Heat Alerts Now Include Boston, NYC, Philly, D.C.

A heat dome of high pressure baking the South and Midwest is now spreading its reach into the Northeast to end the week, where feels-like temperatures will skyrocket into the triple digits.

The National Weather Service has issued extreme heat warnings and heat advisories for millions across the Southern Plains, Mississippi Valley, Ohio Valley, Great Lakes and parts of the Northeast. Those alerts include Boston, Cleveland, Detroit, Memphis, New York City and Washington, D.C., just to name a few cities.

Here are the main takeaways from the forecast:

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(TRACK THE HEAT: Temperatures | Heat Index)

How Hot Will It Get?

-Northeast: Increasing humidity and heat arrives on Thursday. By Friday, highs in the mid-90s will spread into the Northeast from Boston to New York City and Washington, D.C. A few daily record high temperatures will be threatened, potentially including New York City and Philadelphia. The heat index will be in the triple digits along the Interstate-95 corridor on Friday as well. Temperatures will drop over the weekend, especially north of the mid-Atlantic, but then another brief burst of summertime heat returns to all of the region early next week.

-Midwest: After Thursday, some brief relief will arrive for the southern Great Lakes, including Chicago and Detroit, as the Ohio and mid-Mississippi valleys continue to sear in feels-like temperatures near or above 100 degrees, including St. Louis. Over the weekend, a much larger area stretching as far north as the upper Midwest and Great Lakes will see temperatures and humidity spike once again.

-South: This is where the most long-duration, intense heat will be entrenched through the first half of next week. The Southern Plains and lower Mississippi valley are the epicenters for the worst heat as the weekend ends. But over the weekend and through middle of next week, highs will climb into the middle to upper 90s and even some low 100s across all of the South. The feels-like temperature will be well over 100 degrees in most of the region in that time. Some daily record highs are possible in the Southeast states this weekend, possibly including Raleigh and Jacksonville.

(MAPS: 10-Day US Forecast Highs and Lows)

How Long Will This Last?

- Unfortunately, this heat wave may have staying power in the South, so get used to the sweltering temperatures for the foreseeable future.

- After temperatures and humidity spike again in the Midwest and Northeast late this weekend into early next week, significant heat relief is expected by the middle of next week, according to longer-range outlooks. Temperatures could trend below average in those regions during that time.

(MORE: Should You Turn Off Your A/C Or Leave It On When You Leave Home?)

What You Should Do

- Heat is the No. 1 weather killer in the U.S., with hundreds of fatalities each year.

- The combination of heat and humidity lingering into the night for several days in a row is dangerous for those without access to air conditioning, especially in the cores of larger cities.

- Check on family, friends, the elderly and those with chronic illness who live without access to air conditioning.

- If possible, avoid being outdoors for long in the peak heat of the afternoon.

- If not, take frequent breaks, avoid strenuous activity, drink more water than usual and wear loose, lightweight clothing.

(MORE: Heat Advice From An ER Doctor)

Why So Hot?

- A strong bubble of high pressure aloft known as a heat dome has developed.

- Sinking air near the center of this heat dome suppresses clouds, allowing the intense sunshine this time of year to heat the ground and air above it most efficiently.

- Southerly winds on the western periphery of the heat dome pull increasingly hot and humid air north, then may circulate that oppressive air eastward on its northern rim, sometimes fueling clusters of thunderstorms with flooding rain and/or high winds.

Jonathan Erdman is a senior meteorologist at weather.com and has been covering national and international weather since 1996. Extreme and bizarre weather are his favorite topics. Reach out to him on Bluesky, X (formerly Twitter) and Facebook.