[This post has been updated.]

Brace yourself for another extinction event.

The massively popular dinosaur hall at the Smithsonian’s National Museum of Natural History will be closed as of Monday for a $48 million, five-year overhaul. As I reported earlier, most of the more popular specimens won’t reappear until 2019, when the high-traffic 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 Nation’s T. rex,” one of the most complete Tyrannosaurus rex skeletons ever unearthed. (In Washington on a long-term loan from the Army Corps of Engineers, the T. rex was delivered to the Mall from Montana last week, via FedEx.)

From my earlier story: “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.”

Dinosaurs aren’t disappearing completely from Washington’s top tourist attraction. The Nation’s T. rex is being examined, cleaned, reinforced and scanned, piece by brittle piece, in an exhibition space dubbed the “Rex Room” over the next few months.

The museum’s full-size T. rex replica known as “Stan” and a massive Triceratops cast, “Hatcher,” will be displayed on the museum’s second floor by Memorial Day. A stopgap exhibition, “The Last American Dinosaurs, Discovering a Lost World,” will open in November, two days before Thanksgiving. The interim exhibition will feature Stan, Hatcher and several skulls (including an Edmontosaurus) and will remain open until the Fossil Hall reopens.

A cast of the Nation’s T. rex skull will remain on display, too, in the museum’s Constitution Avenue lobby. Fun fact about the cast: It lived in then-Rep. Newt Gingrich’s office back when he was speaker of the House.

Still, even with the interim exhibitions, Johnson told me earlier this year, the dinosaur experience “won’t be as rich as we presently offer and will offer” at the kid magnet on the Mall, which trails only the Louvre in museum visits, according to the 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. During the closure, Johnson said, 25 million to 30­ million people “will miss the opportunity to see the hall, and that’s a shame.”

But the five-year closure is necessary because each of the specimens currently on display must be dismantled, cleaned and remounted — a laborious and potentially fraught process. There is also work to be done in and around the hall itself, with “construction to the guts of the building and taking the architecture back to its original 1910 splendor,” Johnson said.

“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 museum, by the way, will close later than usual on Saturday and Sunday — 7:30 p.m. — to give visitors one last chance to visit the dinosaur hall.

There are also several public programs planned for the weekend to help send off the dinosaurs, including a Dino Film Fest (with post-screening Q&As) and “red carpet” photo opps with the fossils, which will be uploaded to the museum’s Fossil Hall Fotorama! gallery.