(Reuters) - The worst drought to hit the United States in a half century expanded in the upper Midwest and northern Plains states in the past week due to warmer- and drier-than-normal weather, but loosened its grip on some central and southern areas of the country.
Nearly 65 percent of the contiguous United States were under at least "moderate" drought as of September 18, up from about 64 percent a week earlier, according to the Drought Monitor, a weekly compilation of data gathered by federal and academic scientists.
However, the portion of the country under "exceptional" drought - the most dire classification - declined slightly to 5.96 percent from 6.23 percent a week earlier.
"This U.S. Drought Monitor week saw some improvements in drought conditions in portions of the lower Midwest, Southern Plains, and the South while drought-stricken areas of the Northern Great Plains and upper Midwest continued in a dry pattern leading to some minor degradation of conditions," David Simeral of the Western Regional Climate Center, said.
Conditions in southern Illinois and Indiana and eastern Kentucky, some of the worst-hit drought areas this summer, improved with heavy rainfall over the past month, including precipitation from a Hurricane Isaac.
But warmer and drier-than-normal conditions parched central and northern Minnesota and Wisconsin - states that dodged the worst of the summer's drought.
Overall, the nine-state Midwest region saw its moderate-or-worse drought area shrink to 39.25 percent, from 40.80 percent a week earlier, the Drought Monitor showed.
Moderate drought expanded in the Dakotas amid below-normal precipitation across most of the central and northern Plains, although conditions improved in southeastern Kansas, which received rain.
The area of the High Plains consumed by moderate drought or worse inched up to 82.81 percent from 82.77 percent a week earlier, but the area gripped by exceptional drought shrunk to 23.71 percent, from 25.10 percent.
In the south-central United States, the drought grew more dire in the Texas and Oklahoma panhandle region but eased considerably in Arkansas, half of which was under "exceptional" drought just a month ago.
The region as a whole saw its moderate drought area slip to 48.67 percent from 49.00 percent a week earlier and the area covered by exceptional drought held about steady at nearly 9 percent.
Exceptional drought covered 42.09 percent of Oklahoma, up from 39.66 percent a week ago, and 8.74 percent of Arkansas, down from 12.06 percent a week ago and 53.60 percent a month ago.
(Reporting by Karl Plume in Chicago; Editing by Maureen Bavdek)