We’re heading towards the end of the wet season and it finally looks like things are settling down precipitation-wise, with no measurable rainfall at my house five days in a row this week. Hopefully the weeds on my lot won’t grow as fast now that it’s a bit drier! In addition to catching up on the yard work, I figured it’s also a good time to close out my multi-part series of posts on Hawaiʻi’s wet season trends.
I started talking about this back in late January to take a look at why flash flood frequencies in October and November have dropped so much over the last 20 years or so. It turns out that rainfall in October and November over the past 60 years has been trending downward, especially in the west half of the state. I followed up with another post in early March focusing on March and December since these months are #1 and #2, respectively, in terms of flash flood frequency. In this third and final post on wet season trends, I’ll take a look at the overall October through April numbers at the four main airports, and also look at four other sites around the state with long and reliable periods of record. The locations of the eight sites are indicated on the map below.
Map of all the data sites used in this post. Locations are from the Hawaiʻi Climate Data Portal (HCDP).
Similar to the graphs I used in my January 30 and March 6 posts, I plotted the rainfall totals from the 1961 wet season (October 1960 through April 1961) through the 2025 wet season at Līhuʻe Airport, Honolulu Airport, Kahului Airport, and Hilo Airport. The plots also include the trendlines for the 10-year running average and the overall wet season data record.
Plots of October through April rainfall totals for Līhuʻe Airport, Honolulu Airport, Kahului Airport, and Hilo Airport from 1961 through 2025. The amounts are plotted on the year of the January through April portion of the wet season (i.e. the October 1960 through April 1961 wet season is plotted as 1961). The red dashed line is a trendline based on the running 10-year average of the totals. The black dashed line is the linear trendline over the whole time series. Data were pulled from the xmACIS2 database.
All four locations show decreasing trendlines for the wet season totals. Līhuʻe and Honolulu have the most significant drops, while the overall trendlines for Kahului and Hilo are flatter. In fact, the Kahului and Hilo records fail the Mann-Kendall trend test (using the method at real-statistics.com), whereas Līhuʻe and Honolulu pass the test at the 0.05 significance level. These trends are similar to the October + November trends in my January post which have the larger decreases in the west half of the state.
I know I’ve been focusing on the four main airports, but what about other sites across the state from different climate zones. Historically, we’ve had a fairly dense network of rainfall data sites across the state, but finding locations with a continuous period of record going back to the 1960-1961 wet season is not easy. Most of the sites with data in the 1960s are no longer in existence, and the ones that are still available often have significant data gaps or other issues. I was able to scrounge up four, Kōloa (Kauaʻi), Waiāhole (Oʻahu), ʻUlupalakua Ranch (Maui), and Kapāpala Ranch (Big Island), which is one for each county and on the same islands as the four main airports (see map above). None of the four had perfect data records and all had several missing months. To fill these gaps, I used data from the Hawaiʻi Climate Data Portal month-year grids to obtain a continuous data series of monthly data. The graphs below show that all four additional sites have downward trends, but only Kōloa’s is enough to pass the Mann-Kendall trend test at the 0.05 level. ʻUlupalakua Ranch was close to passing the trend test but didn’t quite make it.
Same as the above graphs, but for Kōloa, Waiāhole, ʻUlupalakua Ranch, and Kapāpala Ranch. The map above shows the locations of these sites.
Overall, these results are in general agreement with the Frazier and Giambelluca (2017) study, though they used a longer period of record (1920 – 2012 vs. my 1961 – 2025) and defined their wet season as November through April instead of October through April. It’s interesting that Waiāhole, which is a site exposed to trade wind rainfall, has a flatter trend that is similar to Hilo Airport’s and is quite a bit different from Honolulu Airport on the leeward side of the same island. I don’t know what would explain this difference, though I have some half-baked ideas that I’ll keep to myself at this point.
To close out, I have to say that it has been an interesting exercise to see where the significant declines have taken place both in time and location. I think the next step would be to take a look at the dry season, but I’ll do that down the road a bit. In the meantime, I’ll be outside watching my weeds grow.
Reference:
Frazier, A.G., and T. Giambelluca, 2017: Spatial trend analysis of Hawaiian rainfall from 1920 to 2012. Int J Climatol, 37, 2522-2531.