Kōchi's Pacific location gives it a warmer, wetter climate than the rest of Shikoku. The city receives more rainfall than almost anywhere else in Japan — the southern mountains wring moisture from the typhoon-season air — and summer humidity is high. The upside is mild winters and an early spring.
Spring (March–April) is the best time to visit. Cherry blossoms at Kōchi Castle typically peak in late March, slightly earlier than the rest of Japan. Temperatures are pleasant (15–22°C) and the days are clear before the rains arrive.
May–June brings the rainy season (tsuyu), with overcast skies and frequent downpours. Not miserable, but not ideal either.
Summer (July–August) is hot, humid, and typhoon-prone. The Yosakoi Festival runs on August 9–12, filling the city with dancing teams and live music. It is spectacular if chaotic, and accommodation books up weeks in advance.
Autumn (October–November) is the second-best season: clear skies, cooler temperatures, and good river conditions for rafting in the Niyodo and Shimanto valleys.
Winter (December–February) is mild and quiet. The coast rarely freezes; snow in the city is unusual, though the mountains see snow regularly.