A weeklong heat wave that officials believe has killed at least 25 people continued Sunday, with 40 million people under heat alerts across the East Coast, Southeast and Southwest.

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At least 18 record highs were set on the Fourth of July as celebrations kicked off in cities including Washington, D.C.; Baltimore; Raleigh, North Carolina; Norfolk, Virginia; and Atlantic City, New Jersey.

On Sunday, alerts will continue into the evening on the East Coast as heat index values reach 100 to 105 in cities including Philadelphia, Washington, Baltimore and Raleigh, as well as Charleston, South Carolina, and Jacksonville, Florida. Heat index values reflect what the temperature feels like to the human body.

Temperatures will finally start to drop this week, with highs generally in the 70s to low 90s throughout the East Coast.

In the Southwest, extreme heat watches cover parts of Arizona and California, including Phoenix and Tucson, and go into effect Tuesday through Thursday. Dangerously hot conditions are expected through midweek, with daytime highs reaching as high as 114 degrees.

At least 25 people have reportedly died in the ongoing heat wave, with most of the suspected deaths in New Jersey, where 22 people have died across 10 counties, according to the state’s Department of Health.

Two deaths have been reported in Hinds County, Mississippi, and one in Cook County, Illinois, officials said.

In New York City, more than 378 people have visited emergency rooms for heat-related illnesses, according to the city’s Health Department.

Summer storms

Saturday’s storms produced more than 540 damaging wind gust reports across the central and eastern United States. Some of the strongest gusts reported include 92 mph in Norman, Oklahoma, and 87 mph in Suffolk County, New York, according to the National Weather Service’s Storm Prediction Center.

Sunday’s forecast calls for widely scattered showers and thunderstorms across parts of the Plains, Southeast and mid-Atlantic, where 25 million people are under storm alerts. This risk area includes Philadelphia, Washington, Baltimore and Arlington, Virginia, where conditions are favorable for storms to produce damaging wind gusts over 70 mph and quarter-sized hail. Tornado risk remains relatively low nationwide.

Though some storms may not be severe, scattered thunderstorms still pose a threat of frequent lightning and could cause travel delays Sunday afternoon and evening at major airports in Dallas; Nashville, Tennessee; Chicago; Cincinnati; Charlotte, North Carolina; Atlanta; New York; Philadelphia; and Washington.

Through Monday, 34 million people from Delaware to Connecticut are under flood alerts, though the severe weather risk is noticeably lower. Slow-moving storms carrying ample moisture will raise the risk of localized flash flooding, especially across the mid-Atlantic.

Storms could bring rainfall rates of 2 inches per hour, raising the risk of urban flash flooding in cities including Philadelphia, New York and Hartford, Connecticut. In total, 2 to 8 inches of rain is possible through Monday.