Forecasts for a heat wave of historic proportions in the Pacific Northwest have solidified, and a consensus is building among meteorologists that this could rank among the most extreme events the region has ever seen.

National Weather Service forecast offices in Portland, Ore., Seattle and Spokane, Wash., have all used the word “unprecedented” to describe the expected heat and are warning about its potentially lethal effects, considering significant portions of the population lack air conditioning.

Numerous cities are predicted to approach or surpass their hottest weather ever recorded in June and probably any calendar month. Washington, Oregon and Idaho could challenge state highs, which are close to 120 degrees, according to the National Weather Service.

With projected temperatures 15 to 30 degrees above average, the heat wave will be exceptional for its intensity. The duration, with triple digit afternoon highs and unusually warm nights lasting three to seven days, will also be brutal.

Excessive heat watches and warnings affect over 13 million people, covering parts of northern California and western Idaho and much of Oregon and Washington, except for areas right along the coast. The excessive heat warning in effect for the area around Portland calls for “dangerously hot” temperatures Saturday through Monday.

The temperature in Portland is predicted to reach 109 degrees on Sunday, which would break its all-time record of 107 degrees.

In Seattle, predicted highs Saturday, Sunday and Monday are 95, 99 and 99 degrees and the Weather Service says 100-plus can’t be ruled out. If Seattle manages to reach triple digits, it will mark just the fourth time on record. It needs to exceed 96 degrees to establish a new heat record for the month of June.

While extreme heat is expected to last about three or four days in Portland and Seattle, it could persist for up to a week in eastern Washington.

The event “could rival some of the longest lasting and extreme heat waves in the recorded history of the Inland Northwest,” wrote the National Weather Service in Spokane.

Spokane is predicted to see temperatures top 100 from Saturday through at least July 1. Temperatures are forecast to peak around 110 degrees Monday and Tuesday, which would break its all-time record of 108 and potentially shatter its June record of 105 multiple times.

If triple-digit heat endures as long as predicted in Spokane, it will match or break the city’s record for most triple-digit days in a row: six.

While Portland, Seattle and Spokane are the largest population centers bracing for this heat wave, several other cities in the Pacific Northwest also face the prospect of punishing, record-setting heat:

  • Quillayute, Wash., is forecast to reach 92 on Monday, which would tie its June record
  • Yakima, Wash., is forecast to reach 109 on Sunday, 111 on Monday and 109 on Tuesday. Its highest temperature ever recorded is 110.
  • Eugene, Ore., is forecast to reach 107 Sunday. Its highest temperature ever recorded is 108.
  • Medford, Ore., is forecast to reach 112 degrees Sunday. Its June record is 111 and highest temperature ever recorded is 115 degrees.
  • Boise is forecast to reach 108 degrees Tuesday and Wednesday. Its June record is 110 degrees.

Health concerns

The intensity and duration of the heat wave present major concerns about heat-related illness. Heat is the leading cause of weather-related deaths in the United States in a typical year.

It’s not just the blistering daytime temperatures that are problematic but also potentially record warm nighttime temperatures. Two-thirds of the Seattle-area and nearly one-third of Portland homes lack air conditioning.

“Overnight lows may be the warmest on record in Portland too,” wrote Mark Nelsen, a broadcast meteorologist in Portland, in a blog post. “Low-mid 70s Sunday and Monday mornings. That means homes/apartments will remain dangerously warm with no chance to cool off. Typically we see reasonable overnight temps in our area.”

Populations most vulnerable to excessive heat are older adults, the sick, homeless and socially isolated. Outdoor workers can also be in danger if not given frequent breaks.

“This won’t just be one day in the 100s. Multiple days in a row of these temperatures will make heat illness all the more likely if precautions aren’t taken because the heat will continue to stress the body each day,” wrote the Weather Service in Spokane.

What’s causing the heat

The skyrocketing temperatures can be traced to a sprawling ridge of high pressure colloquially known as a “heat dome.” That high deflects inclement weather to the north, bringing copious sunshine, sinking air and clear skies.

The intensity of this heat dome, based on projected atmospheric pressure at high altitudes, “is forecast to be near the strongest on record at Quillayute, WA upper-air station,” tweeted the Oregon Climate Office.

The Weather Service office in Seattle wrote: “temps aloft will warm to readings as warm as we ever see — even in mid summer.”

The heat’s intensity will be augmented by what’s known as a “thermal trough,” a zone of low pressure near the ground that increases air flow from east to west or land to sea, effectively cutting off any cooling sea breezes.

Evidence suggests heat domes such as this are becoming more frequent and intense due to human-caused climate change and the buildup of heat-trapping greenhouse gases in the atmosphere.

Drought and fire implications

The blazing hot conditions will also be a concern from a wildfire and drought standpoint. The forthcoming outbreak of heat, made worse by human-induced climate change, will further dry soils and reinforce drought conditions. The drought, in turn, will make future extreme heat events more likely.

About half of the West is experiencing an extreme or exceptional drought — the two most severe categories — as scant precipitation and above-average temperatures wreak havoc on agriculture and water resources.

“The heat will unfortunately be bad news for farmers, too. Low relative humidities and no precipitation will worsen the already bad drought,” wrote the Weather Service in Spokane. “Plus, fuel models suggest record dryness this early in the season, so wildfires are a concern.”

As of Thursday, large fires were active in 13 states. So far this year, more than 29,000 fires, the most in a decade, have burned more than 1.1 million acres, according to the National Interagency Fire Center.

Unprecedented heat to surge into Canada

It’s not just the United States that will be affected.

The heat wave will bulge north into western Canada, threatening to establish records for the highest temperatures ever observed in Alberta and British Columbia. It’s even possible the record high temperature for all of Canada is beaten.

The Weather Network, a Canada-based weather news organization, cautioned the heat wave could produce “the warmest day in history for some cities in B.C. [British Columbia]” on Sunday or Monday.

“This feature will be particularly perilous because it’s so rare in this part of the world,” the Weather Network wrote. “Sixty percent of British Columbians do not own an air conditioner in their households.”

The latest blast amid a blistering hot summer

This next blast of heat comes on the heels of a historic heat wave that set nearly 4,000 records in the West last week. Phoenix hit at least 115 degrees for a record six days in a row, and Tucson topped 110 for a record eight straight days. The heat wave expanded into Mexico, where a location just south of the U.S. border soared to 124.5 degrees, the highest temperature ever recorded in the country during June.

“[A]ll-time maximum temperature records fell at locations in seven different states,” wrote Climate.gov.

At the moment, a historic heat wave is simultaneously torching Eastern Europe and Russia, as climate change reveals itself all around the globe.

Matthew Cappucci contributed to this report.