A punishing heat wave is set to scorch millions across the West Coast, with forecasters warning of “lethal temperatures” and a rare lack of overnight cooling.

The most severe impacts are centered in Northern California, where an “Extreme Heat Warning” runs from mid-Sunday morning through Tuesday evening.

The alert covers the Sacramento Valley, Shasta County, and surrounding foothill regions, all facing the brunt of a prolonged heat surge.

Temperatures in the hardest-hit inland areas could climb as high as 110°F.

But what has officials especially alarmed is what happens after sunset: overnight lows are expected to remain in the 70s, offering little to no relief from the daytime heat.

That persistent warmth raises the risk of heat exhaustion and heat stroke, particularly for those without reliable cooling.

The National Weather Service says the region is facing a “Major Heat Risk,” meaning even healthy people can be affected if they are exposed for too long or unable to stay properly hydrated and cool.

The peak of the heat is expected Sunday and Monday, with only a modest easing beginning Tuesday. The National Weather Service is part of the National Weather Service.

Farther north, conditions are also turning dangerous.

The Portland metro area, along with southwest Washington communities including Vancouver and the Columbia River Gorge, is expected to see highs between 95°F and 100°F.

Overnight temperatures there will only dip into the low-to-mid 60s, limiting recovery time between hot days.

The National Oceanic and Atmospheric Administration warns that prolonged extreme heat without nighttime cooling can strain health systems, infrastructure, and essential services, particularly when exposure is widespread.

Officials stress that while inland California often sees triple-digit summer heat, the duration of this event combined with persistently warm nights makes it especially dangerous.

Heat illness can escalate quickly under these conditions, particularly for older adults, young children, and people without access to air conditioning.

Residents are being urged to take strict precautions: stay in air-conditioned or cooled indoor spaces whenever possible, drink plenty of water, avoid alcohol and excessive caffeine, and wear lightweight, loose clothing.

Authorities also advise limiting outdoor activity during peak afternoon hours, keeping blinds closed during the day, and ventilating homes overnight when temperatures allow.

People are also being told to check on neighbors who may be vulnerable and never leave children or pets inside parked vehicles.

Cooling centers, typically public spaces such as libraries and community buildings, are being made available for those without access to air conditioning, according to the California Department of Public Health.

Health officials are also warning of a less visible but serious threat: cold-water shock.

As people try to escape the heat in rivers and lakes, frigid snowmelt-fed waters can trigger rapid gasping, loss of coordination, and drowning risk within minutes, even for strong swimmers.

While the heat dominates the northern half of the region, Southern California faces a separate coastal hazard.