In a statement published on Wednesday, May 18, Pagasa Administrator Vicente Malano said the western sections of Luzon and Visayas have experienced rainfall in the past five days.
"The presence of frontal system and the occurrence of severe thunderstorms have brought widespread rains during the last five days in areas under Type 1 climate and other parts of the country. Moreover, southwesterly wind flow was also observed during the past few days and this satisfies the criteria of the start of the rainy season," Malano said.
According to the weather bureau, Metro Manila and the western sections of the country are expected to experience "intermittent rains associated with the Southwest monsoon."
"However, breaks in rainfall events, also known as monsoon breaks, which can last for several days or weeks may still occur," Malano said, noting that the country should also anticipate above normal rainfall conditions in the coming months because of the ongoing La Niña.
According to the Department of Health, La Niña is a weather phenomenon characterized by unusually cold ocean temperature in the Equatorial Pacific which causes increased numbers of tropical storms in the Pacific Ocean.
Diseases common during this season are those brought by mosquitoes, such as dengue and malaria, and water-borne infections, including acute gastroenteritis, typhoid fever, cholera, hepatitis A, and leptospirosis.