Sweltering days ahead for East, Midwest ahead of July Fourth weekend

Unusually hot and potentially dangerous days are ahead for the central United States heading into the Fourth of July weekend, and meteorologists warn some areas won’t see much overnight relief, either.

A heat dome is forecast to build and expand over the region as the week progresses, with daytime highs approaching triple digits in some places. The system is expected to spill over into parts of the East for several days, AccuWeather said in an online forecast.

"People who mind the heat will be very uncomfortable if they don’t have air conditioning," AccuWeather meteorologist Alex Duffus said.

The timing means visitors marking the nation's 250th birthday in Washington, DC, could face perilously hot conditions.

Along with widespread highs in the 90s, the system will bring high humidity to the Mississippi and Ohio valleys, Duffus said, with more than two dozen states affected at its peak.

Urban areas, which retain heat through solar energy absorbed by pavement, brick and concrete, compounded by the long daylight hours of June and July, may see overnight lows persist above 80 degrees.

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

Perilous heat in the Midwest

Chicago residents will see highs around 90 degrees for four to five days in a row, AccuWeather said.

In St. Louis, where highs are typically in the high 80s this time of year, temperatures will soar into the 90s for at least eight straight days, approaching 100 degrees.

Kansas City, too, will see highs in the mid to upper 90s, according to the National Weather Service.

Dew points will eclipse the 70-degree threshold as humidity climbs. That’s where things get dicey. As AccuWeather explains, higher dew points mean perspiration is slower to evaporate, making it difficult for the body to cool off; when the effects are prolonged, the danger of heat exhaustion and heatstroke rises.

Triple digits in the East?

The one-two punch of heat and humidity is anticipated to reach Eastern states by midweek, AccuWeather said, possibly lingering through the Independence Day weekend.

In Philadelphia and Washington, highs could hit 100 degrees for several days at least – well above their average highs in the mid to upper 80s.

New York City could also see temperatures reach the century mark as the weekend approaches. Newark, New Jersey, is even more likely to see triple digits. The effects for both will be worsened by humidity and cloud cover, with heat indices pushing 110.

The region isn’t expected to cool down until sometime during the holiday weekend, according to the weather forecaster.

Lingering Southwest wildfire conditions

Elsewhere, thunderstorms expected to track along the heat dome’s edges from the Rockies to the northern Plains, with others forming along the Gulf Coast, could bring temporary afternoon relief, AccuWeather said.

Meanwhile, in the short term, wildfire conditions are still critical for the Southwest, particularly in the Four Corners area, and portions of the Great Basin, the National Weather Service said. The agency’s office in Grand Junction, Colorado, said 40 to 50 mph wind gusts would produce another day of "strong fire behavior" with smoke from regional fires affecting air quality and visibility.

A shift in the jet stream over the Fourth of July weekend should force the heat dome to retreat to the Plains, finally ushering in cooler air for the sweltering East.