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Pacific Northwest sizzles, a precursor to a nationwide heat wave

Next week an enormous heat dome may swell over the country

High temperatures on next Thursday as simulated by the National Weather Service. (Pivotal Weather) (Pivotal Weather)
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Heat alerts blanketed the Pacific Northwest on Friday, with 11 million people under excessive heat warnings, including residents of Seattle and Portland. Another 12 million are under heat advisories, as temperatures spend at least one more day peaking around dangerous levels.

It’s the prelude to an even bigger heat wave building across the rest of the Lower 48, which will bring sweltering conditions to much of the Plains, Midwest and Corn Belt as the calendar flips to August.

There are already signs that temperatures well over 100 degrees could be ubiquitous across the central United States by late next week, the raw temperatures combining with steamy summertime humidity to yield heat indexes pushing 110 degrees in spots.

Next week’s heat wave could be a long-duration event over the Central United States, potentially extending into mid-August.

Grueling heat in the Pacific Northwest

It’s been a hot week in the Pacific Northwest, and the heat isn’t winding down yet. Seattle hit 94 degrees on Tuesday, 91 on Wednesday and 91 on Thursday. Friday’s predicted high is 93 degrees, and Saturday is projected to hit 92.

Five days is the longest streak on record at or above 90 degrees in Seattle. That occurred in early July 2015. At present, Seattle is expected to tie that record on Saturday, but there’s a decent chance that Sunday could hit 90, too. That would be a record for longest stretch of consecutive 90-degree days. The National Weather Service is forecasting 88 degrees on Sunday.

The stretch could also be among the top 5 warmest five-day windows in Seattle. Based on the Weather Service’s current predictions, the five-day average high could be 92.6 degrees, tying with Aug. 9-13, 1977, for fifth place.

“Extreme heat will significantly increase the risk of heat-related illnesses for much of the population, especially those who are heat sensitive and those without effective cooling or adequate hydration,” wrote the National Weather Service in Seattle.

Portland is close to shattering its record longest streak at or above 95 degrees — six days, set in 1981 and 1941. So far it has been four days — 99 degrees on Monday, 102 on Tuesday (a daily record) and 96 on Wednesday and Thursday. Friday is projected to hit 99 degrees before reaching 101 on Saturday and 96 on Sunday.

The city of Portland could easily be hotter if a phenomenon called “downsloping” were occurring. That would involve air subsiding down the Cascades before going through what’s called “adiabatic compression” as the air heats up and dries out.

Instead, a “thermal low,” or a strip of low pressure induced by hot, rising air, is straddling the Cascades. That prevents downsloping but also fends off a cooling sea breeze. The result? Portland is caught in between a cooling influence and a heating effect, sitting right on the fence at about 100 degrees.

Farther inland, temperatures are even hotter — Kennewick, Wash., could be near 111 degrees on Friday, Saturday and Sunday. Temperatures will settle back to around the century mark by Monday.

In the northern reaches of California’s San Joaquin Valley, highs will peak around 115 degrees in spots. Heat alerts also spread into the Great Basin of Nevada, the Columbia River Basin and Idaho.

On Thursday, numerous locations in interior parts of Northern California, Oregon and Washington saw record highs of 100 to 115 degrees, including Medford, Ore. (111), Redding, Calif. (115), and Yakima, Wash. (109).

A heat dome, or a ridge of high pressure, is fueling the warmth — parked in the extreme northeast Pacific and adjacent western British Columbia. That brings hot and dry sinking air that squashes any cloud cover and allows further heating. Highs will settle back closer to seasonal norms by the start of next week.

A new heat dome to dominate the Lower 48

Next up on the nationwide weather map is an even bigger heat wave that’s set to bring sweltering weather to much of the Lower 48. Signs point to a virtually coast-to-coast event, with a sprawling heat dome that could exist for a week or more.

The heat dome will deflect the jet stream into Canada, allowing hot weather to build in to its south. The eastern half of the country will also be facing tropical humidity, with dew points nearing 70 degrees. That could lead to heat indexes in the 100-to-110-degree range.

The heat will begin to build markedly toward the middle of next week. Highs between 100 to 105 degrees will spread over Texas, Oklahoma, Kansas, Nebraska and Iowa on Wednesday; that could include Dallas, Fort Worth, Austin, San Antonio, Oklahoma City, Tulsa, Wichita, Kansas City, Omaha and Des Moines.

Upper 90s are more likely in the Upper Midwest, including Chicago, and throughout the Mississippi River Valley. On the East Coast, lower to mid-90s could reach all the way to the Mason-Dixon Line.

By Thursday, upper 90s could make it up the Hudson River Valley to near Albany, and Boston could be looking at a high around 96. Bangor, Maine, could end up around 91 degrees — somewhat of a rarity that far north.

There are signs that the heat could peak across the Lower 48 between Aug. 5 and 7, before consolidating over the Great Plains. Thereafter, active weather, in the form of severe thunderstorms, could roll through the Great Lakes, Northeast and Mid-Atlantic as “ridge running” thunderstorms crest over and ride along the northern periphery of the heat dome.