Two unforgiving heat waves are roasting opposite corners of the Lower 48, prompting the issuance of heat alerts for more than 150 million people. Excessive-heat warnings or heat advisories stretch nearly 1,500 miles, and some of these alerts will remain in effect until the weekend.

The most intense heat is set to roast the Pacific Northwest, which has already endured several blasts of abnormally high temperatures this summer, including an “unprecedented” event in late June when temperatures in Portland, Ore., soared to 116 degrees and Seattle hit 108, both all-time highs. Experts estimated the episode was made 150 times more likely thanks to human-induced climate change.

Portland and Seattle are again under an excessive-heat warning, which covers much of western Oregon and Washington state. It also extends into parts of Northern California, home to the Dixie Fire, the state’s second-largest blaze on record. At half a million acres burned, the fire is still only 30 percent contained and growing. The top eight largest wildfires on record in California have occurred in the past four years.

In the East and Midwest, a sprawling area of heat advisories covers the zone from East Texas and portions of the Mississippi Valley to southeastern Michigan and large parts of the Mid-Atlantic and Northeast along the Interstate 95 corridor and parts of New England.

Cities under heat advisories include Little Rock, Nashville, Raleigh, N.C., Indianapolis, Washington and Boston. Some areas could even see harsher conditions with “extreme” heat and humidity, prompting an excessive-heat warning for cities such as Kansas City, St. Louis, New York and Philadelphia. The cities face a greater risk since asphalt and concrete can trap heat and boost temperatures, a phenomenon known as the “urban heat island effect.

The dual heat waves are the result of a pair of high-pressure systems or heat domes flanking both U.S. coastlines. One is moving into the Pacific Northwest and British Columbia from the eastern Pacific Ocean, while the second skirts the southeastern U.S. coast and is pumping hot air westward and northward.

Climate change is intensifying the frequency and intensity of extreme heat as increasing concentrations of greenhouse gases from fossil fuel burning warm the atmosphere. On Monday, the landmark review of climate science from the Intergovernmental Panel on Climate Change warned that exceptional heat events will strengthen in the coming decades. A noticeable rise in heat events has already been observed. Heat is the leading cause of weather-related fatalities in the United States.

Pacific Northwest heat wave and fire risk

The heat wave commenced in parts of the interior Pacific Northwest on Tuesday when Medford, Ore., soared to 101 degrees, about 10 degrees above average. Wednesday’s reading reached 104 degrees, and Thursday could be hotter still, with a high of 105. The city is under an excessive-heat warning through Saturday, when high temperatures are expected to settle back into the upper 90s.

This heat wave is not nearly as intense as the historic blast in late June, but it would be a notable event in most other summers.

The heat is forecast to peak in Portland on Thursday, with highs near or at 100 degrees, some 15 to 20 degrees above average. Temperatures could actually be held back somewhat due to wildfire smoke; otherwise they would climb to near 105 degrees in the Willamette Valley.

“With the widespread heat expected, there is an enhanced risk to sensitive groups. Drink lots of water, check on neighbors, friends and family often, and locate to a cool space during the peak heating time frames,” the Weather Service in Portland wrote.

Portland will see temperatures gradually moderate over the weekend. The excessive-heat warning remains in effect until Saturday, with forecast highs in the mid-90s, but temperatures will slip into the 80s by Sunday.

Instigating the heat is a sprawling high-pressure system colloquially known as a “heat dome.” It will remain centered offshore of the Pacific Northwest, bringing sinking, warming air and a flow out of British Columbia. That pumps in hot air from over the land, reinforcing the anomalously toasty temperatures and helping subdue the cooler marine layer lapping at the coast.

In Seattle, the heat begins in earnest Thursday, when the Weather Service predicts a high near 91 degrees, compared to an average of 78. Friday could see a high of 94. Elsewhere in western Washington state, the Weather Service is predicting highs of 96 in Olympia and 92 in Bellingham, both of which would be records for Aug. 12.

Friday is forecast to be Seattle’s hottest day, besting the previous Aug. 13 record of 92 degrees. Temperatures then moderate into the upper 80s Saturday and more refreshing 70s on Sunday.

Spokane, Wash., is predicted to see highs close to 100 degrees through Saturday and is under an excessive-heat warning.

The combination of high temperatures and low humidity is expected to elevate the wildfire risk in Northern California and interior Oregon and Washington for the next several days. The Weather Service in Medford wrote that its index for predicting fire plume growth is in the 95th percentile or higher through Thursday. In addition, a few dry thunderstorms that emit lightning but little rain are possible, which could ignite new blazes.

Heat wave in the Central U.S. and East

It’s not just the Pacific Northwest experiencing yet another brutal scorcher. Much of the Midwest and East Coast is stuck beneath another heat dome languishing and delivering a summertime steam bath. While this high-pressure ridge is less impressive from a meteorological standpoint, it’s more strategically positioned. That helps it to truck a soupy, sultry air mass northward.

The out-of-bounds system, often referred to as the “Bermuda High,” is banked unusually far west — helping clockwise winds to spread a southerly breeze over most of the eastern half of the nation.

Heat advisories blanket most of the Midwest and Mississippi Valley, where the combination of exceptional heat and tropical humidity will yield heat index values well into the 100- to 105-degree range. In cities, paved surfaces, concrete and other materials associated with the “urban heat island” effect could preclude nighttime lows from falling appreciably, posing a danger to vulnerable populations without access to adequate cooling.

That’s why places like Kansas City and St. Louis are under their own localized excessive-heat warnings. Kansas City can expect “dangerously hot conditions with heat index values up to 110 degrees,” according to the Weather Service. “Overnight lows will only dip down to about 80 degrees.”

The heat will be equally unforgiving east of the Appalachians, too. Washington is projected to see a high close to 100 degrees on Thursday. If it manages to hit the century mark, it would be the first time since Aug. 15, 2016.

That’s dependent on the exact wind direction, since the sensor is located at Reagan National Airport, adjacent to the Potomac River. A westerly wind would keep any breeze off the river at bay.

New York City is looking at a high of 95 degrees, and Boston could hit 97, their hottest high since June. Both Boston and the Big Apple should remain equally hot Friday, although Washington, D.C., may cool to “only” 96 degrees.

An excessive-heat warning is in effect for New York City and the Tri-State area through Thursday. It warns of “extreme heat and humidity” and “dangerously hot conditions with heat index values up to 108 degrees.”

Some semblance of moderation will come Saturday when the Interstate 95 corridor will climb into the upper 80s rather than 90s as a weak cold front swings through.