NEW YORK—Grains and fuel prices finished an erratic trading session lower Thursday as commodities traders kept a close watch on the weather near the Gulf Coast and in the Midwest.

Consumers aren’t in for a break in gasoline and grocery costs, however. Refineries have been ratcheting up wholesale gasoline prices ahead of the Labor Day weekend, and meanwhile, drops in the price of agricultural futures usually take a long time to work through the system to retail prices.

Furthermore, the markets have seen relatively thin volumes ahead of the holiday, which can make trading...

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