September has arrived with typical conditions for what is historically one of Costa Rica’s wettest month across most of the country. As we enter peak rainy season, weather patterns become more predictable yet more intense, driven largely by the position of the Intertropical Convergence Zone (ITCZ).
The ITCZ plays a particularly dominant role during September, with its atmospheric moisture and instability. Adding to that, this Monday kicked things off with Tropical Wave #27 moving through and moderate trade winds, which have brought even more rain (we got hammered with afternoon downpours on the central Pacific).
ITCZ influence peaks: During September the Intertropical Convergence Zone typically exerts some of its strongest influence over Costa Rica’s weather patterns as it moves back south over the country. Learn more about this crucial weather driver in our guide to Costa Rica’s Intertropical Convergence Zone.
After Tropical Wave #27 passes and trade winds weaken mid-week, the ITCZ’s positioning maintains favorable conditions for heavy rainfall across the Pacific regions, Northern Zone, Central Valley, and Caribbean mountains. This is a classic September pattern where multiple weather drivers align to create consistent afternoon and evening storm activity.
Monday’s tropical wave impact: Tropical Wave #27 and moderate trade winds create widespread instability and rainfall potential across most regions.
Tuesday’s transition: Trade winds remain moderate, particularly affecting northern Guanacaste, while storm systems continue developing.
Mid-week ITCZ dominance: Weakening trade winds allow the Convergence Zone to fully establish its influence, driving consistent daily storm cycles.
Weekend consistency: The established pattern continues with reliable afternoon thunderstorm development across multiple regions.
Partly cloudy conditions typically develop across Pacific regions and the Central Valley, offering the most favorable weather windows of each day. Tuesday brings moderate winds to northern Guanacaste areas. The Caribbean and Northern Zone may experience rain and shower activity during morning hours, though this varies by specific location.
The Central Valley and Pacific transform from partly cloudy to cloudy as daytime heating triggers showers and thunderstorms. The Caribbean and Northern Zone experience variable cloudiness with enhanced rainfall concentrated in mountain ranges where terrain amplifies the ITCZ’s effects. The Caribbean coast, may remain much drier.
Northern Pacific and Central Valley areas maintain partly to completely cloudy skies with possible rain extending into early nighttime hours. Limón and northern regions see continued shower development. Central and Southern Pacific coastal areas experience consistent evening storm activity with coastal showers becoming well-established.
Pacific Regions and Central Valley: Experience classic September behavior with morning clearing followed by increasingly reliable afternoon thunderstorm development. The weakening trade wind pattern allows storms to develop more consistently than during drier months.
Caribbean and Elevated Areas: Mountain regions see enhanced rainfall as the ITCZ’s moisture interacts with topography. However, Caribbean coastal plains and northeastern areas often experience their driest conditions during September and October, as seasonal wind patterns shift moisture away from these regions toward the Pacific side of the country.
Northern Zone: Benefits from multiple moisture sources, experiencing both trade wind-driven precipitation and ITCZ-enhanced afternoon storm development throughout the seven-day span, especially in the mountains.
September temperatures reflect increased cloud cover and storm activity, with pleasant morning warming giving way to storm-cooled afternoons.
High: 30° (86°) | Low: 13° (55.4°)
High: 35° (95°) | Low: 19° (66.2°)
High: 32° (89.6°) | Low: 14° (57.2°)
High: 32° (89.6°) | Low: 16° (60.8°)
High: 31° (87.8°) | Low: 18° (64.4°)
High: 32° (89.6°) | Low: 16° (60.8°)
This forecast is based on the weekly outlook shared by Costa Rica’s National Meteorological Institute (IMN) and other sources.
