State and city officials were keeping a wary eye on a tropical storm moving up the East Coast ahead of the July 4 holiday, as updated security measures were disclosed for the annual celebration on the Charles River Esplanade.
Authorities and organizers of the Boston Pops Fireworks Spectacular said they would hold off on making any weather-related decisions until at least Wednesday, as the combined threat of a cold front and the storm loomed.
Mayor Martin J. Walsh and others did not rule out the possibility of moving the big event up a day; a preshow concert is already scheduled for Thursday. But forecasters said that Thursday could be worse than Friday, as the front threatened heavy rains and even flooding.
Tropical Storm Arthur formed off the Florida coast Tuesday. It is expected to steam along the coast and could bring heavy rain to New England by the Fourth of July.
“It’s going to be a tough haul,” said William Babcock, a National Weather Service meteorologist. “Unfortunately for everyone, the two days Boston looks to have the most effects are Thursday and Friday, the prime day and the backup. That’s just the way the weather works sometimes, I guess.”
Babcock put the chance of rain Thursday and Friday at just above 60 percent, noting that heavy cloud cover could obstruct views of the fireworks.
Whenever the fireworks are set off, those looking to enjoy them from the Longfellow Bridge will have to search elsewhere. Viewing from the bridge is prohibited due to construction, said Colonel Timothy Alben, superintendent of the Massachusetts State Police.
In a new restriction, bicycles will not be allowed into the oval and the lagoon/island areas of the Esplanade. Police said bikes in other locations must be attended to and cannot be locked to fences, poles, or other structures. Less bike traffic will allow the crowd to easily disperse if an evacuation becomes necessary, Alben said.
“The greatest partner that we have in these major events is the public,” he said. “We really need people to be vigilant, to pay attention to what’s going on around them.”
Boats will not be allowed between the Massachusetts Avenue Bridge and the fireworks barges, which have been moved west for the show. Backpacks, firearms, explosives, coolers on wheels, glass containers, cans, premixed drinks, and alcoholic beverages will be prohibited.
Liquids must be carried in sealed plastic containers less than two liters, and no grilling will be allowed. Bags and purses will be searched.
Between 600,000 and a million spectators are expected to attend the celebration, which Walsh called one of the country’s greatest patriotic traditions.
“It brings people together to celebrate not only the birth of our nation, but the spirit of our community,” he said at a press conference.
New additions to the list of performers were announced Tuesday.
Joey Mcintyre of New Kids on the Block will join the Beach Boys and Broadway star Megan Hilty on stage. Governor Deval Patrick will narrate a Boston Pops arrangement of “The Star-Spangled Banner.’’
“I would like to do it today if we could assemble the crowd fast enough, mostly because I know we can predict the weather for at least the next 12 hours,” Pops conductor Keith Lockhart said.
On a set of barges forming a T on the Charles River on Tuesday, endless arrays of black tubes pointed skyward like cannons. Shawn Allison spent the day dropping shells into the mortars and plugging their orange wires into a system of 240 computers.
With rain forecast, much of the equipment was covered in tarps, and a dozen crew members worked to weather-proof the setup throughout the day.
“What we’re trying to do is load the show as if it was going to go on the third,” said Matt Shea, vice president of Atlas PyroVision, which is doing the fireworks display. “Not that it was going to go on the third, but we all know that the weather is not going to be the best.”
The show will be similar to last year’s, with the traditional playing of the “1812 Overture” followed by another fireworks display to a soundtrack that will begin at 10:30 p.m.
Shea said the barge was turned and moved west to give audiences a broader depth of field to view the pyrotechnics. A new feature will give the illusion of lollipops in the sky, as a multidimensional fan of comets illuminates the horizon.
About 12,000 fireworks weighing at least 3 tons will have been set off by the time the show concludes.
• Tropical Storm Arthur threatens Florida coast
Faiz Siddiqui can be reached at firstname.lastname@example.org.