Two seasons, set by the ITCZ
Panama does not have four seasons; it has two. A wet season runs broadly from May into November, and a dry season (which Panamanians call verano, “summer”) runs from roughly December into April. The mechanism is the Intertropical Convergence Zone, or ITCZ, the band near the equator where trade winds from the two hemispheres meet and force warm, moist air upward into thunderstorms. Over the year, that band migrates north and south following the sun, and Panama sits close enough to the equator that the ITCZ passes overhead twice. When the ITCZ is parked over or north of Panama, the country gets its wet season; when it is pushed south and the northeast trade winds dominate, the country dries out into verano.
The asymmetry between the two coasts is a direct consequence. Moisture for Panama’s wet season flows westward from the Caribbean, so the Caribbean side of the country receives much higher rainfall than the Pacific side, a pattern documented across the national water planning and Smithsonian literature. A traveler notices it quickly: Bocas del Toro and the Guna Yala coast are wet nearly year-round, while the Azuero peninsula and the area around Panama City have a pronounced, dusty dry season.
The overlay that makes it volatile: ENSO
The two-season cycle would be predictable if it ran on its own. It does not. Panamanian rainfall is strongly modulated by the El Niño–Southern Oscillation, the basin-wide sloshing of warm water and atmospheric pressure across the equatorial Pacific. When the central and eastern equatorial Pacific runs warmer than normal (El Niño), Panama tends toward drier conditions and a weaker wet season; when it runs cooler than normal (La Niña), Panama tends toward wetter conditions. The 2023–2024 El Niño, one of the strongest on record, is the recent example everyone felt: it contributed to the drought that forced the Panama Canal to restrict transits and reduce the draft ships could carry.
The practical importance of ENSO to Panama is therefore not abstract. A strong El Niño means a dry wet season, lower lake levels, canal draft restrictions, and water-supply stress on the Pacific slope. A La Niña means heavier rain, higher landslide risk on the steep Caribbean slopes, and fuller reservoirs. The oscillation is the single biggest source of year-to-year weather variability in the country.
How El Niño and La Niña are actually defined
The terms get used loosely in weather reporting, so it is worth stating how the authorities define them. The U.S. National Oceanic and Atmospheric Administration’s Climate Prediction Center identifies El Niño and La Niña episodes using the Oceanic Niño Index (ONI): a three-month running mean of sea-surface temperature anomalies in the Niño 3.4 region of the central equatorial Pacific (the band from 5°N to 5°S, 120°W to 170°W), computed from the ERSST.v5 dataset.[1] An episode is declared when the ONI exceeds a threshold of ±0.5°C (that is, when the running mean is at least half a degree above (El Niño) or below (La Niña) the long-term average) and when that threshold is met for at least five consecutive overlapping three-month seasons.[1]
That definition matters because it is a deliberately high bar. A single warm month does not make an El Niño; the anomaly has to be sustained across more than a year of overlapping seasons. The threshold and the persistence requirement together filter out short-term noise and reserve the El Niño/La Niña labels for events that are large and long enough to drive real teleconnections, including, at the other end of the chain, Panama’s rainfall.
NOAA has also updated its monitoring basis. The agency now uses the Relative Oceanic Niño Index (RONI) for official monitoring, per National Weather Service Public Information Statement 26-05, which adjusts the anomaly to account for the long-term warming of the tropical Pacific so that El Niño and La Niña are identified against a moving baseline rather than a fixed historical average.[1] The substantive definition, a sustained ±0.5°C anomaly in the Niño 3.4 region, is unchanged; the RONI refines how the anomaly is computed.
The recent sequence: a strong El Niño, then a weak La Niña
The most consequential recent ENSO event was the 2023–2024 El Niño. It was one of the strongest on record, peaking at an ONI of roughly +2.1 in December 2023, then weakening through 2024 (the peak monthly value in January was around +1.9) as the Pacific cooled back toward neutral.[1] That event’s dry footprint on Panama is the proximate cause of the canal’s 2023–2024 crisis: Gatún Lake fell to historically low levels, the canal authority reduced daily transits and maximum draft, and shipping companies faced multi-day queues and surcharges. The El Niño did not act alone (longer-term rainfall variability also contributed), but it was the dominant driver.
As the El Niño faded, conditions moved through neutral territory and into the cool phase. Through 2025, the ONI ranged roughly from 0.0 to −0.3 for much of the year before tipping into a weak La Niña by year-end, with ONI values around −0.5 to −0.6 in late 2025.[1] A weak La Niña is a modest event: it nudges Panama toward wetter conditions but does not guarantee flooding or a crisis in either direction. The interpretive point is that the ENSO state is a continuous, evolving quantity, and a page like this is date-stamped: the figures above describe the state at the time the source was captured, and a reader wanting the current state should consult NOAA’s Climate Prediction Center directly.
