WASHINGTON, D.C. --- Atlantic hurricanes are sometimes a major factor contributing to any late-summer surge in gasoline prices, but this has not been the case so far this year, the Energy Department said. Instead, key marketplace factors have included continued erosion in gasoline inventories, with stocks reaching an all-time low in terms of days of supply, combined with sporadic refinery problems and low imports. Pressure from these sources has been particularly acute in the Midwest, where gasoline prices were the highest in the nation over the latter part of August to mid-September, reported the Energy Department's Energy Information Administration (EIA). "These developments may be signaling that continued vulnerability to upward pressure on gasoline prices may persist for a while, at least longer than normal for this time of year," EIA said. "Once again, gasoline markets may be at crossroads leading to somewhat atypical price paths over the next month or so." The U.S. average retail price for regular gasoline decreased 3.1 cents during the week of Sept. 17, averaging $2.787 per gallon. That's 29.0 cents higher than the same week last year, EIA said. Regional retail regular gasoline prices were mixed, with East Coast prices 0.2 cent higher at $2.732 per gallon. The Midwest price plummeted 12.0 cents to $2.859 per gallon, but that was still 54.0 cents per gallon above the price for the same period last year. Prices for the Gulf Coast lost 1.4 cents to settle at $2.679 per gallon, the lowest regional price. The Rocky Mountain region price dropped to $2.824 per gallon, down 1.2 cents this week but 3.2 cents per gallon higher than last year. West Coast prices strengthened 4.4 cents to $2.860 per gallon, the highest in the country. The average price for regular grade in California was up 6.5 cents to $2.904 per gallon, 5.6 cents per gallon over the previous year. Retail diesel prices climbed to $2.964 per gallon, 4.0 cents more than the previous week and 25.1 cents per gallon higher than this time last year. Diesel prices rose in all regions of the country. The largest increase, 4.9 cents, occurred on the East Coast where prices settled at $2.961 per gallon. In the Midwest, diesel prices rose 4.2 cents to $2.984 per gallon, 36.0 cents per gallon more than a year ago. The Gulf Coast's average diesel price increased 3.8 cents to $2.903 per gallon. The Rocky Mountain region's diesel price increased 1.7 cents to $2.972 per gallon. The West Coast price grew by 1.6 cents to $2.998 per gallon. California prices went up 2.8 cents, settling at $3.023 per gallon.