ST. LOUIS (AP) — 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 Wednesday, 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.