Heat wave engulfs East Coast with temperatures approaching 100 degrees

By Saturday, temperatures are predicted to surge into the 90s for millions along the heavily populated Interstate 95 corridor from Richmond to Boston. The scorching temperatures — about 20 degrees above normal — combined with moderately high humidity will increase the risk of heat-related illness.

When the heat peaks on Saturday, scores of daily record highs are predicted; a few locations in the Northeast could register their highest temperature ever observed during May and their hottest weather recorded so early in the calendar year. Some locations in the interior Northeast could see temperatures higher than they typically see through an entire summer.

Heat advisories are in effect for much of the Acela Corridor in the Northeast, including Philadelphia, Newark, parts of the New York City metro, Hartford, Conn., Providence, R.I., and Boston, where heat indexes — reflecting how hot it feels factoring in the air temperature and humidity — could range between 100 and 105 degrees. It’s the first heat advisory issued during the month of May by the National Weather Service in New York dating back to at least 2006; the same is true for the Boston office.

The hot weather is coinciding with a busy weekend for outdoor activities, including many college commencements and the 147th running of the Preakness Stakes in Baltimore, where record highs in the mid- to upper 90s are expected.

The first instance of excessive heat is often the most dangerous as people are not yet acclimated to high temperatures. Outdoor workers, older adults, young children and people without access to air-conditioning are most vulnerable to heat-related illness.

“The abrupt beginning of hot temperatures early in the season after a relatively cool spring brings an increased risk of heat illnesses unless proper precautions are taken,” the National Weather Service serving Washington and Baltimore wrote in a special statement. “Reschedule strenuous activities to early morning or evening. Plan to wear lightweight and loose fitting clothing, and seek shade from the sun. Schedule frequent rest breaks in shaded or air-conditioned environments.”

The spell of hot weather is arriving just as the National Weather Service issued its summer temperature outlook, which projects substantial odds of hotter than normal conditions for much of the Lower 48 states and Alaska. The Northeast faces some of the highest odds of abnormally hot weather. In other words, the weather through the weekend may be a mere sneak preview of punishing heat to come.

A staggering 125 million Americans are slated to experience temperatures topping 90 degrees through Sunday. Already the heat has gathered across the southern United States, the product of sinking air, high pressure and a “zonal” west-to-east jet stream that’s allowed toasty temperatures to build northward.

Parts of Central Texas made it above 105 degrees on Thursday, and mid- to upper 90s were ubiquitous across the South. Dallas is having its hottest May on record so far, and Abilene has seen a record number of 100-degree days during the month. San Antonio has notched more 100-degree days this May than it did in the entirety of last summer.

Now the air flow ahead of a strip of low pressure approaching from the Midwest and Ohio Valley will allow the skyrocketing temperatures to surge up the East Coast, with 90s from the Gulf Coast of Florida to northern Vermont and the Canadian border.

The heat will reach a climax on Saturday before being shunted to the immediate Interstate 95 corridor ahead of a cold front on Sunday. By Monday, highs about 15 to 20 degrees cooler — more commensurate with seasonal norms — will return.

In Boston, a 91-degree high is forecast for Saturday and 94 degrees on Sunday. But just a few miles inland in the Merrimack Valley, highs might climb into the upper 90s. It’s not out of the question that, with westerly winds and downsloping air, one or two locations may nick 100 degrees. Downsloping is the process by which air descending from a high elevation to a low one compresses and warms, drying out and heating up further.

That’ll likely net a record in Boston for Sunday, where the current number to beat is 93 degrees, set in 1959.

Hartford is expected to hit 97 or 98 degrees on Saturday and 96 on Sunday. That will smash daily records if realized and threaten an all-time monthly record of 99, last set in 2010. The average high for mid- to late May is 73 degrees.

Average high temperatures during the month of May in Hartford have risen 1.5 degrees since 1950, and 90-degree days are nearly twice as common, boosted by human-caused climate change.

Worcester, Mass., is slated to shatter its monthly record two days in a row. Since bookkeeping began in 1892, the city hasn’t jumped above 94 during the month of May. It is forecast to hit 97 on Saturday and 97 on Sunday.

Meanwhile, the New York City metro area will see highs climb into the upper 90s just west of the city on Saturday and Sunday. Central Park might hover around 91 degrees, falling shy of the 93 and 96 records for Saturday and Sunday, respectively, that have stood since 1996.

In D.C., a high of 94 is projected for Friday and 96 on Saturday. Sunday will be a hair cooler, likely in the lower 90s. Saturday’s reading could snag a record and, if it reaches 97 degrees, become the hottest temperature observed in D.C. so early in the year.

Philadelphia also has a strong chance to break its record of 95 Saturday.

Across the Southeast, upper 90s are expected on Friday, with a few 100-degree readings possible in central North Carolina. Even at the beaches, Wilmington, N.C., could reach 95 degrees. Mid- to upper 90s will fill most of the Gulf Coastal states and reach all the way west to Texas.

Temperatures will finally drop after this weekend, as a strong cold front plows through the region.

Signs point to a renewed warm-up into the middle of next week.