(NewsNation) — As summer gatherings ramp up and Americans head outdoors, the National Weather Service issued a warning Sunday about a heatwave sweeping across the American Southwest.

Dangerous heat over the weekend is forecast to continue across the Southwest before cooler temperatures arrive Monday, according to the NWS.

Phoenix, Las Vegas, Denver and California’s Death Valley all posted record temperatures Saturday.

The National Weather Service in Phoenix reported a temperature of 114 degrees Fahrenheit, tying the record high for the date set in 1918.

Las Vegas tied a record for the day set in 1956, with temperatures soaring to 109 F.

In Colorado, Denver hit 100 F, tying a record set in 2013 for both the high temperature and the earliest calendar day to reach 100 F.

Excessive heat warnings and heat advisories currently cover an expansive portion of the U.S., stretching from the Southwest to the Mississippi Valley, according to the NWS.

Another night of record-setting heat is expected across those areas, according to the NWS, with nighttime temperatures forecast to bottom out in the upper 70s and 80s in many locations.

The NWS said the heatwave will shift farther north and east, expanding across the Central Plains and into the eastern U.S., on Monday and Tuesday.

The temperatures forecast for Chicago the rest of the week are in the high 90s.

Heat is part of the normal routine of summertime in the desert, but weather forecasters say that doesn’t mean people should feel at ease. Excessive heat causes more deaths in the U.S. than all other weather-related disasters — including hurricanes, floods and tornadoes — combined.

Scientists say more frequent and intense heat waves are likely in the future because of climate change and deepening drought.