Mauka Showers

Last week I talked about the Kona slopes annual rainfall cycle. As a natural follow-on, this week I planned on covering the windward Big Island annual rainfall cycle and how drought manifests itself on that side of the island during the summer. So much for well-laid plans.

On August 14th, last Thursday, NOAA’s Climate Prediction Center (CPC) issued a La Niña Watch. A La Niña Watch is “issued when conditions are favorable for the development of La Niña conditions within the next six months.” So to quote Bloat at the end of the movie “Finding Nemo”, “Now what?”

La Niña is the cool phase of the whole El Niño/Southern Oscillation (ENSO) cycle, and like its warm counterpart El Niño, it can play a significant role in determining what our October through April wet season weather will be like in the Hawaiian Islands. The CPC’s probabilities for August (graph below) show that La Niña conditions are slightly favored for the fall of 2025. The graph also shows that ENSO-neutral (gray bars) is favored already in early 2026 so CPC doesn’t expect this La Niña event to last long if it does indeed develop.

NOAA CPC’s 3-month probabilities for El Niño (red), La Niña (blue), and ENSO-neutral (gray). The x-axis shows 3-month seasons from July-August-September (JAS) 2025 through March-April-May (MAM) 2026.

Just last month, CPC’s probabilities favored ENSO-neutral for the rest of the year so what changed? The graphs below show forecasts of sea surface temperature (SST) anomalies for the “Niño 3.4” region of the equatorial Pacific from several dynamical and statistical models. Many climate experts focus on this region to help determine the ENSO phase. When the 3-month SST anomaly in Niño 3.4 is at or above +0.5 degrees C, it’s an indicator that El Niño is occurring. If the anomaly is at or below -0.5 degrees C, then it’s an indicator of La Niña. Anomalies between the two thresholds are considered to be ENSO-neutral territory. In the April 2025 forecast, the consensus of the guidance from long range climate models showed ENSO-neutral persisting through the end of 2025. The spread in the forecasts was also quite large, with fall projections ranging from moderate-to-strong La Niña to moderate-to-strong El Niño. However, in July and August, the guidance trended toward La Niña (cooler), and the spread of forecasts tightened (higher confidence). The forecast guidance, along with observed changes in the ocean and atmosphere, led CPC to issue the La Niña Watch.

Map showing the location of the Niño 3.4 region used to monitor ENSO conditions.

The graph from the Columbia Climate School’s International Research Institute (IRI) showing the April 2025 model forecasts for SST anomalies in the Niño 3.4 region. Each line is a different climate model. Lines with filled icons are the dynamical models, and those with unfilled icons are statistical models. The dynamical consensus is the bold, dark red line, and the statistical consensus is the bold, dark green line. The pink vertical bar has been added to highlight the SON through NDJ periods where La Niña probabilities are favored.

Same as above, but for the August 2025 forecast.

So here’s where the “now what?” portion comes in. If a La Niña does develop, there’s a general expectation that the Hawaiian Islands should have a wet season with above average rainfall, as seen in the graphic below. This may have been true for your grandparents’ La Niña, but it’s not so simple now.

Map from climate.gov showing temperature and precipitation patterns for La Niña during the Northern Hemisphere winter.

There are a couple of factors mucking up the deal here. First is that La Niña events have been drier in the Hawaiian Islands in recent decades (O’Connor et al. 2015), with the shift occurring around the early 1980s. The second factor is that the strength of the La Niña appears to make a difference. The figures below from Sutton et al. (2023) show precipitation changes in the January-February-March period in the year following La Niña onset. The first figure is for weak La Niña events, and it shows most of the main Hawaiian Islands having above average precipitation, especially Kauaʻi and Oʻahu. The second figure is for moderate-to-strong La Niña events. The area of above average precipitation is shifted toward the northwest, leaving the main Hawaiian Islands in an area of below average precipitation. The data used in these maps are from the period after the shift identified by O’Connor et al. (2015), so they incorporate the recent trend toward drier La Niñas.

Map of precipitation changes from the average across the tropical central and west Pacific during weak La Niña events. The image covers January through March in the year following La Niña onset and was pulled from Sutton et al. (2023).

