Story highlights

NEW: Flood waters have inundated many towns in central Luzon, CNN affiliate reports

A boy was killed by a falling tree and a woman died after being hit by a collapsing wall

"We've had extremely ferocious wind, torrential rain," says storm chaser in Philippines

CNN  — 

Flooding unleashed by a deadly, slow-moving typhoon has invaded more towns in the northern Philippines, forcing people to clamber onto rooftops to await rescue.

Typhoon Koppu began battering the region over the weekend, driving tens of thousands from their homes.

The fierce storm has been lumbering around the country’s main island of Luzon at an excruciatingly slow pace, setting off floods and landslides across the rugged terrain.

Floodwaters have engulfed San Antonio in central Luzon, Mayor Antonino Lustre said Monday, according to CNN affiliate ABS-CBN. Rescuers were unable to reach some areas, where residents were stranded on roofs, the broadcaster reported.

Floodwaters have wreaked havoc in other towns in Nueva Ecija province, Gov. Aurelio Umali said, according to ABS-CBN.

The storm came ashore in the early hours of Sunday morning at super typhoon strength, ripping the roofs off buildings and uprooting trees in the coastal province of Aurora.

“Through the night, we’ve had extremely ferocious wind, torrential rain,” storm chaser James Reynolds told CNN from the town of Maddela. “The building I’m in, the water’s been coming in the windows.”

Initial estimates suggest the storm has caused at least 5.3 billion pesos, or $114 million, in damage to infrastructure and agriculture, though Pama noted that number will likely rise.

Towns reportedly cut off by landslides, floods

Flooding and landslides have cut off roads and communications in three towns in Aurora province, including Casiguran, where the typhoon made landfall, authorities reported.

“Based on the report of the Philippine army, there were many houses destroyed and trees uprooted in the three towns,” the official Philippines News Agency said. The army and other agencies worked to clear the routes to Casiguran, which has about 25,000 people, and to Dinalungan and Dilasag, it reported.

Roads have also been cleared in Baler, another town in Aurora, where CNN Philippines reporter Paul Garcia reported flooding in several neighborhoods. Surprised local residents said that while storms are common in the area, flooding is not, Garcia reported.

The heavily populated region around Manila, the Philippine capital, wasn’t in Koppu’s direct path, but still got lashed with wind and rain.

One of those killed – a 14-year-old boy struck by a falling tree – was in Quezon City. Another was a 62-year-old woman hit by a wall that collapsed in Subic, northwest of the capital.

Tens of thousands displaced

More than 20,000 people have been displaced across Luzon, with the majority of them taking shelter in evacuation centers, the Philippines’ disaster management agency said late Sunday.

That number is expected to rise as the storm, known in the Philippines as Lando, continues to drench northern Luzon.

The storm was packing maximum sustained winds of 240 kph (150 mph) when it slammed into the eastern coast of Luzon, according to the U.S. military’s Joint Typhoon Warning Center, although the Philippines’ national weather agency measured the winds as being significantly weaker, at 185 kph.

The typhoon has since lost strength as it has moved over Luzon. By Monday morning, it had maximum sustained winds of around 140 kph, according to the U.S. warning center.

Huge rainfall expected

But much of the concern about Koppu’s impact centers on the extreme amounts of rain it’s expected to unleash.

“The big story out of this storm is definitely going to be the rainfall totals,” said CNN meteorologist Allison Chinchar. Some areas were forecast to receive more than 1 meter (39 inches) of rain by the end of Wednesday.

The typhoon was predicted to dawdle across northern Luzon for days because of a ridge of high pressure over China blocking its progress farther north.

“That’s where the problem with the flash flooding comes in, because when you have all of this rain that keeps coming down over the same places over and over, that is likely to trigger mudslides and landslides in addition to flash flooding problems in … some of the low-lying areas,” Chinchar said.

Officials reported dozens of flight cancellations, thousands of people stranded in ports and many municipalities without power.

‘The bowling alley for typhoons’

Situated in the Western Pacific, the Philippines is frequently hit by typhoons.

“They’re located in the belt basically between the equator and the subtropics. You might consider it the bowling alley for typhoons moving across the Pacific,” said Bob Henson, a weather and climate science blogger for Weather Underground.

“It’s considered to be the most vulnerable large nation on Earth for tropical cyclones,” he said.

The deadliest storm to hit the country in recent years was super Typhoon Haiyan, which left more than 7,000 people dead or missing in November 2013.

Haiyan, one of the strongest storms ever to make landfall, generated a devastating storm surge that flattened entire neighborhoods in the densely populated coastal city of Tacloban before moving quickly over other areas of the central Philippines.

CNN’s Jason Hanna, Kim Hutcherson and journalist Arlene Espiritu contributed to this report.