As I mentioned in my wet season outlook post from a couple of weeks ago, an ongoing El Niño event continues to strengthen. As a result, Hawaiʻi should have a drier than normal wet season. This is what usually happens in Hawaii during El Niño events, and this is consistent with the forecasts by the latest dynamical and statistical models. Does this mean that the whole October through April wet season is drier than normal during an El Nino? If not, what part of the wet season would the driest conditions occur?
To show how the El Niño rainfall conditions in Hawaiʻi evolve during the wet season, I thought it would be useful to show October through April monthly rainfall totals from different locations across the state. El Niño events vary in strength, so I pulled totals from the most recent events in each strength category. These categories are weak, most recently in 2018-2019, moderate (2023-2024), strong (2009-2010), and very strong (2015-2016). I also included totals from the last ENSO-neutral event (2019-2020) and the recent weak La Niña in 2025-2026 for comparison purposes. “ENSO” means El Niño Southern Oscillation, which is the name for the large-scale climate feature in the Pacific and encompasses the full range of events from El Niño to La Niña. ENSO-neutral is between El Niño and La Niña states. Check out this blog post from climate.gov for the basics on ENSO.
Note that the El Niño events and their strength categories are based on CPC’s Relative Oceanic Niño Index (RONI). A portion of the RONI table is included below. It was recently updated to include the August-September-October value of +2.0C. Since there have been five straight months of RONI values at or above +0.5C, the ongoing event is now officially considered to be an El Niño event. Additionally, a value of +2.0C makes it a “very strong” event.
CPC’s RONI table values. RONI values in bold red font are El Niño events, bold blue values are La Niña events, and the remainder are ENSO-neutral. The x-axis shows 3-month seasons starting with December-January-February (DJF) on the left to November-December-January (NDJ) on the right. The table was split and cropped for easier viewing.
Before talking about the general tendencies, I need to point out that every ENSO event is different, even if they’re within the same strength category. For example, two moderate El Niño events may share the same general characteristics but will differ in the details. These differences can result in variations in rainfall at different times during the wet season and at different locations. That said, there are some tendencies that can be teased out to give you a general idea of the overall conditions to expect. For the following discussion points, please refer to the October through April rainfall graphs included below.
Here are some of the tendencies I see:
- The early portion of the wet season, October and November, can be quite wet, especially during strong and very strong El Niño events. Presumably this is due to an extension of the wetter than normal conditions during the summer. It’s probably related to more late season tropical cyclone activity in the Central Pacific basin. This makes it harder to sound credible during climate outlook briefings in the early fall because you’re trying to tell your audience it’ll become much drier than normal while it’s pouring rain outside! This happened to me multiple times.
- The El Niño dry pattern can start to become established in the second half of December, but if it was still rainy early in the month, the signal in the statistics can be unclear.
- You win the bets in January and February, especially during strong and very strong El Niño events. The dry pattern over Hawaiʻi becomes well established and is very persistent. This time of the year can be very wet during weak La Niña and ENSO-neutral events so you clearly see and feel the contrast between these events and the strong/very strong El Niño events.
- March and April can still be drier than normal, but the signal may not be as strong as during January and February.
- Emphasis is often on whether or not we’re in El Niño or La Niña. However, don’t ignore ENSO-neutral, especially if you’re interested in rainfall impacts. Hawaiʻi’s wet season can be very wet during ENSO-neutral conditions. A while back I looked at wet season rainfall and found that 7 out of 10 of the wettest wet seasons occurred during ENSO-neutral conditions. It would be interesting to revisit this with newer and more comprehensive data.
Map of data locations used in the following graphs.
Plots of monthly rainfall as percent of average values for the October through April wet season for the most recent weak La Niña (2025-2026, blue bars), ENSO-neutral (2019-2020, green bars), weak El Niño (2018-2019, yellow bars), moderate El Niño (2023-2024, orange striped bars), strong El Niño (2009-2010, red stippled bars), and very strong El Niño (2015-2016, maroon bars) events. Plots are available for Līhuʻe Airport, Honolulu Airport, Kahului Airport, and Hilo Airport. Plots for Kapāpala Ranch and Waikoloa on the Big Island, and Waimea Heights on Kauaʻi are included for increased representation of non-windward sites. The rainfall data are from the Monthly Precipitation Summary data tables on the Honolulu Forecast Office’s website and archived personally over the years. Average values are from the Hawaiʻi Climate Data Portal for Waikoloa and Waimea Heights, and the National Centers for Environmental Information for the rest.
Climate data and forecasts are often broken down by 3-month seasons, such as November-December-January, or NDJ. However, because the El Niño dryness in Hawaiʻi can be less clear in December and March, December-January-February or January-February-March rainfall signals can be muddied by rain events on the fringes. I thought it would be interesting to just show the January and February rainfall numbers for the different types of ENSO events.
In the graphs below, you can see that January and February rainfall during strong and very strong El Niños is below 50 percent of average at most of the sites and in some cases below 20 percent of average. This applies to both windward and leeward locations, meaning its statewide dryness. Weak El Niño events can be “leaky”. They tend to be dry, but heavy rain events sometimes occur so it ends up not being as dry as during strong or very strong events.
Same as the October through April graphs, but showing only the January and February rainfall when the El Niño rainfall signal is the strongest.
Even though October this year was rather wet, with all this monsoon trough business, I wouldn’t expect it to continue. Watch and see what January and February are like. Previous events point toward “very dry”.