The Pacific Northwest is once again in the midst of a heat wave after already seeing its worst such event on record this summer. Temperatures are soaring into the low 100s in some areas, while dangerous heat is also affecting the South Central states and Gulf Coast.
Why it matters: The occurrence of yet another heat wave during a drought in the West is ratcheting up wildfire risks. The heat itself is a major public health risk, as extreme heat is typically the biggest annual weather-related cause of mortality in the U.S.
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The big picture: Heat watches, warnings and advisories are in effect across 19 states, from Portland, Oregon, east to St. Louis, and running all the way south to New Orleans. Temperatures of between 10°F and 15°F above average in these areas, along with high humidity, pose a public health threat.
This heat wave, which is forecast to continue through the weekend in many areas, is part of a series of extreme heat events that have turned deadly this summer across the U.S. and Canada.
In Oregon and far north-central sections of California, high temperatures could hit 110°F in a few spots, the National Weather Service warned, with the predicted high in Portland soaring to just under the century mark on Friday.
In inland portions of Washington state, unusually hot conditions have prompted the issuance of heat advisories through Saturday.
The heat is aggravating fire weather conditions, and "Red Flag" warnings are in effect in areas where some large blazes — such as the Bootleg Fire, the nation's largest — are already burning.
A total of 82 large wildfires are burning in the West amid extreme heat and drought, routinely forming towering pyrocumulus clouds above the blazes. Upper-level winds are carrying smoke more than 1,000 miles east, fouling air quality in the Northeast and mid-Atlantic states and sending smoke across the Atlantic Ocean and into Europe.
The heat is being exacerbated by an ongoing extreme drought — the worst so far this century — in the West. Dry soils allow incoming solar radiation to heat the air more efficiently, thereby drying out the environment even more and adding to warming in a feedback loop.
The Pacific Northwest heat wave in June killed hundreds and bore the fingerprints of human-caused global warming, scientists found.
In parts of Kansas and Mississippi, heat indices of up to 110°F are forecast Thursday through Friday, causing the Weather Service to warn of "significant heat stress" conditions.
The prolonged nature of this heat wave may act to heighten public health impacts, as warm overnight temperatures prevent those without access to air conditioning from cooling off. Overnight lows in the Pacific Northwest are likely to stay in the upper 60s to low 70s, while lows in the upper 70s to low 80s will be more common in the South.
The Weather Service forecast office in Jackson, Mississippi, is warning that portions of the state face an "extreme risk" of heat stress conditions through Saturday, with air temperatures hovering near 100°F and heat indices climbing to 115°F.
How it works: This latest heat wave comes courtesy of a "heat dome," which is an area of high pressure aloft that helps lock in place hot, dry weather. The latest heat dome is sitting over the Plains, encouraging sinking air. Its influence is being felt across much of the country.
As the air descends, it warms up and also squelches any showers and thunderstorms that might temporarily break the heat.
That is not the case along the periphery of the high-pressure area, however. With elongated heat domes like this one, there tends to be a strong jet stream flowing along the boundary between hot and cooler air to the north.
This can create an ideal environment for severe thunderstorms, which meteorologists refer to as a "ring of fire" weather pattern, since the storms erupt on the edges of the high-pressure area.
A series of damaging storms with high winds swept across Wisconsin overnight, and the Storm Prediction Center designated the mid-Atlantic states as being in a Level 3 of 5 severe weather threat zone on Thursday. This is mainly due to the likelihood of well-organized clusters of storms that can bring severe wind damage to several states.
The bottom line: The extreme heat seen this summer is a clear sign of things to come, scientists say. Heat waves are among the clearest consequences of a warming planet, with their likelihood, severity and duration already increasing as the planet's average temperature climbs due to human-caused greenhouse gas emissions.
A study published Monday found that heat extremes such as the Pacific Northwest event, during which Portland hit an all-time high of 116°F, are likely to be far more common in coming years as the rate of global warming quickens.
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