(Bloomberg) -- Brace yourself for the hottest day of the year. It could reach 100 degrees Fahrenheit (38 Celsius) on July 20 as the U.S. East Coast slides into the weekend. When you add in the humidity, it could feel like 110.

The East Coast isn’t alone, according to the National Weather Service. A “dangerous and widespread” heat wave is forecast for two-thirds of the U.S., with a high pressure dome holding the heat in place. At night, temps will struggle to fall below 80.

The heat wave will stress Midwest crops and boost energy demand, including in New York where an outage darkened part of Manhattan last weekend. Anyone outdoors should stay hydrated and seek shade when possible, the elderly and sick should be monitored and pets and children shouldn’t be left unattended in cars, the weather service warned.

Chicago could peak at 98 degrees Fahrenheit Friday, with New York’s Central Park reaching that on Saturday. Washington will touch at least 100 this weekend.

“Excessive heat warnings cover a pretty considerable number of states today,” said Alex Lamers, a forecaster with the U.S. Weather Prediction Center in College Park, Maryland. “Then we are expecting the heat wave to ramp up on the East Coast in the next couple of days.”

The high heat will test regional electric grids from New York to Illinois, with extended power demand likely to “stress the grid like it hasn’t been stressed in a long, long time,” according to Jim Rouiller, chief meteorologist at the Energy Weather Group near Philadelphia.

“Somebody in Brooklyn is going to touch 100,” he said.

Shallow Roots

For agriculture, the floods of the spring that delayed U.S. planting have meant plants have shallower roots and this is exacerbating the impact of the heat.

Normally, it would take four or five days of temperatures in the 90s to stress crops, according to Don Keeney, senior agricultural meteorologist at Maxar. “When the root systems are as shallow as they are this year it only takes a day or two,” he said.

By July 25-29, the heat will begin to ease as cooler air drops out of Canada across the Great Lakes, with temperatures ranging 1 to 2 degrees warmer than normal across the Ohio Valley, mid-Atlantic and South.

While the jump in air conditioning use will drive demand for natural gas to fuel power plants, record shale production from basins across the U.S. is refilling stockpiles at a rapid clip, allaying concern about a potential winter supply crunch and helping to keep prices near a three-year low.

In addition to the U.S., heat warnings are also in place across New Brunswick and Nova Scotia, according to Environment Canada.

(Adds potential stress on East Coast electric grid in sixth and seventh paragraphs.)

--With assistance from Joe Richter.

To contact the reporter on this story: Brian K. Sullivan in Boston at bsullivan10@bloomberg.net

To contact the editors responsible for this story: Luzi Ann Javier at ljavier@bloomberg.net, Reg Gale, Steven Frank

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