PHOENIX (AP) — Extreme heat spread across Arizona, New Mexico and parts of Texas, Colorado and Kansas as severe weather swept across many parts of the U.S. on Sunday. There was unseasonable cold in the Pacific Northwest, snow headed to the northern Rocky Mountains and heavy rainfall forecast from the northern Plains to the Upper Midwest.

The National Weather Service estimated that more than 63 million people were under heat advisories on Sunday, stretching from the Southwest northward up through Denver and into Chicago.

Temperatures in Phoenix, which hit 112 degrees Fahrenheit (44.4 Celsius) on Saturday, were expected to reach close to that on Sunday. Weather service forecasters say the first two weeks of June in Phoenix already have been an average of 5.6 degrees Fahrenheit hotter than normal, making it the hottest start to June on record.

“We have already seen some pretty significantly high temperatures in our area,” said Ted Whittock, a meteorologist with the National Weather Service in Phoenix. “We are recommending that everyone reduce their time outdoors between 10 a.m. and 6 p.m., stay hydrated and wear light, looser fitting clothing.”

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Whittock said the heat in metro Phoenix will ease a bit Monday through Wednesday, with the highs pushing back up as the week progresses, likely prompting another excessive heat warning.

The heat has been especially dangerous in recent years in metro Phoenix, where 645 people died from heat-related causes in 2023 — a record.

The city and Maricopa County have adopted additional measures this year in hopes of keeping people safer, including two new overnight cooling centers where people can rest in air conditioning after the sun goes down. There are more than 100 other cooling centers that have been open since May 1 where people can get cold water and sit in a cool space during daytime hours.

In neighboring New Mexico, a heat advisory was in effect over the weekend for the Chavez County plains including Roswell, where the high was forecast to hit 107 degrees F (41.6 degrees C) on Monday. The high for Albuquerque was forecast for 99 degrees F (37.2 C) on Sunday, cooling slightly to 96 degrees F (37.6 C) on Monday. Highs were expected to approach 105 F (40.6 C) in El Paso, Texas, which has now opened five cooling centers.

Temperatures from the 90s to nearly 100 degrees F (37.7 C) were expected in metro Denver and areas to the south. Thunderstorms were possible in communities north of Denver.

The heat wave was moving eastward Sunday into the Plains and the Great Lakes area and was expected to arrive in the Northeast by Tuesday. The threat of thunderstorms with potential high winds and heavy rainfall was increasing in the Chicago area, even as heat indices were forecast to reach near 100 degrees F (37.7 C) through the middle of the week.

As the heat wave spreads eastward, temperatures in Washington and the rest of the mid-Atlantic as well as New England were likely to see highs in the mid- to upper 90s as the week goes on, with excessive humidity making it feel even more oppressive.

The U.S. last year saw the the most heat waves, consisting of abnormally hot weather lasting more than two days, since 1936.

While much of the country swelters, late season snow was forecast for the northern Rockies Monday and Tuesday. Parts of Montana and north-central Idaho were under a winter weather watch, with as much as 6 inches (15 centimeters) of heavy, wet snow expected in the mountains around Missoula, Montana. As much as 20 inches (51 centimeters) was predicted for higher elevations around Glacier National Park.

Meanwhile, a fresh batch of tropical moisture will bring an increasing threat of heavy rain and flash flooding to the central Gulf Coast late Sunday into Monday. Heavy rain is expected to start Monday morning, with the moisture shifting toward the Gulf Coast by Tuesday.

The intense flooding from heavy rains continued to dissipate in southern Florida, where some areas in and around Miami and Fort Lauderdale were left underwater in recent days as storms dumped up to 20 inches (50 centimeters).

That unnamed storm system coincided with the early start of hurricane season, which this year is forecast to be among the most active in recent memory.