(Reuters) - The worst U.S. drought in a half century loosened its grip on the Midwest in the past week, helped by rain and cooler temperatures, but the drought grew more dire in the northern Plains, a report from climate experts said on Thursday.
But the improved Midwest weather arrived too late for crops in major farm states such as Kansas, Iowa, Illinois and Indiana, where severe corn and soybean yield losses have already been realized.
The portion of the contiguous United States suffering from at least "severe" drought fell to 42.34 percent from 44.03 percent over the prior week, according to the Drought Monitor, a weekly synthesis representing a consensus climatologists.
The percentage of the Midwest in that category slipped to 49.96 from 51.06 the previous week, with the most notable improvement in Indiana, 64.07 percent of which was under severe drought or worse, down from 81.48 percent a week ago.
Top corn and soybean producer Iowa remained fully under severe drought or worse, but the portion of the state deemed under "extreme" drought fell to 58.30 percent from 67.54 a week earlier.
Illinois, the No. 2 corn and soy grower, was gauged to be 69.56 percent under extreme drought or worse, down from 76.72 percent last week.
Conditions also improved considerably in Michigan while marginal improvements were noted in Ohio, Missouri and Minnesota.
"After a very dry summer, some areas have been in a wetter and cooler pattern over the last several weeks," said Brian Fuchs of the National Drought Mitigation Center.
"Where the heaviest rains occurred, improvements were made, but it should be noted that many of the row crops will not benefit from these rains and pastures have had minimal improvement so far," he added.
More rain from tropical depression Isaac, which came ashore in Louisiana as a category 1 hurricane on Tuesday, was expected over the next five days in Arkansas, Missouri, Illinois and Indiana.
Key farming and ranching states Kansas, Nebraska and Oklahoma received some rain in the past week, but the majority of the central and northern Plains remained dry.
The six-state High Plains region was 79.12 percent under severe drought or worse, up from 76.96 percent a week ago.
Extreme drought or worse covered 90.14 percent of Kansas, down from 96.43 percent a week ago, while the area of the country's top winter wheat state under "exceptional" drought, the highest category, dropped to 55.18 percent from 66.93.
Nebraska, which remained fully under severe drought or worse, saw only a slight improvement in the far southeast corner of the state. Meanwhile, severe drought expanded across the Dakotas.
Reporting by Karl Plume in Chicago; Editing by Marguerita Choy