These US cities broke heat records on Sunday

Record high temperatures were set in at least four U.S. cities over the weekend amid a deadly heat wave that spanned much of the country.

The National Weather Service (NWS) warned on Sunday of "searing heat" across the Mid-Atlantic and northeast regions of the country, with 100 degree or higher temperatures also expected in Kansas, Oklahoma, southern Missouri and northern Arkansas this week.

Although most of the nation struggled with heatwaves starting last week, the northeastern U.S. bore the brunt over the weekend.

These cities broke records:

Boston

The NWS station in Boston, Mass., reported that Boston recorded a temperature of 99 degrees on Sunday, slightly higher than the 98 degree record set in 1933.

The local NBC affiliate reported that temperatures hit 100 degrees at Logan Airport.

"Sunday Scorchaaa in the City!" tweeted meteorologist David Bagley.

Providence

Providence, R.I., logged a temperature of 96 degrees Fahrenheit on Sunday, surpassing the previous high set in 1933 and tied in 1987, according to the Boston NWS station.

However, the city was still at least two days short of its all-time record of seven consecutive days above 90 degrees.

Newark

Newark, N.J., set a new daily temperature record at 100 degrees on Sunday. It was the fifth consecutive day of the temperature reaching 100 degrees in the city, creating the longest running streak ever recorded.

And Sunday had cooled down since earlier in the week. Temperatures hit 108 degrees in Newark on Friday.

Reading

Reading, Pa., also notched a high-heat record for the day, at 97 degrees.

Philadelphia, about an hour away, was expecting a possible record-breaking 100 degrees on Sunday, but fell short with temps topping out at 98 degrees, according to the NWS.

Heatwaves began spreading across the U.S. last week, putting more than 85 million Americans under an excessive heat watch or heat advisory.

Heat is the deadliest weather-related event in the U.S., responsible for killing 190 Americans last year, according to NWS.

Heatwaves, defined by a period of two days or more with unusually hot weather by the National Oceanic and Atmospheric Administration, are typically caused by trapped, warm air that simmers over a geographical area and bakes it like an oven. The trapped warm air is created by high pressure systems moving in and forcing warm air down.

In the south, Texas and other states are trying to stay cool as hot temperatures are expected to continue into the next week.

The Pacific northwest is also bracing for prolonged deadly heat waves this week, with NWS stations in Portland, Ore., and Seattle, Wash., readying for temperatures in the high 80s to low 90s through Friday.

In California, heat helped fuel the Oak Fire, which has burned through nearly 15,000 acres in Mariposa County and is zero percent contained, according to California's fire agency. California Gov. Gavin Newsom (D) declared a state of emergency in Mariposa County on Saturday.

Last week, record high temperatures were also set in Europe, including the United Kingdom and France.