'Unusual' Dense Fog Nearly Covers Entire Span of US

Dense fog spanned nearly the entire U.S. on Tuesday morning as weather advisories were issued from Montana south to the Gulf of Mexico in Texas, an "unusual" occurrence, according to one meteorologist.

Weather patterns are changing drastically across the U.S. this week as many states return to normal temperature ranges after enduring bone-chilling cold caused by a polar jet stream. The arctic blast plunged some areas of the country into subzero temperatures that were worsened by brutal windchills. A subtropical jet stream is now bringing more moisture up from the south, which creates advection fog.

National Weather Service (NWS) Weather Prediction Center meteorologist David Roth told Newsweek that this fog occurs when warm air condenses as it passes over colder ground.

Fog is not uncommon in January, particularly along the Gulf Coast as water temperatures drop. However, Roth called the extent of the dense fog on Tuesday an "unusual" occurrence.

Visibility was reduced to a quarter-mile or less in most areas. The dense fog advisories were in place for 14 states. The biggest stretch of fog encompassed Montana, Wyoming, South Dakota, Nebraska, Iowa, Kansas, Missouri, Colorado, Oklahoma, Texas and Arkansas. There also were dense fog advisories in Oregon, Washington and California.

NWS meteorologist John Pike told Newsweek that the fog will be a pattern across parts of Oklahoma for a few days.

The advisories warned motorists to slow down and use headlights, as the fog creates hazardous driving conditions. "Leave plenty of distance ahead of you," one advisory said.

Photos of the dense fog were shared on X (formerly Twitter).

"I can confirm the dense fog in the Henderson foothills. I can't say I've ever seen it like this here," one user posted with a photo of the fog.

"Dense Fog this morning continues in the Cache Valley - check out these images from USU (normally there's a building visible in the image on the right!)," the NWS office in Salt Lake City posted with eerie photos of the fog.

The advisories also warned any mariners out in the fog along the Gulf Coast.

"Inexperienced mariners, especially those operating smaller vessels, should avoid navigating in hazardous conditions," the NWS said. "If you must navigate, proceed with caution. Use proper fog signals. Make sure all running lights are on. Remember to use your radar and compass."

Some states faced a freezing fog advisory, including Oklahoma, Texas and Nebraska. Motorists under those advisories were warned to watch out for slick spots, particularly on elevated surfaces like bridges.