Mauka Showers

In my September 26, 2025 post (“Flash Floods in Hawaiʻi – By the Numbers”), I mentioned how during the period from 1960 through 2000, November was a clear #1 in terms of flash flood frequency and October was #2. Over the past couple of decades, however, fewer heavy rain events have been occurring early in the October through April wet season. As a result, in the period from 2004 through 2025, November slipped to #5 and October was tied with September at #7.

Number of flash floods in Hawaiʻi by month for the period from 2004 through 2025. Data for the graph are from my own log of flash flood events. For the official counts, please contact the Honolulu Forecast Office.

Just from experience, it doesn’t feel like October and November in recent years are as active as they used to be in the past. I remember while growing up in Hilo or working in Honolulu during really big heavy rain and flash flood events such as:

• October 1981: 10+ inches on Oʻahu, 25+ inches on Maui, 20+ ft flood wave in Waiawa Stream (central Oʻahu).
• October 2000: 24+ inches over east Maui. Flash flooding from Keʻanae to Hāmoa. 
• October 2004: Mānoa Flood (picture below), with nearly 9 inches of rain in 5 hours in an urbanized area.
• November 1979: 25+ inches over the windward side of the Big Island and widespread flash flooding from Waipiʻo Valley to Pāhala. I remember school was canceled that day. Yay!
• November 1996: Over 12 inches in leeward Oʻahu affected Election Day, followed a week later by heavy rainfall that flooded Mākaha Valley.
• November 2000: 37+ inches (graph below, 22 inches in 6 hours!) in the Kaʻū District and over 24 inches in portions of the South Hilo and Puna Districts of the Big Island. Widespread flash flooding and several bridges washed out.

Picture of the upstream side of the bridge over Mānoa Stream at Woodlawn Drive. I took the picture during a post-event damage survey the following day.

Plot of hourly rainfall totals from the Kapāpala Ranch Hydronet gage during the November 1-2, 2000 flash flood event. I used this plot in numerous presentations, mainly to highlight the 22 inches in 6 hours.

It just doesn’t feel like we get events like this as often anymore. However, feeling is one thing, but what do the data show? Taking a look at data from a large number of rain gages, while providing a more representative picture, would be a significant project and would take a while to complete. I’m retired, so I told me that I don’t want to right now, and me agreed with that logic. For the purpose of this blog, I thought it would be useful to take a look at the numbers from the four main airport climate data sites across the state. While not the wettest areas in the state, Līhuʻe Airport, Honolulu Airport, Kahului Airport, and Hilo Airport, do provide a long and stable period of record.

I pulled data for October and November back to 1960, which is the start of the monthly flash flood event record, and plotted the totals for each site. Trendlines using the running 10-year average were also included. The graphs for the four airports are shown below.

Plots of October+November rainfall totals for Līhuʻe Airport, Honolulu Airport, Kahului Airport, and Hilo Airport from 1960 through 2025. The red dashed line is a trendline based on the running 10-year average of the totals. Data were pulled from the xmACIS2 database.

Naturally, there are significant year-to-year variations, but the trendline is downward by about 50% at Līhuʻe, Honolulu, and Kahului. Interestingly, the trendline at Hilo is essentially flat. There appears to be a hint of a decadal signal with an increase in rainfall from the late-1980s to around 2010. This signal is most prominent at Hilo and Kahului, but less pronounced at Līhuʻe and Honolulu. I don’t know for sure what changes are occurring in the large-scale pattern to force this, especially since it’s not affecting the entire state uniformly.

The long-term decrease in October+November rainfall at the four airport sites is consistent with the decrease in flash flood events in both months. If this trend continues, it would mean that the Hawaiian Islands dry season effectively runs from May through November with a wet season compressed to just December through April. Definitely not good for agriculture or water supply! The data are also generally consistent with the results from Frazier and Giambelluca (2017) which found a general drying trend from 1920 through 2012 over 90% of the state.

Of course, using data from only four sites produces more questions than answers. For example:

• Is the downward trend also seen at other rain gages in the state?
• What are the trends in other portions of the wet season? Flash flood frequencies have increased in December and March, so have the rainfall totals increased in these months? It would seem to be obvious that it should increase but it’s better to take a look at the numbers than just assume it is the case.
• Is the lack of downward trend in October+November rainfall at Hilo Airport also reflected in a lack of trend in flash flood frequency?
• Flash flooding is produced when intense rainfall of sufficient duration falls over an area with hydrologic conditions primed for rapid runoff. This may not necessarily line up well with 2-month rainfall accumulations. That said, is there a decrease in the amount of intense rain events during October and November?

As Willy Wonka said in the movie, Charlie and the Chocolate Factory, “So much time and so little to do. Wait a minute. Strike that. Reverse it.” I’ll probably look at some of these questions in later blog posts because they’re pretty interesting to me. I just don’t want to tackle them at one time. Also, I suspect most people don’t want to read a post that long.

Reference:

Frazier, A.G., and T. Giambelluca, 2017: Spatial trend analysis of Hawaiian rainfall from 1920 to 2012. Int J Climatol, 37, 2522-2531.