"The spend on bathing suits and flip flops has already pretty much happened," said Kathy Grannis, a spokeswoman for the National Retail Federation. Barbecue spending will probably also hold steady, with people moving planned parties indoors if Arthur crashes.
If anything, Arthur-generated bad weather would spur consumer spending. Instead of indulging in free or low-cost activities like lounging on the beach or watching fireworks, more consumers might go to the movies or visit the mall, Grannis said.
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Or shop online. "I'm willing to bet retailers have already put in motion email campaigns targeted to people on the East Coast, with mention of the weather," she said.
Arthur, the first named tropical storm of the season, could slightly shift consumers' Fourth of July plans as it travels up the East Coast this week. Meteorologists predict the storm will become a Category 1 hurricane Thursday, before tracking off to the East somewhere north of North Carolina. "As the forecast continues to unfold, it looks more and more like we're just going to get a glancing blow," said Paul Walsh, vice president of weather analytics at The Weather Channel, an NBCUniversal company. NBCUniversal is also the parent of CNBC. "I don't think it'll ruin your Fourth of July weekend."
That bodes well for most of the 41 million Americans that AAA expects will travel 50 miles or more over the holiday. (That figure is up 1.9 percent from last year, and is 14 percent higher than this year's Memorial Day weekend traffic.)
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"We don't expect people to cancel their plans," said Heather Hunter, a spokeswoman for AAA. In previous years with bad weather, she said, drivers usually opt to time their trip to avoid the worst of the storm─heading out early or waiting a day. But they rarely just opt to stay home. "Americans are ready for their summer vacation," she said.
One thing they won't be happy with: gas prices are expected to hit the highest in six years for this weekend.
—By CNBC's Kelli Grant