Falling diesel futures and ballooning supply have alarmed U.S. refiners about when, or even if, demand may pick up ahead of the peak winter season, potentially hurting crude oil prices amid the year-long rout. Futures plunged 16 percent in July, stockpiles are at four-year highs and demand is down 3.7 percent, all signs refiners’ gains from strong summer gasoline demand may not be sustained this winter. Over the past five years, diesel futures prices have risen by an average of 4 percent in July and a further 4 percent in August, ramping up ahead of the fall and winter when demand from farmers and heating oil customers rises. “That was one area that impacted us negatively in the mid-continent,” said George Damiris, executive vice president at HollyFrontier Corp. […]