can be extremely dangerous for vulnerable people, including children and the elderly. Warmer overnight low temperatures can make a heat wave especially dangerous, since they mean the body doesn't get as much of a chance to cool down. "Your body needs a reprieve," Ashley Ward, director of the Heat Policy Innovation Hub at Duke University, told The Associated Press. "You don't get that overnight, we start the next day at a deficit." When overnight temperatures stay above about 75 degrees Fahrenheit, she said, "You start to see some pretty extraordinary outcomes with respect to heat illness and heat stroke, and even mortality." Climate changeis making extreme temperatures and heat waves more frequent and longer-lasting, with the number of days above 95 degreesexpected to increaseacross the U.S. How to stay safe during a heat wave Heat is the leading cause of weather-related deaths, and heat-related illnesses increase significantly during extreme heat and high humidity events. The simplest way to way toavoid heat-related illnessesis to stay out of the sun and avoid extended time outdoors in extreme temperatures, experts say. If strenuous outdoor activities are unavoidable, you should dress for the heat and wear lightweight, loose-fitting and light-colored clothing,the National Weather Service recommended. You should alsostay hydratedby drinking plenty of water, even if you're not thirsty. It's also important to be aware of any symptoms of overheating, which include muscle cramps, unusually heavy sweating, shortness of breath, dizziness, headaches, weakness or nausea, according to theCenters for Disease Control and Prevention. If you experience these symptoms, one major sign to seek care is if the symptoms don't improve after getting into a cooler environment or hydrating,experts say. Marine veteran's wife arrested by ICE, still in custody Reporter's Notebook: Some good news about AI West Virginia's "coal country" still recovering from February floods
