Why this summer may be especially hot in the United States (2024)

This forecast sets the stage for bouts of record-challenging high temperatures throughout the nation and the possibility of the hottest summer ever observed. In the central states and Rocky Mountains, the combination of heat and an expectation for drier-than-normal weather will increase drought potential. It will also raise the fire threat in some areas.

Weather Service officials and the Centers for Disease Control and Prevention on Monday stressed the importance of preparation for the heat as they unveiled a heat forecast guide that will be used across the United States for the first time this year. Already used for years in the West, the HeatRisk forecast will use a color-coded scale to describe the health risks expected from heat waves over the coming seven days.

“It’s giving us knowledge we can use to take steps to protect our health if we need to,” said Aaron Bernstein, director of the CDC’s National Center for Environmental Health. “We’ll be able to know how hot is too hot for health.”

While La Niña has a small cooling effect on the planet overall, it has boosted summer heat in the United States, especially in recent years when human-caused climate change has also fueled higher temperatures.

The three La Niña summers from 2020 to 2022 were all historically hot. The nation’s summer average temperature of 74 degrees in 2021 was tied for the hottest on record; 2022 and 2020 marked the third- and sixth-hottest summers, respectively.

“A common feature in summer during a developing La Niña is a semi-permanent upper-level ridge over the middle of North America,” wrote DTN, a forecasting company based in Minneapolis. “Ridges are notorious for hot and dry conditions.”

These ridges, referred to as heat domes, are common in summer but tend to be most persistent during La Niña.

A hot summer often goes hand in hand with drought, as high temperatures increase evaporation, which strips moisture from the land surface.

The driest weather compared to normal is expected to stretch from western Texas into the northern Rockies. The Weather Service predicts drought will persist or develop in much of this region.

Wetter-than-normal conditions are predicted in the Mid-Atlantic and Northeast, which are already off to a wet start this year. A combination of above-normal temperatures and rainfall in this zone could increase the threat of flooding.

The hot and dry conditions predicted in the Rockies would ordinarily support an elevated summer fire threat. However, large parts of this region are coming out of a wet winter, so current outlooks show limited areas of higher-than-normal fire potential.

While recent La Niña summers have been hotter than normal, the potential for a scorching summer is especially high when La Niña comes on the heels of a strong El Niño event, like we just experienced.

Of the eight strongest El Niño winters since 1950, the following summer was hotter than the summer before it by about 1.2 degrees. If that turns out to be the case this summer, it would rank as the hottest on record.

The forecast comes as the Weather Service seeks to improve public understanding of how and when extreme heat can pose health risks, especially for sensitive groups such as older adults, children, people with asthma and people who are pregnant.

There are many measures of extreme heat, such as the heat index — which factors in both air temperature and humidity levels — and the wet-bulb globe temperature — which adds in the effects of winds, sun angle and cloud cover. The HeatRisk forecast combines that meteorology with known information about the prevalence of heat illnesses in an area to determine risk categories on a color-coded scale ranging from green to magenta.

“This is a way to simplify things,” Weather Service Director Ken Graham said at a news conference Monday.

CDC Director Mandy Cohen added that “90 degrees in Miami is not the same as 90 degrees in Portland, Maine.” The HeatRisk scale “gives you a more complete picture of what heat might mean for you.”

Although the country is in the midst of a late-season cool spell, featuring frosty mornings from the Midwest to the Northeast, April will probably still become the 11th straight month of record-breaking warmth for the globe.

El Niño continues to exert a warming influence on global temperatures even while waning.

The National Oceanic and Atmospheric Administration projects that “it is virtually certain” that 2024 will rank among the 10 warmest years on record and gives 2024 a 55 percent chance of topping 2023 as the warmest year.

Jason Samenow contributed to this report.