Storm Arthur, which is moving up the East Coast and may scrape North Carolina as a hurricane, is expected to block a cold front from the west, bringing rain, thunderstorms and the threat of flooding, the U.S. Weather Prediction Center said.

As Arthur passes, the front will be able to move, and New York City’s weather should start improving in time for the 35th Macy’s fireworks show July 4.

“In a nutshell, it will be wet Thursday and Friday and sunny and nice on Saturday,” said Jeffrey Tongue, a National Weather Service meteorologist in Upton, N.Y. “I don’t know if it’s hope or optimism that for the fireworks in New York the rain will be out to the east by then.”

Most of the East Coast’s foul weather will come from the front Arthur will block as it moves up the coast. The Fourth of July holiday is traditionally a day of barbecues, beach outings and fireworks displays in cities including New York, Boston and Washington.

AAA projected July 26 that 41 million Americans would travel at least 50 miles over the holiday, 34.8 million of the travelers by car. The nation’s largest motoring group defines the Independence Day weekend as running from Wednesday, July 2, through Sunday, July 6.

Severe thunderstorms may break out starting Wednesday across the mountains from Maine to North Carolina as the front moves in and rain will spread across much of the region tomorrow, said the Weather Prediction Center in College Park, Md.

Flash flooding from thunderstorms and rain is possible tomorrow from southern New Jersey into Virginia, the center said.

“That cold front is trying to move through and it can’t move through until the tropical storm moves by,” said Rob Carolan, owner of Hometown Forecast Services Inc. in Nashua, N.H.

North Carolina’s Outer Banks may be the only part of the coast that takes a direct hit from Arthur, according to the U.S. National Hurricane Center. The west side of the storm may scrape Dare County where as many as 300,000 people are expected to be vacationing.

County officials met on Tuesday to discuss the storm and will decide Wednesday if evacuations are needed.

A hurricane watch has been issued for part of coastal North Carolina and tropical storm watches are posted from Florida to Virginia.

Carolan said Arthur is a lopsided storm with most of its power on its northern and eastern side. If the storm tracks as forecast, only its weakest side will touch the North Carolina coast.

Timing and location will determine the quality of weather on the Fourth of July, he said. The farther south and west, the nicer the weather, Carolan said.

New York will have rain during the day on July 4 with the possibility of improving conditions as the day goes on, Tongue said. The Macy’s fireworks show begins at 9 p.m.

“The rain should be ending by Friday from west to east,” Tongue said.

Montauk on eastern Long Island will be the last to clear, he said.

Washington will likely have conditions improve earlier than in New York, Carolan said. As the storm passes, it will pull cooler and drier air in behind it, so by the time fireworks start there temperatures could be in the 60s Fahrenheit (upper teens Celsius) under clearing skies.

Boston, farther to the north and east, will have showers linger longer, Carolan said. The concert by the Boston Pops and the fireworks show is scheduled to start at 8:30 p.m. July 4 on the banks of the Charles River.

The weather service said there is 50 percent chance of showers in Boston then.

On Cape Cod, the part of Massachusetts that juts out into the Atlantic, the popular tourist destination is expected to deal with tropical storm conditions through Saturday, July 5, according to the weather service.

For most of the eastern U.S., Saturday will dawn as a nice day and conditions will improve through the rest of the weekend, Carolan said.

“People who were planning to travel may change their plans,” said Phil Flynn, senior market analyst at the Price Futures Group in Chicago. “It will certainly put a damper on gasoline demand.”

2014 Fireworks Displays

More fireworks

• Our interactive map showing public fireworks displays includes rain dates
• How to get the best (and safest) bang for your buck when buying fireworks