More than 65 million years after they went extinct, dinosaurs are about to disappear again — at least from public view in Washington.

The Smithsonian’s National Museum of Natural History said Friday that its high-traffic dinosaur hall will close April 28 for a previously announced $48 million makeover. Most of the popular specimens won’t reappear until 2019, when the Fossil Hall at the world’s second-most-visited museum is reopened.

Once completed, the 31,000-square-foot hall will have a new centerpiece: the Wankel T. rex, one of the most complete Tyrannosaurus rex skeletons ever unearthed.

“Those five years are going to fly by,” promised museum director Kirk Johnson, who didn’t sound fully convinced. “It’s definitely going to be traumatic for me, because I’m a paleontologist. . . . I’m also thinking of the kids who won’t be able to see the dinosaurs.”

The announcement generated consternation among the dino-obsessed — or among their parents, nannies, teachers and grandparents, anyway.

The current exhibition of dinosaurs is seen at the Smithsonian National Museum of Natural History, which is acquiring the bones of a Tyrannosaurus rex on a loan from the Army Corps of Engineers. (Sarah L. Voisin/The Washington Post)

“Shocking,” said Kathleen Simons, a federal contractor with six grandchildren, ages 7 months to 14 years. “Five years is so long. That just floors me.”

She worries, she said, that Ella, who is just turning 2, and Kelsey, a mere 7 months old, won’t get to roam the Fossil Hall “during their magic years for dinosaur love.”

Not all of the dinosaurs will be gone: On Thursday, the museum installed a cast of the Wankel T. rex skull in its Constitution Avenue lobby. Next year, it will add a temporary exhibit featuring fossils from the last days of the American dinosaurs. The museum is also planning a series of paleontology programs and events.

But the dinosaur experience, Johnson said, “won’t be as rich as we presently offer and will offer” at the Mall’s kid magnet, which trails only the Louvre in museum visits, according to the 2013 global attractions attendance report by the nonprofit Themed Entertainment Association.

An estimated 8 million people visited the Natural History Museum last year. Two-thirds of them — nearly 5.5 million people — passed through the Fossil Hall, according to Smithsonian officials. During the closure, Johnson said, 25 million to 30­ million people “will miss the opportunity to see the hall, and that’s a shame.”

In the grand scheme of things, five years isn’t much time; after all, some of the fossils on display are well over 100 million years old.

But the Smithsonian’s National Museum of African American History and Culture, which broke ground in February 2012, is expected to open on the Mall in November 2015 after fewer than four years. That museum will be more than 10 times the size of the dinosaur hall, whose makeover will take five years.

“That’s too long,” said Amber Suders, who was visiting from Yorktown, Va., with some members of her dance studio, including two mothers with grade-school-aged girls who spent part of the afternoon with the dinosaurs.

Why will the renovation be so slow? Well, you try dismantling, cleaning and remounting a Diplodocus quickly.

The enormous plant eater, dead for more than 145 million years, has been on display at the Natural History Museum since 1931, when the exhibit space was known as the Hall of Extinct Monsters. Removing the 90-foot dinosaur’s heavy old bones from the original mount will be a potentially fraught process.

“It’s not uncommon when you’re dismantling these things that you have breakage,” Johnson said. “No problem, because we have this stuff called glue that works pretty well. But there are some dramatic moments when you take these things down.”

Then, the Diplodocus will be examined and prepared for exhibit using modern techniques — a process that will be repeated by teams of conservators with every ancient specimen in the exhibit, including skeletons mounted on the wall, some 30 feet up.

“It’s not a trivial process to dismantle and remount all these things,” Johnson said. “We’ll do a lot of cleaning and paleontology on the specimens to really understand what we have. A lot of them have been on display for so long, we don’t truly know how much is real fossil bone and how much is plaster or clay or whatever.”

The traditional technique, he said, was to fill in missing parts “and then paint the whole thing the same color, so it would be difficult to see what was real and what was reconstructed.” Now, “you fill it in, but you make a clear distinction between that and the actual bone.”

Besides the daunting task of removing, then remounting, the dinosaurs, there is work to be done in and around the hall itself, with “construction to the guts of the building,” Johnson said, “and taking the architecture back to its original 1910 splendor.”

“I would dearly love to do it faster,” he said, “but I have been convinced by the staff that it’s going to be a five-plus-year project, with lots of people working very hard for that full amount of time.”

During the Fossil Hall closure, the rest of the signature items in the museum’s massive collection will remain on display — from the Hope Diamond, which sits on a rotating pedestal behind bulletproof glass, to the giant African bush elephant that towers over visitors in the rotunda.

The daily tarantula feedings will continue. The North Atlantic right whale replica won’t swim away.

“It will be disappointing not to be able to see the dinosaur hall for five years, but there’s always something closed here,” said Tim Krepp, a Washington tour guide.

The Lincoln Memorial Reflecting Pool was fenced off for a $34 million reconstruction project from 2010 until 2012. The National Museum of American History closed for a two-year, $85 million makeover in late 2006. The National Portrait Gallery was shut down for more than six years. The Washington Monument has been undergoing repairs since it was damaged in the August 2011 earthquake; it is expected to reopen this year.

The Fossil Hall’s five-year closure, Krepp said, “won’t leave a huge hole because we have surplus capacity. There’s way more stuff in Washington than an average family or tour group can take in. Even in Natural History itself, there’s a lot of other great stuff to see.”

For a period of time, beginning just before the Fossil Hall closes, museum visitors will even be able to watch conservators uncrate and examine the Wankel T. rex, bone by bone, in a hall just off the rotunda.

The marquee specimen is coming to Washington on a long-term loan from the Army Corps of Engineers, which owns it, and the state of Montana, which has had it since the late Cretaceous Period.

The dinosaur was supposed to have arrived at the Natural History Museum in October, but its cross-country trip was delayed by the government shutdown. Currently stored in a Museum of the Rockies facility, the T. rex bones are scheduled to arrive by truck in Washington on April 15.

The 35-foot-long skeleton will be inspected, cleaned, restored, scanned and studied in Washington, then sent to Toronto, where it will be mounted in a lifelike pose before going on display in 2019.

“When we reopen, the new place is going to be so spectacular, you won’t even believe it,” said Johnson, the director. “It will be worth the wait.”

Patrick Svitek contributed to this report.

The Smithsonian’s Top Five museums in 2013

National Museum of Natural History: 8 million visitors

National Air and Space Museum: 7 million visitors

National Museum of American History: 4.9 million visitors

National Zoo: 2 million visitors

National Museum of the American Indian: 1.4 million visitors