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MANILA -- A low pressure area is expected to bring rains over large swaths of the country, which may ease the declining level of Angat Dam, the primary water source of Metro Manila, the state weather bureau said Monday.

At 3 a.m., the brewing storm was located 595 kilometers northeast of Borongan City, Eastern Samar, said PAGASA weather forecaster Meno Mendoza.

The weather system is unlikely to intensify into a storm in the next 24 hours, but it could bring rains to Central Luzon, where Angat Dam is located, as it heads towards Taiwan later this week, he told radio DZMM.

On Monday, the LPA's trough or extension will bring rains that may spawn floods or landslides in Metro Manila, Visayas, Calabarzon, Mimaropa, Bicol, Caraga, Davao Region, and Aurora province, PAGASA said.

The rest of the country will experience hot and humid weather that may be marred by localized thunderstorms in the afternoon or evening, added Mendoza.

The water level in Angat Dam as of 6 a.m. slid to the 159.43-meter mark, below the 160-meter critical level, according to the PAGASA website.

Water supply in Metro Manila was slashed to 36 cubic meters per second from 40 m³/s over the weekend, said National Water Resources Board chief Sevillo David Jr.

Manila Water, the concessionaire in Metro Manila's east zone, lost some 350 to 400 million liters of supply allocation, which is equal to the water demand of 3 cities, said the firm's corporate communications head, Jeric Sevilla.

Some customers of Manila Water may experience 12 to 17 hours of rotational supply interruption due to this, he said.

Maynilad, which services Metro Manila's west zone, is also implementing supply outages in the wee hours, the schedule of which is posted on the firm's Facebook page, said corporate communications head Jennifer Rufo.

"Magtulungan po tayong lahat kasi lahat po tayo may magagawa para ma-preserve po iyung tubig sa Angat," she said.

(Let's help each other because we can all do something to preserve the water in Angat.)

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The dam's declining water level, lack of rains due to El Niño, and high demand were blamed for a water supply shortage that brought misery to more than a million households in Metro Manila in March.

PAGASA earlier this month declared the start of the rainy season.

However, PAGASA also said weak El Niño conditions which were felt in the tropical Pacific since the last quarter of 2018 is "likely to continue" until August.