Golfito is hot and humid year-round (average highs 30–32°C). The dry season runs from December to April; the rainy season from May to November brings afternoon downpours that can last for hours and sometimes flood the lowland roads. September and October are the wettest months.
For most travellers the December to April window is the easiest: dry trails, clear water for whale watching, and the active billfish season. For whales specifically, August and September (Southern Hemisphere whales calving) are the unmissable moment — the wettest months but also the moment when the gulf has its highest concentration of cetaceans.