The chaotic weather map includes the possibility of severe thunderstorms developing in between hot and cold fronts. Forecasters said the colliding fronts could lead to areas of flash flooding between eastern Nebraska and northern Wisconsin on Saturday night, as well as strong storms across parts of eastern Montana into North and South Dakota.

Meanwhile a plume of tropical moisture will reach the central Gulf Coast during the next couple of days, with heavy rain expected to start Monday morning, according to the National Weather Service.

Forecasters said the threat of heavy rains in Florida continues to dissipate, but some thunderstorms could cause local flooding given the already saturated soil. Some areas between Miami and Fort Lauderdale were left underwater in recent days as storms dumped up to 20 inches (50 centimeters) in southern parts of the state.

The damaging no-name storm system coincided with the early June start of hurricane season, which this year is forecast to be among the most active in recent memory amid concerns that climate change is increasing storm intensity.

Across much of the southern parts of the country, temperatures were rising Saturday.

In Atlanta, where highs were forecast to near 100 degrees Fahrenheit (38 degrees Celsius) both weekend days, city officials opened a cooling center to provide relief. The city also postponed a “Family and Friends Field Day” due to the heat.

And in the west Texas city of El Paso, Saturday highs were expected to approach 105 F (40.6 C), and the National Weather Service issued a heat advisory through Monday morning for the region. The city has opened five cooling centers that will operate daily until further notice.

The National Weather Service said temperatures in Phoenix, where an excessive heat warning was in effect, were forecast to reach 113 F (45 C) on Saturday afternoon. That would be short of the record for June 15 — 115 F (46 C) — set in 2021.

Though Arizona is entering its three-month monsoon season, when a shift in wind patterns typically pulls moisture in from the tropical coast of Mexico, no rain is forecast for most of the coming week.

“No chances of rain across the state,” National Weather Service meteorologist Ted Whittock said, while noting that there is a 30% chance of showers in southeastern Arizona next Friday.

In Tennessee, tens of thousands of revelers at the Bonnaroo Music & Arts Festival braved a hot, sunny weekend to take in more than 150 performances at the 700-acre farm campground and concert venue that hosts the annual event. While medical crews treated various heat-related conditions, some fans constructed elaborate canopy and tent combinations for shade. Others had their sunscreen confiscated upon entry because of restrictions on full-size bottles and aerosol cans, The Tennessean reported.

Temperatures in the Mid-Atlantic and New England will likely peak in the mid- to upper 90s next week, which is “nothing to sneeze at even in the middle of the summer, let alone this early in the summer,” National Weather Service meteorologist William Churchill said. The high humidity will make it feel even hotter in many places, he added.

Last year the U.S. had the most heat waves — abnormally hot weather lasting more than two days — since 1936. In the South and Southwest, last year was the worst on record, according to the National Oceanic and Atmospheric Administration.

Next week’s heat wave will ramp up Sunday in the center of the country before spreading eastward, the National Weather Service said, with some areas likely to see daily record highs. The heat wave could last into the weekend in many places.

While most of the country bakes, parts of Montana have been placed under winter storm watches with a potential for wet snow Monday night.

Churchill said the northwestern cold front is connected to the heat wave because one extreme is often accompanied by the other.

Heavy rain and sporadic thunderstorms were expected in western Washington into Saturday evening. In Edmonds, where an outdoor art festival was underway, the National Weather Service warned in the early afternoon that a stronger thunderstorm was headed in.

Hikers and other outdoor enthusiasts in the state’s Cascade Mountains and Montana’s Rocky Mountains were also likely to see heavy snow at lower elevations. The National Weather Service issued a winter weather watch for north-central Idaho and western Montana from Sunday through Tuesday.

The agency warned of a hypothermia risk and said backcountry roads could become impassible due to the expected snowfall and possible downed trees and branches.

As much as 6 inches (15 centimeters) of heavy, wet snow was expected in the mountains around Missoula, Montana, and as much as 20 inches (51 centimeters) was predicted for higher elevations around Glacier National Park.