"Isang" (International name: Hato) has intensified into a tropical storm as it moves in a westward direction, according to state weather bureau PAGASA.
In its 11 p.m. advisory, PAGASA said Isang was last seen at 185 kilometers east of Basco, Batanes.
It has maximum sustained winds of 65 kilometers per hour with gustiness of 80 kilometers per hour.
It is forecast to move west at the speed of 17 kilometers per hour, said PAGASA.
Tropical Cyclone Warning Signal No. 1 is currently raised over Batanes and the Babuyan group of islands.
Isang is expected to exit the Philippine Area of Responsibility (PAR) by Tuesday night or Wednesday morning.
Moderate to occasionally heavy rains, which may trigger flashfloods and landslides, are expected over Northern Luzon.
Isang may also enhance the southwest monsoon or the habagat, which may bring rains over the western section of Luzon and Visayas.