Who measures Panama’s weather
Observation is the unglamorous foundation of all of this, and Panama’s weather and hydrology are monitored by a network operated by several institutions in parallel. The Empresa de Transmisión Eléctrica (ETESA) is the national authority for electricity transmission and the principal hydro-meteorological operator; the Autoridad del Canal de Panamá (ACP) maintains the dense instrumentation in the canal watershed, because the canal’s operations depend on knowing lake levels and rainfall in real time; and the Smithsonian Tropical Research Institute (STRI) operates research-grade stations, particularly at its field sites such as Barro Colorado Island, Bocas del Toro, and Galeta.[2] These stations are visible as a mapped network through the STRI GIS portal, which draws on ETESA, ACP, and STRI data sources.[2]
The institutional pluralism is a strength. ETESA handles the operational forecast and warning responsibility; ACP’s watershed data are the gold standard for the canal basin specifically; and STRI’s long-term research stations provide the continuous, carefully quality-controlled records that climate science requires. For a reader, the practical entry point for current Panamanian weather and warnings is ETESA’s Dirección de Hidrometeorología; for the canal-specific conditions that drive draft restrictions, it is the ACP’s hydrological reports.
Connecting weather to water and climate
This page deliberately stays on the mechanics of weather, the wet-dry cycle and ENSO, rather than the broader climate averages or the practical travel-planning implications. The general climate of Panama (temperatures by altitude, the meaning of the wet season for a visitor’s itinerary) is on the climate-of-panama page, and the regional variation (why Boquete is cool, why Bocas is wet, why the Azuero dries out) is on the climate-by-region page. The link to water resources runs through the same ENSO mechanism described here: the El Niño-driven drought of 2023–2024 that stressed the canal is the same phenomenon that, in aggregate, pulls Panama’s renewable water availability below its long-run average in dry years. And the link to risk runs through the wet season: heavy La Niña-enhanced rainfall is what drives the landslide and flooding hazard that civil protection manages each year.
The dry-season trade wind and Pacific upwelling
The dry season is not just the absence of rain; it has its own active mechanism. During verano, the northeast trade winds blow across Panama, and where those winds are not blocked by mountains, notably across central Panama and the Azuero peninsula, they push Pacific surface water offshore, which draws cold, nutrient-rich water up from depth in a process called upwelling. That upwelling is the reason the Gulf of Panama and the Gulf of Chiriquí turn biologically productive in the dry season: the cold, nutrient-rich water feeds phytoplankton, which feed the small fish, which in turn draw the seabirds, whales, sharks, and rays that concentrate in the eastern tropical Pacific. A weather phenomenon, the seasonal wind, thus drives one of Panama’s great biological spectacles. The upwelling also cools the Pacific coast locally, which is why a dry-season visitor to the Azuero or the Pearl Islands can find the air and water cooler than the latitude would suggest. The asymmetry between the wet Caribbean and the seasonally dry, upwelling-cooled Pacific is one of the clearest ways Panama’s weather shapes its ecology and its coast.
Reading an ENSO outlook for Panama
For a reader trying to plan around ENSO rather than just understand it, the practical sequence is to check the current ONI state at NOAA’s Climate Prediction Center, read the CPC’s seasonal outlook for the probability of El Niño, La Niña, or neutral conditions over the coming months, and then map that onto Panama’s calendar: a forecast for El Niño during the May-to-November wet season signals drought risk and possible canal draft restrictions, while a La Niña forecast signals heavier rain and higher landslide risk on the Caribbean slopes.[1] ENSO is probabilistic, not deterministic: an El Niño tilts the odds toward dryness but does not guarantee a drought, and La Niña tilts them toward wetness without guaranteeing floods. The canal authority’s hydrological reports and ETESA’s seasonal outlook are the two Panama-specific sources that translate the central-Pacific signal into local expectation. The shift to the Relative ONI (RONI) matters here mainly as a technical refinement (it adjusts the anomaly baseline for long-term warming) and does not change how a reader interprets the ±0.5°C / five-season threshold.[1]
What to verify before relying on this
This page covers the two-season cycle, the ENSO definition (ONI, Niño 3.4, the ±0.5°C / five-season threshold, the RONI), the 2023–2024 El Niño and 2025 La Niña sequence, and the observation network. The ENSO values cited are point-in-time and the ONI updates monthly, so any specific figure here is the value at the time the source was captured and should be re-verified against NOAA CPC for the current state. This page does not provide a forecast or a seasonal outlook; for those, consult ETESA and NOAA CPC directly. The page also stops short of the Panama-specific rainfall-teleconnection science, the detailed statistical relationship between Niño 3.4 and rainfall at particular Panamanian stations, because a peer-reviewed Panama-specific teleconnection source was not available in this pass; that relationship is described here at the level the regional literature supports. Practical wet-season and hurricane-risk guidance for visitors is on the rainy-season and climate pages.
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