Extreme heat could stop July 4 travel plans

A dangerous heat wave has settled over the central and eastern U.S. right as one of the busiest travel weeks of the year hits its peak. Temperatures of up to 105 degrees Fahrenheit are expected, according to the National Weather Service, with some daily, monthly and all-time records possible. More than 160 million Americans are under heat alerts.

The cause is a heat dome, a stalled high-pressure system parked over the Northeast that traps hot, humid air in place. The humidity is the part that turns uncomfortable into dangerous. Temperatures climbed into the upper 90s and low 100s from Washington, DC, to Boston, with the heat index pushing above 110 degrees in spots. At least 20 locations broke or tied record high temperatures on Thursday. New York's Central Park hit 100 degrees, tying a record set in 1966, its first triple-digit reading in 14 years.

The grid is under real strain. More than 200,000 customers were without power Thursday evening, including at least 19,000 Con Edison customers around New York City and upstate. The Department of Energy issued emergency orders to increase electricity generation within the PJM Interconnection grid, which serves much of the Mid-Atlantic. Energy Secretary Chris Wright directed data centers in the Mid-Atlantic to run on backup power instead of pulling from the public grid, partly to keep enough capacity for residential air conditioning.

Travel is taking a hit. Amtrak said trains may need to run at reduced speeds and could see delays between 11 a.m. and 7 p.m. through July 4 due to the heat across the Northeast, Southeast and Midwest. Some Northeast trains were canceled outright over the effect of the heat on the tracks. New Jersey Transit trains into New York were also expected to face delays and cancellations from heat damage to equipment. On the air side, Delta issued a high heat advisory for New York's LaGuardia Airport through July 4, citing hot weather compounding with operational constraints.

The holiday itself is getting reworked around the weather. Philadelphia canceled its Friday Independence Day parade, one of the largest in the country this weekend, over the extreme heat. Fireworks displays were canceled or postponed across several DC suburbs. Washington activated an extreme heat alert through July 5 and expanded cooling centers, hydration stations and emergency medical resources as visitors arrive for holiday events.

The health risk is the headline underneath all of it. Heat is the deadliest weather hazard in the U.S., with a higher annual average toll than tornadoes, hurricanes and lightning combined. The nights are the quiet problem. Overnight lows are only falling into the 70s in many places and struggling to drop below 80 in urban areas, leaving the body little chance to recover before the next day. Eastern Virginia could see its most significant heat wave since July 2012, which caused more than 30 heat-related deaths across four states.

There's a climate signal in it too. A World Weather Attribution analysis published Friday found the combined intensity of this week's heat and humidity would have been virtually impossible without fossil fuel pollution.

Relief is coming, but slowly. Parts of the Midwest should start cooling later in the weekend before the break spreads into the Northeast, while much of the South stays hotter than normal into next week. CNN
If you're traveling, build in buffer time and check your carrier before you leave. If you're outdoors, the standard advice holds: water, shade, frequent breaks inside, and don't leave kids or pets in a parked car.