The brief respite from high gasoline prices is over. After falling in June and July, gasoline prices are surging again ahead of the Labor Day weekend, according to industry observers. Supply concerns in the Midwest have pushed wholesale prices to their highest level in more than two months, and consumers could be in for a rough few months at the pump, analysts say. Trouble at several refineries, along with dwindling U.S. gasoline stockpiles and unusually high summer demand, fueled the sharp increase in prices. The September gasoline futures contract on the New York Mercantile Exchange closed at 83.82 cents a gallon, up 10.6 cents since July and at the highest level since June 19. Pump prices already are on the rise. Nationally, regular unleaded is averaging $1.49 a gallon, up six cents from a week ago, according to the U.S. Energy Information Administration.