Millions Across U.S. Face Dangerous Heat

A dangerous, widespread heat wave continues to bake much of the central and eastern U.S., bringing the hottest temperatures of the year and uncomfortably muggy air from the Midwest and the Northeast that in some areas could last into much of this week.

Already, daily record highs were set Thursday in Salt Lake City (104 degrees), their first triple-digit high of the year. On Saturday, Mitchell, South Dakota, broke their daily high temperatures record at 104 degrees, smashing their previous record of 101 degrees. High temperature records were also set on Saturday across Minnesota and Wyoming. On Sunday, Marquette, Michigan, broke their daily high temperature record with a high of 93 degrees.

(MORE: Download The Weather Channel App To Track The Heat Dome)

Here are the main takeaways from the forecast:

Heat Alerts

- The National Weather Service has issued extreme heat warnings, watches and heat advisories for large swaths of the central and eastern U.S., as you can see in the map below. Some cities in these alerts, including Chicago, Kansas City, St. Louis, Detroit, Cincinnati, Pittsburgh, Philadelphia, Washington D.C. and New York City.

- The highest level heat alerts are now called "extreme heat" watches and warnings. You can find more about those changes here. These alerts indicate that heat illnesses are expected to spike when they are in effect.

(TRACK THE HEAT: Temperatures | Heat Index)

Heat Timing

Here's when we expect the worst of the heat:

- Mid-Mississippi and Ohio Valleys: Could last through much of this week

- Northern Great Lakes: Relief will arrive by Tuesday

- Northeast: Extreme heat through Tuesday. Relief will gradually arrive in New England and New York by Thursday.

-Mid-Atlantic and Appalachians: Heat could last much of this week

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

How Hot?

- Dozens of locations, especially in the Northeast, will be within striking distance of daily record highs, especially through Tuesday.

- A few locations could approach or top 100 degrees during the heat wave. Boston, New York City, Philadelphia and Washington, D.C. could flirt with the century mark during the peak of the heat wave early this week. Some of those locations could do so multiple days during the heat wave.

- New York's Central Park hasn't reached 100 degrees since July 18, 2012, according to the National Weather Service. Philadelphia hasn't reached 100 degrees in June since 1994. Boston's last 100-degree high was on July 24, 2022.

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

Humid, Too

- Oppressive humidity will accompany the heat wave.

- This will particularly be the case in the mid-Mississippi and Ohio Valleys, but also into the Great Lakes and mid-Atlantic and Northeast.

- That means the heat index, or feels-like temperature, will be in the triple digits on one or more days for some cities in the Midwest and Northeast early this week.

- This also means that overnight temperatures will not cool off or provide any relief from the daily heat.

(MORE: Why The Dew Point Temperature Matters)

What You Should Do

- Heat is the number one 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 could be 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 over the eastern U.S.

- 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 and southwesterly winds are also tapping hot and humid air from the South into the Midwest and Northeast.

- This heat dome will only slowly weaken and "flatten out" in the East in the week ahead.

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.