MANILA, June 2 -- A huge part of the country will likely continue experiencing high heat index levels, possibly until mid-June, despite the rainy season, the weather bureau's head said on Tuesday.

The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) declared the start of the southwest monsoon (habagat) last May 30.

Occasional to frequent rains and thunderstorms are expected, PAGASA said in an advisory, adding the possibility of the onset of rainy season in the coming days.

In an interview with the Philippine News Agency, PAGASA Administrator Nathaniel Servando said it is important to know that high heat index levels, or what the temperature feels like to the human body when relative humidity is com...