A bone-dry October is pushing nearly half of the United States into a flash drought, leading to fires in the Midwest and hindering shipping on the Mississippi River.

More than 100 different long-term weather stations in 26 states, including Alaska, are having their driest October on record, through Sunday, according to records by the Southern Regional Climate Center and Midwest Regional Climate Center. Cities that have had no measurable rain for October include New York, Houston, Dallas, Philadelphia, San Francisco and Sioux City, Iowa, along with normal dry spots such as Los Angeles, Las Vegas and Phoenix, National Weather Service records show.

“This is on pace for a record dry October,” said Allison Santorelli, acting warning coordinator for the National Oceanic and Atmospheric Administration’s Weather Prediction Center in College Park, Maryland. This includes the Southeast, some of which experienced deadly flooding just the month before in Hurricane Helene.

In June, less than 12 percent of the country was experiencing drought. Now it’s almost 50 percent and growing, according to the U.S. Drought Monitor.

That fits the definition of “flash drought,” which is different than normal slowly developing dry spells, said U.S. Department of Agriculture meteorologist Brad Rippey, a drought monitor author. A study last year showed that a warming world from the burning of coal, oil and gas is causing more frequent and damaging flash droughts, such as a $30 billion one that hit America in 2012 and a devastating 2022 drought in China.

In one-eighth of the area of the continental United States, no rain has been reported for the first 28 days of October. About 93 percent of the continental U.S. is seeing below-average rain in October, most with less than an inch, according to climate center data analyzed by The Associated Press.

Cities like Washington are forecast to hit 80 degrees on Halloween, right after Chicago and Detroit flirt with those summer-like temperatures. “That’s wild” for the end of October, said meteorologist Ryan Maue, a former chief scientist at NOAA who recalled trick-or-treating in snow in Michigan.

Santorelli said a dome of high pressure has blocked moisture from moving north from the Gulf of Mexico, keeping dry much of the U.S. from the Plains and Midwest over to the Eastern seaboard.

“We’ve been stuck in this blocking pattern for almost two months,” Rippey said.

Studies the last decade or so have shown that the jet stream — the currents of air that move weather systems across the world — is wavier and getting stuck more often, attributing it to human-caused climate change’s extra warming of the Arctic, said Rippey. What’s happening now, especially with an extremely warm Arctic and “feverish ocean temperatures across the North Pacific,” fits the theory well, said Woodwell Climate Research Center senior scientist Jennifer Francis, one of the pioneers of the concept.

Stuck weather systems this year have caused “weather whiplash” in places like Sioux City, where downpours in June caused so much rain that it pulled down a railroad bridge and forced people to their rooftops, said climatologist Melissa Widhalm, associate director of the Midwest Regional Climate Center at Purdue University.

Asheville, North Carolina, which was devastated by Hurricane Helene, was deluged with nearly 14 inches of rain in three days in September, but has received only one one-hundredth of an inch in October.

The Mississippi River, a major transporter of crops, is at levels so low that shipping loads have to be constrained, Rippey said. It’s the third straight year of problematic water levels on the river, he said. When Helene struck the Southeast and flooded North Carolina and Tennessee in late September, it provided a burst of fresh water that helped return Mississippi water levels to where they should be, but it didn’t last, Rippey said.

It may hurt the transporting of agricultural products, but luckily for farmers the flash drought came in after harvesting corn and soybeans, Rippey said.

But dry fields mean a situation ripe for wildfires in both the Midwest and East, Rippey said. Farm equipment has started a lot of fires accidentally, he said.

Five large uncontrolled fires Tuesday had burned more than 1,000 acres in the East and Midwest, according to the National Interagency Fire Center.

Relief is on the way for parts of the Midwest as storm systems from the West are forecast to break through with rain, sometimes heavy, Wednesday and Thursday, Santorelli said. But much of the East and Southeast look dry for another week, she said.

Mary Katherine Wildeman in Hartford and Michael Phillis in St. Louis contributed to this report.