110M under heat warnings, advisories Wednesday

Along with the dismal air quality across the upper Midwest, the main weather story across the nation Wednesday was the deadly, record-breaking heat wave scorching the south-central U.S. over the past couple of weeks, killing at least 14 people. Temperatures Wednesday were forecast to approach or surpass 100 degrees from New Mexico to Mississippi.

“These are unprecedented temperatures,” Webb County, Texas, medical examiner Corine Stern said this week. Eleven people have died in Webb County, Stern said. Two others died in Texas while hiking in extreme heat at Big Bend National Park. One person died last week in Louisiana.

And as of late Wednesday, more than 110 million Americans were under some level of heat alert from the National Weather Service. As of Wednesday evening, the alerts span thousands of miles, from California to Florida.

“There may be more danger than a typical heat event, due to the longevity of near-record or record high nighttime lows and elevated heat index readings,” the National Weather Service said.

Meanwhile, much of the upper Midwest continued to face smoke-filled skies Wednesday. The Environmental Protection Agency’s AirNow.gov site showed Detroit in the “hazardous” range and warned that “everyone should stay indoors and reduce activity levels.”

This followed a smoky Tuesday: Chicago and Detroit had the worst air-quality in the world on Tuesday as smoke from Canadian wildfires drifted across the upper Midwest.

Most of the upper Midwest will remain under an air-quality advisory through Wednesday, and as the smoke moves east, air quality is expected to reach unhealthy levels in parts of western and central New York and eastern Lake Ontario, Gov. Kathy Hochul warned.

Here’s a look at Wednesday’s forecast.

Brutal Texas heat wave set to reach Central Plains and Mississippi Valley

The heat wave will be “life-threatening” to those working or spending time outdoors Wednesday, the weather service warned, because of the extreme humidity.

The heat index, which is how hot it feels with humidity factored in, will be in the 110 to 120 degree range across the eastern third of Texas, along the Central Gulf Coast and north through the Lower Mississippi Valley and Lower Tennessee Valley, the weather service said.   

That includes the major metro areas Dallas-Fort Worth; San Antonio; New Orleans; Little Rock, Arkansas; Jackson, Mississippi; Memphis and Nashville, Tennessee; and Montgomery Alabama.

“Take this dangerous heat seriously,” said Weather.com meteorologist Jon Erdman. “Avoid any prolonged, unnecessary outdoor activity, especially in the hottest part of the day. Check on the elderly and anyone else who may be living without air conditioning.”

What is an excessive heat warning?

An excessive heat warning is issued by the weather service within 12 hours of the onset of extremely dangerous heat.

The general rule of thumb for such a warning is when the maximum heat index temperature is expected to be 105 degrees or higher for at least two days and nighttime air temperatures will not drop below 75 degrees. But these criteria vary across the country, especially for areas not used to extreme heat.

That’s important because if you don’t take precautions immediately when conditions are extreme, you may become seriously ill or even die, the weather service warned.

An excessive heat warning means some people can be seriously vulnerable to heat if precautions are not taken. Studies in Canada, Europe and the U.S. have indicated that mortality begins to increase exponentially as the heat increases or stays above a heat index of 104 degrees.

What is a heat advisory?

Just like an excessive heat warning, the weather service issues a heat advisory within 12 hours of the arrival of extremely dangerous heat conditions.

A heat advisory is issued when the maximum heat index temperature is expected to be 100 degrees or higher for at least two days and nighttime air temperatures will not drop below 75 degrees. These criteria also vary across the country, especially for areas not used to dangerous heat. 

US heat index map

Heat, wildfires a concern in parts of California

After a relatively cool couple of weeks across much of California, summer has kicked off with an increasingly hot weather pattern for the interior valleys of California set to start Thursday and continue into the weekend.

In Southern California, wildfire season has begun in Riverside County.

A brush fire called the Juniper Fire has spread 50 acres, burning homes and leading to evacuations Tuesday afternoon in Perris, about 75 miles east of Los Angeles.

Southern California wildfire map

Canadian wildfires deteriorate air quality in Midwest

Poor air quality is set to continue in the Midwest, Great Lakes and Upper Mississippi Valley thanks to Canadian wildfires, according to the weather service. Detroit is under an air quality advisory through Wednesday night.

Chicago and Detroit ranked as the two cities with the worst quality, according to IQAir’s Air Quality Index, switching back and forth between the “unhealthy” and “very unhealthy” categories through Tuesday.

US air quality map

US weather warnings and watches

National weather radar

Contributing: The Associated Press

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