The Intertropical Convergence Zone is sitting directly over Costa Rica this Monday, creating significant atmospheric moisture and instability. This will continue into Tuesday, although with diminishing intensity.
Moisture levels will begin to drop Wednesday, creating a mid-week lull with more isolated rainfall. But this drier spell won’t last long as Tropical Wave #29 arrives Thursday, pumping fresh moisture back into the atmosphere. Following that system’s passage (usually just one day), the ITCZ is going to reposition itself over the country, reestablishing unstable, rain-favorable conditions for the weekend.
Special Note: This weekend brings Costa Rica’s parades of handmade lanterns (faroles) on Sunday and Independence Day next Monday, September 15th. While Sunday may see enhanced rainfall from the repositioned ITCZ, Monday should bring more manageable afternoon thunderstorm patterns for celebrations.
Morning PatternsPartly cloudy skies generally prevail across the country throughout the week, with the best weather windows typically in the early hours. Pacific coastal areas may experience some rainfall, particularly during the more active periods at the beginning and end of the week.
Afternoon DevelopmentThe highest moisture days will be Monday-Tuesday and Friday-Sunday: The Central Valley experiences partly cloudy to stormy conditions with rain and thunderstorms. Central and South Pacific regions, along with the Northern Zone, develop extensive cloud cover with active shower and thunderstorm cycles. Even the typically drier North Pacific sees increased rain chances.
The lowest moisture day is Wednesday: All regions see reduced storm activity, with the North Pacific maintaining partly cloudy conditions and only isolated afternoon development elsewhere.
Transition day is Thursday: Tropical Wave #29 gradually increases atmospheric energy throughout the day, rebuilding storm potential across most regions. Caribbean mountain areas maintain their typical pattern of afternoon showers throughout the week, with enhanced activity during the higher moisture periods.
Evening RainsNight patterns follow the daily moisture cycle. During active periods, the Central and South Pacific experience consistent coastal thunderstorms during early evening hours, while the Northern Zone maintains partly to mostly cloudy conditions with ongoing scattered storm activity. The Central Valley and North Pacific alternate between calm partly cloudy nights and possible rain depending on daily moisture levels.
Caribbean regions generally stay partly cloudy with light mountain rain potential, following their typical September pattern of mountain-focused precipitation.
This week shows us how quickly atmospheric conditions can change in September. The ITCZ’s movements, combined with tropical wave passages, create quick moisture swings typical of peak rainy season weather.
If you have outdoor projects or are planning an activity that needs better weather, Wednesday and early Thursday are your best chance. Otherwise, grab the umbrella and enjoy the tropical rains.
Temperatures reflect the varying moisture and cloud cover patterns, staying moderated during active periods and warming slightly during the Wednesday lull.
High: 31° (87.8°) | Low: 15° (59°)
High: 34° (93.2°) | Low: 20° (68°)
High: 33° (91.4°) | Low: 15° (59°)
High: 33° (91.4°) | Low: 15° (59°)
High: 32° (89.6°) | Low: 20° (68°)
High: 34° (93.2°) | Low: 17° (62.6°)
This forecast is based on the weekly outlook shared by Costa Rica’s National Meteorological Institute (IMN) and other sources.
