Heavy rainfall warnings were raised over Metro Manila and 10 other areas in Luzon on Friday as the southwest monsoon intensified, heightening the risk of flooding and landslides, according to the state weather bureau.

"The effects of the southwest monsoon, or habagat, will continue and will bring heavy to intense rainfall, particularly in areas in the western section of Central and Southern Luzon," said Obet Badrina, a weather specialist at the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA), in the bureau's 5:00 a.m. weather advisory.

The southwest monsoon is a seasonal wind pattern that blows moisture-laden air from the southwest toward the Philippines during the rainy season, which typically runs from June to November. Its rains are often enhanced when a tropical cyclone or low pressure system passes near or over the country, pulling additional moisture into the monsoon flow.

PAGASA said Zambales and Bataan are forecast to experience intense downpours, with rainfall between 100 and 200 millimetres (mm) over the next 24 hours. Under these conditions, widespread flooding is likely, particularly in high-risk areas such as low-lying localities, communities near rivers, and highly urbanized zones.

Metro Manila, Pangasinan, Pampanga, Tarlac, Bulacan, Rizal, Cavite, Batangas, and Occidental Mindoro will receive heavy rainfall of 50 to 100 mm during the same period. PAGASA said localized flooding is possible under this level of rainfall, especially in high-risk areas.

Heavy downpours are expected to persist in these areas until Sunday. Meanwhile, large parts of the rest of Luzon and the Visayas are likely to experience thunderstorms due to the effects of the southwest monsoon.

As of 8:00 a.m., PAGASA reported that the upper to lower portions of the Pampanga River were all above alarm water level. The Pampanga River basin is one of the country's largest, draining a wide swath of Central Luzon including parts of Pampanga, Bulacan, Tarlac, and Nueva Ecija, so elevated water levels across its stretches can affect downstream communities as rain continues.

PAGASA is also continuing to monitor a low pressure area (LPA) and a tropical depression outside the Philippine Area of Responsibility (PAR). The tropical depression, which developed from an LPA on Thursday night, was located 1,770 kilometers east of Extreme Northern Luzon, the bureau said in a press briefing.

"The possibility of it entering the PAR is relatively low," Mr. Badrina said.

The system is also forecast to weaken into an LPA and become part of Tropical Cyclone Krovanh, formerly known as Pilandok. Meanwhile, the LPA formerly known as Saudel was spotted 870 km west-northwest of Extreme Northern Luzon after lashing eastern China.

Residents in the affected areas are advised to monitor PAGASA's weather advisories and follow instructions from local disaster risk reduction offices, as conditions may change while the monsoon persists through the weekend. — Edg Adrian A. Eva