The weather bureau has officially declared on Friday the start of the southwest monsoon, or habagat, indicating the start of the rainy season in the Philippines.

The rest of the country is expected to experience partly cloudy to cloudy skies with isolated rain showers or thunderstorms.

Southwesterly winds will persist until Tuesday, 3 June, bringing scattered rains and thunderstorms over MIMAROPA, Ilocos Region, Abra, Benguet, Batanes, Zambales and Bataan.

From Wednesday to Friday, 6 June, fair weather will occur across most parts of the country, though localized thunderstorms may still occur. However, areas such as the Ilocos Region, Batanes, Bataan, Zambales, Occidental Mindoro and Palawan are likely to continue experiencing scattered rain showers and thunderstorms.

Rains are expected across Luzon on Saturday as the southwest monsoon continues to affect the region, according to the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA).

In its early morning bulletin, PAGASA said monsoon rains will persist over the Ilocos Region and the provinces of Apayao, Abra, Zambales and Bataan. Cloudy skies with scattered rain showers and thunderstorms are also likely over Metro Manila and the rest of Luzon.

Moderate to heavy rains may trigger flash floods or landslides, especially in areas with significant rainfall accumulation.

Elsewhere in the country, localized thunderstorms could bring isolated rain showers. Winds and coastal conditions are expected to remain light to moderate nationwide.

Despite the rains in Luzon, extreme heat continues to grip parts of Mindanao and the Visayas. PAGASA said the heat index could soar to 45°C in Dipolog, Zamboanga del Norte, while 43°C is expected in Zamboanga City and Hinatuan, Surigao del Sur.

Several areas may also experience a 42°C heat index, including Infanta in Quezon, Coron and Cuyo in Palawan, Masbate City, Dumangas in Iloilo, Catarman in Northern Samar, Guiuan in Eastern Samar, and Butuan City in Agusan del Norte.