Dangerously High Temperatures Blanket United States, Expected to Last Into Next Week

— -- A prolonged heat wave is blanketing much of the United States, with dangerously high temperatures believed to be responsible for at least seven deaths, including that of a 4-year-old girl.

The girl died in Williamsport, Pennsylvania, after being left for hours in a hot car as temperatures hit 97 degrees, police told ABC affiliate in Scranton WNEP.

There have also been five heat-related deaths since Wednesday in Roseville, Michigan, where temperatures soared into the high 80s and low 90s. Roseville Fire Chief Mike Holland told the Macomb Daily that five people have died from heart attacks or breathing difficulties, all linked to weather over the past three days.

The high temperatures are also creating deadly conditions in Memphis, Tennessee, where a man was found dead inside his home on Friday. The investigation is ongoing, but police believe the man’s death was heat-related, according to ABC local affiliates.

Heat indices over 100 degrees Fahrenheit were felt across the eastern half of the nation today and are expected to go as high as 100 in New York City and to stay above 80 degrees in urban areas tonight, meteorologists said.

Boston on Friday hit 98 degrees -- the city’s hottest temperature since July 2013.

Sunday is forecast to be slightly cooler in the north and southeast regions of the U.S., with less humidity and temperatures in the low to high 90s. But the worst is yet to come as scorching temperatures are expected to return next week. The feel-like temperature on Monday afternoon is expected to surpass 100 outside of New York City and push to 110 degrees from Philadelphia to Richmond, meteorologists said.

An excessive heat warning is in effect through Monday for northern Delaware, central and southern New Jersey and southeast Pennsylvania.

With the Democratic National Convention set to start in Philadelphia on Monday, officials in that city have put in place a heat plan to help ensure visitors and protesters stay safe and hydrated, according to ABC local affiliate WPVI.

High humidity is exacerbating the extreme heat in the middle of the country.

In Little Rock, Arkansas, the temperature reached 105 degrees Friday -- tying the record for the hottest July 22 on file in the Arkansas capital city and the hottest day overall in the state since 2012. Local hospitals have seen a rising number of people checking in for heat exhaustion symptoms since the beginning of summer, according to ABC local affiliate KATV.

In Illinois, an excessive heat warning for Ford, Grundy, Iroquois, Kankakee, La Salle, Lee, Livingston, Ogle counties is expected to expire at 7 p.m. today. For the Chicago area, maximum heat indices were near 111 degrees this week.

And in Indiana, a heat advisory for Benton, Jasper and Newton counties is expected to expire at 7 p.m. today, according to ABC affiliate in Chicago WLS.

The heat wave is lingering in the southwest today, with temperatures remaining in the triple digits in parts of southern California. Palm Springs could hit 118 degrees, while the Antelope Valley could get up to 109. Los Angeles and Orange counties will see temperatures around 94 degrees. The area will cool slightly on Sunday, remaining in the mid-80s.

Southern California's valleys and Inland Empire will also be brutally hot today, with high temperatures potentially reaching 107 degrees before lowering slightly to the high 90s over the next week, according to ABC affiliate in Los Angeles KABC.

Amid the extreme heat, a fast-moving brush fire broke out Friday afternoon in the hillsides of Santa Clarita, just north of Los Angeles. It had charred 5,500 acres with no containment early Saturday morning, Los Angeles County Fire Department officials told KABC. Mandatory evacuations for some 300 homes in the area were in effect. No injuries or damage to property have been reported so far, KABC reported.

ABC News’ Daniel Manzo and Jessica Ptashenchuk contributed to this report.