Tropical Storm Mangkhut is intensifying into a possible Category 3 or Category 4 typhoon, and the National Weather Service as well as the military's Joint Typhoon Warning Center agree it could hit Guam directly or pass close to the island sometime Tuesday.

NWS meteorologist Landon Aydlett said estimates show the storm could directly hit Guam or the Northern Mariana Islands as a Category 3 or 4 typhoon.

The military's typhoon warning center in Hawaii estimated that by the time the storm becomes a typhoon, it could pass "over Guam/Tinian" on Tuesday. Its initial estimate projects the coming typhoon would make its closest approach around noon Tuesday.

Aydlett said under Category 3, maximum sustained winds could range between 111 and 129 mph; and under Category 4, sustained winds will be 130 mph to 156 mph, with wind gusts of 165 mph or stronger.

“What does that mean for us is – this could be our worst hit in 15 years," Aydlett said in a briefing Saturday afternoon at the Guam Homeland Security and Office of Civil Defense. Guam last saw a massively devastating storm on Dec. 8, 2002, when Supertyphoon Pongsona hit, causing $700 million in damage and disrupting power and water to some areas for days to weeks.

"Our worst-case scenario that we are considering right now is a Category 4," Aydlett said.

Public and Catholic schools are closed Monday. Public schools are being converted into typhoon shelters starting Monday.

Under a Category 3 or 4 storm, people should stay or seek shelter in concrete structures, Aydlett said.

Mangkhut's winds will begin increasing on Monday.

With a Category 4 typhoon, many well-built wooden or tin homes will be damaged or destroyed, and it could spell complete destruction for buildings made of light materials, the NWS warned.

Extensive damage is also expected to nonconcrete roofs, and reinforced hollow-spun concrete and wooden power poles are likely to blow down along with numerous secondary power lines.

The Joint Information Center has urged all residents to have preparations completed and to have families secured in their homes by Monday evening.

Preparations

Island residents heeded numerous warnings on Saturday to prepare for what the NWS said could be the worst storm the island has experienced in more than a decade. Guam Home Center, Benson and Home Depot were packed early Saturday morning with residents filling their baskets with storm-ready supplies, plyboards, buckets and more.

Guam motorists made trips to gas stations, filling fuel tanks as a precaution following a gasoline shortage during Pongsona. During Pongsona, fuel storage tanks at Cabras Island caught fire, which was believed to have been caused by friction during high winds.

Island stores were filled with shoppers buying emergency supplies, including bottled water, canned food items, candles, batteries, flashlights, and butane or propane tanks. Hardware stores were busy selling wood boards to customers who didn't have built-in shutters for windows and doors.

There were waits at some ATMs, too. Cash was king in previous damaging storms on Guam as some point-of-sale systems couldn't process card transactions.

At Guam Home Center, Vince Borja was checking out a generator with the help of a store employee on Saturday. He said he took advantage of the deals the store offered as he prepared his home for the upcoming storm.

Bert Dacanay filled his pickup truck with several sheets of plywood that he intended to use to board up windows at his home.

Dededo Mayor Melissa Savares and her staff spent the day providing sand bags to residents who are concerned about the heavy rainfall Mangkhut is expected to bring. The NWS stated there was a possibility of 10 to 12 inches of rain through Wednesday night.

Savares said she fielded multiple concerns from residents living in substandard homes asking when and what shelters would be open.

Previously, the island’s most populated village designated Astumbo Elementary School and Maria Ulloa Elementary School as primary shelters.

“It’s never going to be enough. We have a lot of families living in substandard homes,” Savares stated. “Especially in the Chamorro Land Trust areas, "zero-down" properties, we have maybe 400 to 500 families living in wood-and-tin structures and even some still living in tents.”

With Mangkhut projected to bring winds possibly in excess of 130 mph, the Dededo mayor is concerned that many of her residents’ homes will not be able to withstand the powerful wind and rain.

“Even Astumbo – it was outfitted to be a second-tier shelter to withstand winds up to 170 mph, but NWS said ... that everyone needs to be in a full, concrete structure. Astumbo is still partial Butler tin,” she added.

Having been through numerous typhoons, Savares has two pieces of advice for residents: pay attention to local media updates, and get to the shelters as soon as they open.

“Nobody is going to rescue you if you wait until the typhoon hits, because we’re risking lives,” the mayor said. “When they say go to the shelters, don’t wait! Preparation really is key.”

Protect pets during storm

Guam Animals In Need reminds the public to protect pets during the storm. While residents are preparing their homes, pet owners are urged to prepare a pet typhoon kit that includes: pet food for a week, your pet’s medication and medical records, ensuring collars and tags are securely fashioned, a recent photo of yourself with your pet (in case they get lost and you need to prove ownership), and comforting items such as blankets and toys.

“Please bring your pets indoors during the storm,” said Cyrus Luhr, GAIN board president. “It’s not just the law, it’s also the right thing to do.”

National Guard facilities

The Guam National Guard facilities at Fort Juan Muna in Harmon, and the Readiness Center in Barrigada, to include the AAFES Express and gas station, will close when Guam moves into Condition of Readiness 2.

According to Maj. Gen. Roderick Leon Guerrero, the adjutant general of the Guam National Guard, “This will ensure the safety of our soldiers, airmen, civilians, tenants and families during the storm.”

Soldiers and airmen are encouraged to contact their respective units should they have any questions, comments or concerns, he stated.

GWA: Start filling water containers

The Guam Waterworks Authority advises customers to start filling water containers in anticipation of the approaching storm now and throughout the weekend. GWA is monitoring production and reservoir levels, and seeks the assistance of the public in avoiding extremely high demands just before the storm's anticipated arrival, GWA stated.

"GWA operations personnel are making all possible preparations for the storm and are making every effort to avoid any loss of service," the agency stated.

Guam National Wildlife Refuge closed

The U.S. Fish and Wildlife Service closed the Guam National Wildlife Refuge, including the Ritidian Unit, on Saturday in preparation of any effects from Tropical Storm Mangkhut. The refuge will remain closed until it is safe for staff and visitors.