Mauka Showers

It’s bad enough that we had a generational kona low in the second week of March, but the atmosphere decided to follow it up with a second storm system. As you’ll see toward the bottom of this post, the amounts were not as high as in the initial kona low, but the flooding impacts were just as severe. The main issue this time around was that soil moisture across most areas of the state was still elevated from the first storm. This resulted in runoff occurring much faster and with greater intensity wherever heavy rain occurred.

The worst impacts from the second storm were on Oʻahu during the early morning hours of March 20th. A mesoscale convective system (MCS) moving toward the east-northeast passed south of Kauaʻi on March 19th then hit the North Shore area of Oʻahu late that night. The infrared (IR) satellite loop below shows deep convection from the MCS as it passed over the island.

NOAA/NESDIS color enhanced infrared image loop centered on the main Hawaiian Islands. The loop covers the period from 9:20 PM HST, March 19 to 2:00 AM HST, March 20, 2026.  In this enhancement curve, blues, greens, and yellows show increasingly colder and higher cloud tops. Lighter gray shades are low clouds, and the darker gray areas are the ocean or land areas.

Many of the past significant flooding events in the Waialua area occur in Otake Camp along the banks of Kaukōnāhua Stream. The floodwaters originate from heavy rainfall over the upper slopes of the Koʻolau Range. Runoff fills Lake Wilson, then overflows into the spillway of Wahiawā Dam. If the flow over the spillway is sufficient, Kaukōnāhua Stream overflows its banks and inundates properties in Otake Camp. It doesn’t always happen this way.

On the night of March 19th, the passing MCS dumped 10 to 15 inches of rain in 6 hours over the lower slopes of north Oʻahu from Mokulēʻia to Turtle Bay. As seen in the figure below, this massive amount of rain occurred below the Wahiawā Dam and affected not just the Kaukōnāhua Stream drainage, but all the drainages along the slopes of the North Shore of Oʻahu. The peak rainfall observation came from a relatively new weather station, Kamananui RAWS, located on the north-facing slope of the Waiʻanae Range. This site recorded a peak 6-hour rainfall total of 11.30 inches from 9 PM HST, March 19 to 3 AM HST, March 20, which exceeded the 500-year return period for this location. Other available rain gages in the North Shore region recorded 6-hour totals that were greater than the 50-year return period threshold. All of this rain took place over soils still soggy from the previous week’s rainfall, resulting in rapid and catastrophic runoff. The impacts were so severe that the National Weather Service’s (NWS) Honolulu Forecast Office (HFO) issued a Flash Flood Emergency for the region. This type of warning is reserved for exceedingly rare and impactful flash flood events.

Multi-Radar, Multi-Sensor (MRMS) radar and rain gage rainfall estimates for the 12-hour period ending at 5 AM HST, March 20, 2026. Light blue lines are streams, and the red line outlines the Kiʻikiʻi/Kaukōnāhua drainage basin. Stream lines and the basin boundary are from the Hawaiʻi Statewide GIS Program and the Wahiawā Dam location is from the U.S. Geological Survey’s Pacific Islands Water Science Center.

In addition to the North Shore region, significant flooding also occurred in the Waiāhole and Waikāne areas of Windward Oʻahu, and Mākaha Valley on the leeward side of the Waiʻanae Range. Flow in Waikele Stream, which originates in the upper slopes of the central Koʻolau Range, surged to levels not seen since the record-breaking flood on December 11, 2008.

After the passage of the MCS, northeast-to-southwest oriented rain bands continued to hit the state, mainly over Maui County from March 20th through March 22nd, then over the Big Island on March 23rd. Rainfall totals for the full 5-day period from March 19 through 23, 2026 are shown in the map below.

Flash flooding occurred on Molokaʻi and Maui on March 21st and 22nd. Fortunately, the Maui flood impacts did not appear to be as widespread and severe as what occurred along South Kīhei Road on March 13th and 14th. Rainfall over the Big Island on March 23rd was intense with strong thunderstorms, but the system was more progressive at that time, which helped limit accumulations and flooding impacts. The exception was the Pāhala area in the Kaʻū District of the Big Island. In this case, the rainfall was a bit more persistent, resulting in flooding that closed a portion of the Belt Highway (Highway 11). The satellite loop below, using GOES Lightning Mapper (GLM) data, shows intense lightning activity as the thunderstorms moved onshore along the Kona and South Kohala Coast. The second loop shows even more lightning activity over the Hilo area later in the day after the rain band shifted to the east side of the Big Island.

Map of gridded rainfall in inches for the 5-day period from March 19th through 23rd, 2026. The data come from the Hawaiʻi Climate Data Portal (HCDP) daily rainfall data grids.

