Punishing heat waves are roasting the Pacific Northwest and central and eastern U.S. simultaneously, placing more than half of the Lower 48 states under alerts from the National Weather Service. Excessive heat warnings or heat advisories are in place for nearly 175 million Americans and some of these alerts 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 the “unprecedented” event in late June when Portland, Ore., soared to 116 degrees and Seattle hit 108, both all-time highs. Portland and Seattle are again under excessive heat warnings, 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.

In the central and eastern U.S., a sprawling area of heat advisories covers the zone from eastern Texas to southeast Michigan and large parts of the Mid-Atlantic and Northeast along the Interstate 95 corridor and parts of the interior New England. Cities under heat advisories include Little Rock, Chicago, Detroit, Raleigh, Washington and Boston. Several excessive heat warnings, for even more extreme conditions, are embedded within these heat advisory zones and include Kansas City, St. Louis, New York City and Philadelphia where the city asphalt and concrete, or urban heat island effect, make temperatures even hotter.

The dual heat waves are due to 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 while a second flanks the Southeast U.S. coast and is pumping hot air westward and northward.

Heat wave resources

More

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 exceptional heat events will strengthen in the coming decades.

Another ‘heat dome’

The heat wave commenced in parts of the interior Pacific Northwest on Tuesday when Medford, Ore., soared to 101 degrees, about 10 degrees above normal. It’s under an excessive heat warning through Saturday when high temperatures are expected to reach the triple digits.

Advertising

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

The heat is forecast to expand over Portland on Wednesday, with highs close to 100 degrees predicted until Friday, some 15 to 20 degrees above normal. Temperatures could actually be held back somewhat due to wildfire smoke; otherwise they could climb to near 105 degrees.

“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 on Thursday, when the Weather Service predicts a high near 94 degrees compared to an average of 78. Elsewhere in western Washington, the Weather Service is predicting highs of 99 in Olympia and 90 in Bellingham, both of which would be records for Aug. 12.

Advertising

Friday is forecast to be Seattle’s hottest day, with a high of 96, which would best the Aug. 13 record of 92 degrees. Temperatures then moderate into the upper 80s Saturday and more refreshing 70s on Sunday.

In Eastern Washington, Spokane 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 are 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 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.

Urban heat islands

It’s not just the Pacific Northwest experiencing yet another brutal scorcher. Much of the central and eastern Lower 48 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.

Advertising

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 of 100 degrees on Thursday. That would mark the first time the nation’s capital has hit the century mark 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 93 degrees, and Boston could hit 96, their hottest high since June. Both Boston and the Big Apple should remain equally hot Friday, though Washington D.C. may cool to “only” 97 degrees.

An excessive heat warning is in effect for New York City and the Tri-State area through Thursday, and 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.