Heat dome bakes millions across northern US before pushing New York, Philadelphia temps toward triple digits

First major heat wave of the season on the way

Millions of Americans could see temperatures above 90 degrees with heat indices in the triple-digits this weekend. High temperatures stretch from Colorado into the Northeast.

WASHINGTON – Millions of people across the northern tier of the U.S. are under heat alerts as a sprawling high pressure system, also known as a heat dome, is forecast to push temperatures in the Northeast toward the triple digits just as astronomical summer arrives.

According to the FOX Forecast Center, the stout system will move eastward across a large swath of the country. This will bring sweltering heat to major cities from Chicago to Detroit to the major urban centers along the Interstate 95 corridor on the East Coast this weekend.

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The prime time for this heat wave will be from Sunday through Thursday, when hundreds of millions of Americans will experience temperatures ranging anywhere from a few degrees above average to 20 degrees above average.

While relatively few locations may actually hit the 100-degree mark, the combination of high temperatures and elevated humidity levels will make it feel significantly hotter. 

Heat indices, which measure how the temperature feels to the human body when factoring in humidity, are forecast to soar well into the triple digits in many areas.

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Among the areas expected to be affected are Denver, Chicago, Detroit, Pittsburgh, Philadelphia, New York City and Washington.

In Denver, temperatures are anticipated to reach the upper-90s on Thursday and climb past 100 on Friday. Triple digital temps will remain for Denver on Saturday.

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New York City has only reached the 100-degree threshold three times during the month of June, while Philadelphia has done so nearly a dozen times, with the most recent instance occurring in the mid-1990s.

The National Weather Service is expected to issue a range of heat-related alerts in the Northeast in the coming days, but a Heat Advisory and Extreme Heat Watch have already been issued for millions of people from Colorado to the Canadian border in Minnesota.

The alerts are intended to warn residents about the dangers of prolonged exposure to high heat, which can lead to serious health impacts such as heat exhaustion, heatstroke and dehydration.

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Heat waves pose a particularly high risk to vulnerable populations, which include children, the elderly, outdoor workers and those without access to air conditioning. 

In communities with heat response plans, cooling centers are usually opened for residents to take advantage of when temperatures reach dangerous levels.

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Temperatures are expected to return to more typical levels by the final days of June and early July as the ridge of high pressure treks eastward, allowing for the entrance of frontal boundaries, which will help induce more cloud cover and rainfall.

The heat likely won’t be gone for long, as the hottest days of the season usually occur in mid- to late July, as the effects of the year’s direct sunlight and solar radiation are felt across the Northern Hemisphere.

This map shows when the hottest day of the year is expected to occur based on the 1991-2020 climatological averages from NOAA's National Centers for Environmental Information.

(FOX Weather)