The worst U.S. drought in 56 years intensified over the past week as above-normal temperatures and scant rainfall parched corn and soybean crops across the Midwest and central Plains, a report from climate experts said on Thursday. The drought became more severe in the southern United States as well, just a year removed from a record-breaking dry spell that ruined crops and wilted grazing pastures across Texas and Oklahoma enough to force an unprecedented northward migration of cattle.