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A slow-moving typhoon that could bring as much as 6 feet of rain to parts of the Philippines grew stronger as it edged closer to the country.

Typhoon Koppu — which is known as "Lando" in the Philippines — had estimated maximum sustained winds of about 120 mph by 11 p.m. ET Friday, according to the U.S. military's Joint Typhoon Warning Center. That's the equivalent of a Category 3 storm.

As of midnight local time Saturday, the storm was located 60 kilometers, or 37 miles, east-southeast of Casiguran, in the Aurora province of the Philippines, according to the Philippine Atmospheric, Geophysical and Astronomical Services Administration.

Light to moderate rain was falling, the agency added, and flash floods and landslides were possible in the coming hours.

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The Weather Channel warned of a potential "catastrophic flood threat" from a days-long deluge.

Weather Channel senior digital meteorologist Nick Wiltgen predicted "prolific rainfall as it grinds across the mountains and valleys of Luzon ... [which is] home to almost half of the country's 98 million people."

He also predicted that Koppu would trace a "painfully slow and dangerously rainy path" across the island.

"Luzon could be looking at 4 to 6 days of heavy rainfall from Koppu/Lando before what's left of it finally drifts farther north," he wrote. "With rain rates in tropical cyclones typically in excess of 2 inches per hour and additional lift for the moist air provided by Luzon's mountainous terrain, extreme storm totals of 20 to 40 inches are likely across much of northern Luzon."

Wiltgen said that two computer models "predicted rainfall totals far exceeding 50 inches in the mountains lining the northwestern coast of Luzon" adding that should the storm stall "highly localized rainfall amounts topping nearly 80 inches would not be unrealistic."

President Benigno Aquino III appeared on national television Friday to warn Filipinos to prepare for the incoming storm, warning that an estimated 7.5 million people would likely need of assistance during and after the storm.

Aquino has not issued a storm warning on national television since 2013 when Typhoon Haiyan ripped through the country, leaving more than 7,300 people dead and missing in its wake, according to The Associated Press.

"Even in the typhoon-prone Philippines, rainfall amounts exceeding 40 inches from a single typhoon are relatively rare, and always extremely dangerous," Wiltgen added. "Far lower rainfall amounts from past typhoons have proven deadly time and again."

He said that at least 15 million people live in the area of northern Luzon north of Manila, "many of them in cities with steep hillsides or flood-prone rivers, and in some cases both."

Metro Manila, which is home to around 12 million people, is due to feel the effects of the typhoon in its later phase.

"Over a foot of rain is not out of the question there, and that would also trigger dangerous flash flooding," Wiltgen added.