Tropical cyclone "Fabian" (international name "In-fa") has intensified into a tropical storm and is expected to continue to enhance the southwest monsoon or "habagat" in the next 24 hours, the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) said in its 5 a.m. bulletin.

As of 4 a.m., the center of tropical storm Fabian was estimated at 1,090 kilometers (km) east-northeast of extreme northern Luzon.

Fabian intensified into a tropical storm at around 2 a.m. Sunday and is now packing maximum sustained winds of 65 kilometers per hour (kph) near the center and gustiness of up to 80 kph.

PAGASA said the cyclone may become a severe tropical storm by Monday afternoon, July 19, and further into a typhoon by Tuesday evening, July 20.

PAGASA also continues to monitor a low pressure area outside the country's area of responsibility, or at 630 km west of Calayan, Cagayan, on Sunday morning.

Both weather disturbances are expected to strengthen the impact of the habagat over the country, PAGASA said.

As such, scattered to widespread moderate to at times heavy monsoon rains may prevail over Palawan, Occidental Mindoro, Aklan, Antique, Capiz, Guimaras, Iloilo, and Negros Occidental within 24 hours.

Metro Manila, Oriental Mindoro, Marinduque, Romblon, Zambales, Bataan, Cavite, Batangas, Laguna, Tawi-Tawi, Sulu, Basilan, Ilocos Region, Bicol, Zamboanga Peninsula, and the rest of Visayas are also likely to experience scattered light to moderate to at times heavy rain showers and thunderstorms.

PAGASA warned against possible flash floods or rain-induced landslides during heavy or prolonged rainfall, particularly in areas that are highly susceptible to these hazards.

"Fabian will remain far from the Philippine landmass throughout the forecast period. The tropical cyclone will maintain a north-northwestward or northwestward heading at a consistent speed until Monday evening (July 19), when it is forecast to exit the Philippine area of responsibility," PAGASA said.

"On the forecast track, the tropical cyclone will pass close or make landfall in the vicinity of the Okinawa Islands in the Ryukyu archipelago on Tuesday afternoon (July 20) and reach the East China Sea on the evening of the same day. A landfall over mainland China may likely occur on Friday morning (July 23)," it added.