* Two-thirds of Midwest under drought, one-third severe
* Worsens in key corn, soy states Iowa, Illinois, Indiana
* Rain in forecast may bring some relief
By Karl Plume
July 12 (Reuters) - The worst drought in a quarter century tightened its grip on the Midwestern United States over the past week as sweltering temperatures and scant rainfall punished corn and soybean crops across the region, a report from climate experts said Thursday.
Nearly two-thirds of the nine-state Midwest region was in some stage of drought in the week ended July 10, up from just over 50 percent a week earlier, according to the Drought Monitor, a weekly report on drought throughout the country compiled by U.S. climate experts.
A third of the region was in severe to exceptional drought, up from about a quarter of the region a week earlier, it said.
"The most anomalous conditions covered the lower Ohio Valley, southern Great Lakes, and middle Mississippi Valley, where daily highs averaged 10 to 13 degrees above normal. Additionally, these areas received scant rainfall if any," said Rich Tinker of the Climate Prediction Center.
"The worst conditions (D3 to D4) were assessed along and near the lower Ohio River and in northeastern Indiana, where rainfall was 7 to 11 inches below normal for the last 3 months," he said, referring to the top two categories of drought, extreme and exceptional.
The deepening drought has dragged the yield potential of the developing soybean crop and the pollinating corn crop, which is in a growth stage when its water needs are highest.
On Wednesday, the U.S. Department of Agriculture slashed production forecasts for both crops due to the drought, joining scores of private forecasts that have sent grain prices to near-record highs.
The whole of Iowa was classified as abnormally dry as of July 10 and 12.7 percent of the top corn and soybean producing state was in severe drought, up from 0.8 percent the prior week.
Harder-hit Illinois, the No. 2 corn and soy state, was 66.28 percent under severe drought or worse, up from 40 percent the previous week.
Severe to exceptional drought covered 80.15 percent of Indiana, versus 68.84 percent the prior week.
Conditions in Missouri also deteriorated, with 82.54 percent of the state un severe drought or worse, compared with 78.83 percent the week before.
Rain was expected to move across the Midwest over the next 10 days, including parched southeastern areas, but coverage may be spotty, forecasters said on Thursday. (Reporting by Karl Plume in Chicago)