The tropical storm threatening the Gulf Coast could drive up gas prices for Labor Day travelers and may push already high D.C. prices above the $4 mark, analysts worried Monday.

Gas prices averaged $3.92 per gallon in the District on Monday, up almost 20 cents from a month ago, according to AAA's fuel gauge report. Gas cost about 20 cents a gallon less in the area's suburbs but has risen sharply in recent weeks. And Tropical Storm Isaac's landfall --expected Tuesday evening --could push prices even higher as Gulf Coast refineries shut down, analysts warned.

"It's conceivable that before this week is over, depending on what happens in the gulf, you might see $4-a-gallon gas to return to D.C. proper this week," said AAA Mid-Atlantic's John Townsend. "I have my fingers crossed, as do most motorists this week. High gas prices are the last thing we need."

Gas prices probably won't go back down until after Labor Day, but they probably won't spike dramatically, said Mindi Farber-DeAnda, an analyst with the Energy Information Administration.

"We've seen markets starting to move upward, but we believe inventories are healthy, and we believe production shutdown at the refineries will not be of long duration. So in all likelihood, prices will trail back," she said.

Farber-DeAnda said Isaac will likely not have the large impact on prices like Hurricanes Katrina and Gustav did in 2005 and 2008.

Gas price hikes are expected to hit the Gulf Coast the hardest; Townsend said Washington travelers headed there for the Labor Day weekend may feel pain at the pump. But vacationers staying close to home will probably not notice especially high prices, Townsend said.

"In our region, in terms of resorts and spots like Ocean City and Virginia Beach, [the storm] will have little impact," Townsend said.

Gas prices for Maryland averaged $3.70 per gallon Monday; Virginia was at $3.60; the Washington Metro area averaged $3.72, according to AAA. Those prices are up more than 20 cents a gallon in the past month.

But despite the high prices, AAA expects 33 million Americans to travel 50 miles or more for the long holiday weekend, with 28.2 million choosing to drive instead of fly -- a 3.1 percent increase over 2011 drivers.