ST. LOUIS – The worst U.S. drought in decades has deepened again after more than a month of encouraging reports of slowly improving conditions, a drought-tracking consortium said this week, as scientists struggled for an explanation other than a simple lack of rain.

While more than half of the continental U.S. has been in a drought since summer, rain storms had appeared to be easing the situation week by week since late September.

But that promising run ended with Wednesday’s weekly U.S. Drought Monitor report, which showed increases in the portion of the country in drought and the severity of it.

The report showed that 60.1 percent of the lower 48 states were in some form of drought, up from 58.8 percent the previous week. The amount of land in extreme or exceptional drought – the two worst classifications – increased from 18.3 percent to 19.04 percent.

“The places that are getting precipitation, like the Pacific Northwest, are not in drought, while areas that need the rainfall to end the drought aren’t getting it,” said Richard Heim, a meteorologist with the National Oceanic and Atmospheric Administration’s National Climatic Data Center. “I would expect the drought area to expand again” by next week since little rain is forecast in the Midwest in coming days.

He said there was no clear, scientific explanation for why the drought was lingering or estimate for how long it would last.

“What’s driving the weather? It’s kind of a car with no one at the steering wheel,” Heim said. “None of the atmospheric indicators are really strong. A lot of them are tickling around the edges and fighting about who wants to be king of the hill, but none of them are dominant.”

The biggest area of exceptional drought, the most severe of the five categories listed by the Drought Monitor, centers over the Great Plains.