NOAA/NESDIS visible image loop centered on the main Hawaiian Islands with Geostationary Lightning Mapper (GLM) data overlaid. The loop covers the period from 7:16 AM to 9:41 AM HST, March 23, 2026. For more information on the GLM data, please click here.

Same as above, but from 12:16 PM to 1:11 PM HST, March 23, 2026.

While the most intense thunderstorms were in the east end of the state on March 23rd, conditions appeared to be more settled over the rest of the island chain with low level winds over Kauaʻi, Oʻahu, and Maui County back to a trade wind direction. Unfortunately, the atmosphere was actually still unstable with a short wave trough aloft to the west of Kauaʻi (map below).

Geopotential heights and winds at 200 millibars from the GFS model’s 00-hr panel valid at 8 AM HST, March 23, 2026. The figure was downloaded from the NCEP Model Analyses and Guidance website. I added a red oval to identify the main Hawaiian Islands and the red dashed line to show the position of the short wave low pressure trough at this level in the atmosphere.

Around midday, showers developed over the windward slopes of the Koʻolau Range on Oʻahu. These showers continued to develop and intensify, eventually becoming thunderstorms that spread into the leeward side of the mountain range. Intense rainfall over the existing wet soils rapidly produced severe runoff in stream basins over the southeast half of the Koʻolaus. Flash flooding occurred (again) in Waiāhole and Waikāne, and there was severe flooding in Mānoa Valley. Mānoa Stream overflowed at the Woodlawn Drive bridge and flooded numerous vehicles. Floodwaters also reached Noelani School, similar to what happened on October 30, 2004. The hydrograph below shows that the water level in Mānoa Stream peaked at a stage value of nearly 13 feet, which is about 9 feet higher than the level earlier in the morning. At a stage value of 12 feet, Mānoa Stream jumps its bank near a home adjacent to the Mānoa Innovation Center. With a peak stage of 13 feet, I’m not surprised that flow got onto Woodlawn Drive. However, I was a bit surprised by the high volume of flow I saw in videos online, which made me wonder if there was a blockage of the channel at the bridge. I saw a post online claiming there was blockage but haven’t seen any confirmation of this. Debris blockage at the bridge would have greatly increased the flow out of the channel similar to what happened in the October 30, 2004 event that also flooded the University of Hawaiʻi at Mānoa campus.

Hydrograph of water level data from the U.S. Geological Survey’s gage at Mānoa Stream near Woodlawn Drive centered on March 23, 2026. The image is from NOAA’s National Water Prediction Service (NWPS).

Here’s a GOES IR satellite loop from the time of the Mānoa Valley flash flood on March 23rd. Look closely for the “puffs” of blue that flare up over the southeast half of the Koʻolau Range. These are the thunderstorms that produced the heavy rainfall, but are easy to miss because of the more intense activity over and around the Big Island and Maui.

NOAA/NESDIS color enhanced infrared image loop centered on the main Hawaiian Islands. The loop covers the period from 9:00 AM to 3:00 PM HST, March 23, 2026.  In this enhancement curve, blues and greens show increasingly colder and higher cloud tops. Lighter gray shades are low clouds, and the darker gray areas are the ocean or land areas.

Taking a look at the MRMS 5-day rainfall analysis shows some interesting features, even though the totals for mid-section of the state are degraded due to the ongoing absence of data from the inoperative Molokaʻi WSR-88D radar. You can see the rainfall swath from the March 19-20 MCS as it tracked south of Kauaʻi and hit the northern portion of Oʻahu. Also evident is the northeast-to-southwest orientation of the rain bands over Maui County.

MRMS 5-day accumulated precipitation over the state covering the period from March 19th through March 23rd. Note that the totals over the mid-section of the state are still degraded because the Molokaʻi radar was inoperative through the entire event. The data in this graphic were obtained from the MRMS repository on the Registry of Open Data on AWS and processed using QGIS software.

The table below shows the top 5 event totals across the state. As in the past, the rainfall totals were pulled from the Hawaiʻi Rainfall Summary (RRAHFO) text product issued by HFO and archived by Iowa State University. Since the RRAHFO product doesn’t include some of the recently installed gages (RAWS and Hawaiʻi Mesonet), I’ve added these as needed based on data retrieved from the HCDP and the WRCC RAWS archive. All of the amounts are midnight-to-midnight totals. Locator maps for each of the gages are included below the table. Since the totals come directly from the RRAHFO product, the HCDP archive, and the WRCC RAWS archive, they have not been examined for errors.

The amounts aren’t as high as in the March 11th through 15th kona low event, but the elevated antecedent moisture conditions lingering from that event really amplified the flooding response across the state. It reminds me a lot of what happened during February through March 2006 (so-called “40 days of rain”, even though it wasn’t 40 days of rain). By late March 2006, the ground was so wet on Oʻahu that it didn’t take much rain to produce significant flooding. Some of the notable floods during this period include bouts of severe flooding in Makiki and the Kāhala Mall flood. Mānoa Stream also overflowed at Woodlawn Drive, but not as bad as on March 23, 2026.

