MANILA, Philippines: A heat index of 41 C to 42 C would likely be felt in Metro Manila until Friday as the ridge of high pressure area (HPA) and the easterlies continue to prevail all over the country, the state-run weather agency said on Thursday.

Weather specialist Benison Estareja of the Philippine Atmospheric Geophysical and Astronomical Services Administration (Pagasa) said the combined effects of the two weather systems were bringing warm and humid weather across the archipelago.

"There would be isolated downpours and thunderstorms but only in some areas and this could happen only in the late afternoon or at night," the Pagasa forecaster said.

Pagasa in its 5 a.m. advisory said that a temperature in the metropolis could reach up to 34 degrees Celsius but the heat index is expected to be much higher.

"By Friday, the heat index in the National Capital Region could go up by 42 degrees Celsius," Estareja said.

The state weather bureau has reiterated its reminder to the public to limit outdoor activities particularly at noon up to mid-afternoon.

For others who could not avoid going out, they should bring protection from the scorching heat and drink plenty of water, it added.

Pagasa said heat index is what the temperature feels like to the human body when relative humidity is combined with the air temperature.