Heat alerts issued; much of U.S. bakes as temperatures rise

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 already has 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 excessiveheat warning, which covers much of western Oregon and Washington state. It also extends into parts of Northern California, where the Dixie Fire, the state’s second-largest blaze on record, rages. At a half-million acres burned, the fire is still only 30% contained and growing. The 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 eastern 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, Tenn.; 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, Mo.; 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 already has been observed. Heat is the leading cause of weather-related fatalities in the U.S.

The heat wave commenced Tuesday in parts of the interior Pacif ic Northwest when Medford, Ore., soared to 101 degrees, about 10 degrees above average. Wednesday’s reading reached 104 degrees, and Thursday forecast 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 was forecast to peak Thursday in Portland, 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.

In Seattle, the heat was to begin in earnest Thursday, when the weather service predicted a high near 91 degrees, compared with an average of 78. Today could reach a high of 94.

Today 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.