The Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) has issued a heat index forecast of dangerously high temperatures in at least 19 areas nationwide on Sunday, May 25, with Dagupan City, Pangasinan expected to experience a sweltering 46°C.

According to PAGASA's May 24 forecast, the heat index — or the "feels-like" temperature based on actual air temperature and humidity — could reach the "danger level" in several locations across Luzon and Visayas.

Meanwhile, 18 other areas are expected to experience “danger level” heat index, including:

- Laoag City, Ilocos Norte; Aparri, Cagayan – 45°C

- Bacnotan, La Union; CBSUA-Pili, Camarines Sur – 44°C

- Sinait, Ilocos Sur; Tuguegarao City, Cagayan; ISU Echague, Isabela; Iba, Zambales; Sangley Point, Cavite City – 43°C

- MMSU Batac, Ilocos Norte; Baler (Radar) and Casiguran, Aurora; Cubi Point, Subic Bay; Ambulong, Tanauan, Batangas; Infanta, Quezon; San Jose, Occidental Mindoro; Virac, Catanduanes; Dumangas, Iloilo – 42°C

PAGASA said a “danger level” heat index — ranging from 42°C to 51°C — poses serious health risks.

Prolonged exposure at this level may result in heat cramps, heat exhaustion, and in severe cases, heat stroke.

In Metro Manila, heat indices remain high but under the “extreme caution” category. PAGASA forecasts 40°C in NAIA, Pasay City, and Science Garden, Quezon City.

The heat index refers to the apparent temperature—how hot it actually feels to the human body when humidity is factored in with the actual air temperature.

PAGASA advises the public in the affected areas to take precautions such as staying indoors and limiting outdoor activities between 10 a.m. and 4 p.m.; keeping hydrated by drinking plenty of water; wearing light and breathable clothing; and using umbrellas, hats, or sunscreen when outdoors.

The public is also advised to watch for signs of heat-related illnesses, especially among the elderly, children, and those with pre-existing health conditions.