Station	Mar 19	Mar 20	Mar 21	Mar 22	Mar 23	5-day total
Kauaʻi
Kilohana	0.43	0.30	0.00	2.01	2.21	4.95
N Wailua Ditch	3.41	0.46	0.00	0.14	0.16	4.17
Waiʻaleʻale	2.41	0.18	0.00	0.47	0.72	3.78
Līhuʻe Airport	3.27	0.19	0.01	0.03	0.06	3.56
Waiahi	3.22	0.15	0.00	0.02	0.01	3.40
Oʻahu
Kaʻala	7.59	12.84	1.26	0.10	3.32	25.11
Kamananui RAWS	7.39	10.11	0.52	0.12	0.41	18.55
Punaluʻu Stream	2.38	11.46	1.06	0.00	3.13	18.03
Hakipuʻu Mauka	1.65	10.81	0.90	0.01	4.61	17.98
Moanalua Rain Gage	1.09	7.92	1.47	0.22	7.21	17.91
Schofield East	2.43	11.64	1.83	0.06	1.95	17.91
Maui County
Honoulimaloʻo	0.64	5.36	6.00	7.35	2.90	22.25
Keōpukaloa	0.56	2.86	5.35	9.21	0.64	18.62
Hanaʻula	0.85	5.14	8.83	0.83	1.49	17.14
Puʻu Kukui	0.79	3.58	8.87	1.19	2.54	16.97
Puʻu Aliʻi	0.58	4.17	4.80	3.64	3.37	16.56
Big Island
Pāhala	0.09	0.02	0.04	0.38	7.12	7.65
Kapāpala RAWS	0.11	0.01	0.31	0.46	4.00	4.89
Kapāpala Ranch	0.13	0.01	0.88	0.46	3.38	4.86
Saddle Quarry	0.16	0.03	2.56	0.63	0.77	4.15
Pali 2	0.25	0.00	2.18	0.56	0.72	3.71

Maps of locations for the top 5 rainfall totals on Kauaʻi, Oʻahu, Maui County, and the Big Island. Station location data are from the HCDP and Mesowest.

An analysis of various rainfall durations at sites with significant rainfall showed extreme rates over the North Shore of Oʻahu. Rainfall rates from the Nuʻuanu and Mānoa areas on March 23rd were high but didn’t really move the needle in terms of return periods. Based on the flooding severity, this points to the important role of high levels of antecedent moisture in the Mānoa event. The table below shows the peak rainfall amounts (in inches) for different durations over the March 19th – 20th and March 23rd events. Return periods (in years) are in the rows below the rainfall amounts. The data are from the Hawaiʻi Mesonet archive and the WRCC RAWS archive. Return periods for each site are from NOAA Atlas 14.

For the 4th time(!) so far this year, rain rates exceeded the 500-year return period somewhere in the state. Some of this is due to increased sampling from the much larger number of automated rain gages now in the field. However, there’s no denying that we’ve had a lot of intense rainfall in the second half of the 2025-2026 wet season.

Station	15–min	1-hr	3-hr	6-hr	12-hr	24-hr
Kamananui RAWS	N/A	3.64	8.30	11.30	13.69	16.59
		50	500	500-1000	200-500	100-200
Dillingham	N/A	2.40	5.69	7.37	9.21	10.13
		10-25	50-100	50-100	50-100	25-50
Kaʻala	0.95	3.17	7.03	10.16	14.43	19.67
	2-5	10-25	100-200	100-200	200-500	200-500
Pūpūkea RG	1.16	3.35	6.61	7.85	9.34	9.97
	2-5	10-25	25-50	25-50	10-25	10
Kalaheʻe Ridge	1.07	3.25	6.48	7.82	9.18	9.75
	2-5	25-50	100	50-100	25-50	10-25
Lyon	1.06	2.26	4.24	4.94	5.24	5.79
	< 2	2	2-5	2-5	< 2	< 2
Nuʻuanu Res. 1	0.71	1.84	3.68	4.02	4.41	4.41
	< 2	2-5	2-5	2-5	< 2	< 2
Kalāwahine	0.63	1.57	3.51	3.96	4.38	4.70
	< 2	< 2	< 2	< 2	< 2	< 2

Map of locations for the sites included in the rainfall frequency analysis. Station location data are from the HCDP and Mesowest.

Although many of us are tired of all this rain, we’re not done with the wet season yet. We still have April to go through. Remember that the Waipā, Kauaʻi storm that produced the U.S. record for 24-hour rainfall occurred in mid-April 2018. So continue to stay vigilant, be prepared, and have a plan in case there’s another flash flood event.