A sprawling heat dome settled over the central and eastern United States on Wednesday, placing more than 200 million people under extreme heat alerts, according to the National Weather Service (NWS). Air temperatures were forecast to reach 100°F (38°C) in multiple cities, with humidity pushing the heat index as high as 115°F (46°C) from the Mississippi Valley to the Northeast. The NWS warned of a “high risk” of heat-related illness, particularly for the elderly, children, and those without adequate cooling. In Illinois, several roadways buckled under the thermal stress, local authorities reported, while three firefighters died battling wildfires that erupted in several states, according to federal monitoring agencies.

Across Mexico, the national meteorological service (SMN) attributed heavy to torrential rains across 25 states to two cyclonic circulations interacting with low-pressure channels. Accumulations of 50–75 mm were expected in a dozen states, with isolated totals reaching 150 mm in southern Veracruz. The SMN cautioned that the downpours could trigger landslides, river flooding, and flash inundations in low-lying areas, while a separate heatwave maintained temperatures above 40°C in Sonora and Chihuahua.

In South America, contrasting extremes took hold. Argentina’s SMN issued a yellow alert for severe storms in the northeastern provinces of Misiones, Corrientes, and Formosa, where hail, intense electrical activity, and rainfall of up to 70 mm were anticipated. Simultaneously, a cold front brought a sharp temperature drop to twelve central and western provinces, with snow alerts—orange in Mendoza, yellow in Neuquén—and wind gusts exceeding 100 km/h in the high Andes. In southern Brazil, civil defence officials warned of persistent heavy rain and severe thunderstorms across Rio Grande do Sul, Santa Catarina, and southwestern Paraná, driven by a stationary front and a deepening low-pressure system; river overflows and power outages were reported in several municipalities.

Emergency measures were activated in major US cities. New York Mayor Zohran Mamdani announced an “unprecedented” heat emergency plan, opening hundreds of cooling centres and deploying mobile units to distribute water and conduct wellness checks. Chicago, Philadelphia, and Washington D.C. took similar steps, with the capital bracing for three consecutive days near 100°F during Independence Day celebrations. Authorities across the affected regions urged residents to limit outdoor activity, stay hydrated, and monitor vulnerable neighbours. The NWS said the heatwave would persist through the holiday weekend, while meteorological agencies in Latin America maintained their alerts as the unstable weather patterns remained in place.