Same as above, but for moderate-to-strong La Niña events.

The Niño 3.4 graphs shown above point toward a weak La Niña event, if it manages to develop at all. Based on the precipitation change map, this would favor above average rainfall for the upcoming October 2025 through April 2026 wet season. This is also supported by the latest available precipitation outlook from the North American Multi-Model Ensemble (NMME). The map below shows probabilities favoring above normal precipitation over the Hawaiian Islands for the January through March 2026 (JFM) period. Maps for November 2025 through January 2026 and December 2025 through February 2026 (not shown) are similar.

Precipitation probabilities for January through March 2026 from the North American Multi-Model Ensemble (NMME) August 2025 forecast. Brown shades are areas where probabilities favor below normal precipitation. Green areas favor above normal precipitation. The red oval indicates the location of the Hawaiian Islands.

Based on the climatology and the NMME output, you may think that it’s pretty much guaranteed that we’ll have above average rainfall for the upcoming wet season. Not so fast, young man! There’s still quite a bit of uncertainty that makes the forecast anything but guaranteed. Here are a couple of recent examples to illustrate the point.

Back in the summer of 2017, a weak La Niña was in the process of developing. The September forecast from the NMME showed probabilities slightly favoring above normal precipitation for the upcoming October through April wet season. The January through March 2018 forecast map is shown below.

Same as above, except for JFM 2018.

We ended up with a wet season that ranked 3rd wettest in the last 30 years. A map of the 6-month Standardized Precipitation Index (SPI) for the state ending in April 2018 shows large portions of the island chain categorized as “moderately wet” to “extremely wet”. It looks like the forecast was on target for wet conditions, just like the NMME and climatology said it would.

SPI analysis from the Hawaiʻi Climate Data Portal (HCDP). The SPI is mainly used for monitoring drought, but is applicable to all rainfall conditions. The map shown here covers the 6-month period ending in April 2018.

But (and there’s always a “but”) not all wet season outlooks end up successful with pats on the back and high fives to go around. You just need to look back at the last wet season to see a nice big serving of humble pie. Last summer, it looked like a weak La Niña would develop late in 2024. The NMME precipitation forecast from September 2024 was consistent with this expectation, showing probabilities favoring above normal precipitation from around December 2024 through April 2025. The JFM 2025 NMME panel is shown here. The probabilities not only favored above normal precipitation, they were higher than what was forecasted in September 2017. I was thinking “guarans-ball-barans” (small kid time Hilo language) gonna be wet!

Same as above, except for JFM 2025

Here’s the SPI that helps capture what actually played out. We ended up with the 2nd driest wet season in the last 30 years! The large-scale pattern in December 2024 and into mid-January 2025 looked more like an El Niño event with persistent stable and dry conditions over the state, a strong zonal jet stream in the North Pacific, and North Shore surf big enough to hold the “Eddie”.

Same as the SPI map above, but for the 6-month period ending in April 2025.

So to answer the “now what?” question, I think there’s a direction we can lean toward right now (wetter than normal), but don’t lean too far because there’s a lot of uncertainty in the outcome, and a lot can change over the next several months. Over the years, I’ve always wished there was a higher degree of confidence in the long range forecasts, but it just isn’t possible right now.

P.S. – For those who are unfamiliar with the whole La Niña, El Niño, ENSO-neutral thing, you can check out the El Niño/Southern Oscillation (ENSO) page by the National Centers for Environmental Information and the “What is El Niño?” page by the Pacific Marine Environmental Laboratory. They do a nice job of explaining the basics of these large-scale climate features.

References

O’Connor, C.F., P-C Hsu, and K. Kodama, 2015. Variability of Hawaiian winter rainfall during La Niña events since 1956. J. Climate, 28, 7809-7823.

Sutton, J., N. Luchetti, E. Wright, M.C. Kruk, and J.J. Marra, 2023. An El Niño Southern Oscillation (ENSO) based precipitation climatology for the United States Affiliated Pacific Islands (USAPI) using the PERSIANN climate data record (CDR). NOAA. https://www.ncei.noaa.gov/sites/g/files/anmtlf171/files/2023-08/ENSO_Rainfall_Atlas.pdf