Mauka Showers

In my blog post on March 27, I mentioned that despite all the rain, we were not done with the wet season and still had April to go through. Mother Nature then decided to send a reminder to make sure we got the message. For more than a month, the North Pacific has been locked in a pattern with strong high pressure anomalies over the Aleutian Islands and Kamchatka Peninsula. A related effect to that is a low pressure anomaly over the central North Pacific, including the Hawaiian Islands. The image below shows 500 millibar height anomalies (at roughly 18,000 ft) in 14-day increments. The red and orange areas are anomalous high pressure areas and the blues are anomalous low pressure areas (unsettled weather). Note the persistent red over the far North Pacific and persistent blue since late February over Hawaiʻi (small red ovals).  Yes, we’ve been stuck in overall unsettled conditions for a while. I can’t recall a persistent pattern like this since February-March 2006 (the so-called “40 Days of Rain”, even though it wasn’t).

Map of 14-day 500 millibar height anomalies from February 10 through April 10, 2026. Positive anomalies are in shades of yellow/orange/red. Negative anomalies are in shades of blue. The map was produced by NOAA’s Climate Prediction Center in their weekly ENSO update.

In this latest episode of heavy rainfall, a strong upper tropospheric low pressure system dropped into the area northwest of the main Hawaiian Islands and developed a large area of low pressure at the surface. This weather system remained in the same general area and was responsible for multiple bouts of heavy rainfall that affected all four counties in the state over the course of six days. Fortunately, the rainfall overall was not as intense as the March events, so impacts were limited mainly to the usual flood prone areas of the state.

Heavy rainfall started on April 7th with an upper tropospheric trough north of the state and a westerly jet stream axis over the Big Island. A highly localized area of intense rainfall fired up over the Hāna area of east Maui during the morning of April 7th. This bout of heavy rain was followed that evening by another localized but more persistent area of heavy rain over the same area. The heavy rain from this second episode continued into the early morning hours of April 8th. Nearly 8 inches of rain fell within 12 hours at the Hāna Airport during the second episode. Radar estimates showed 9.5 inches in 12 hours south of the airport where the cell was more intense. All of this was confined to an area between Nāhiku and Kīpahulu. The image below shows the intense rain cell early in its evolution over east Maui at 6:12 PM HST, April 7.

Image of Seamless Hybrid Scan Reflectivity (SHSR) data at 6:12 PM HST, April 7, 2026 from the NOAA National Severe Storms Lab’s Multi-Radar/Multi-Sensor (MRMS) system website. The image from the website was cropped for visual clarity. In this image, the red color indicates an area of intense rainfall.

A few hours after the east Maui evening cell developed, a band of heavy rain also fired up over the east side of the Big Island. This band of rain had a much broader coverage than the Maui cell and dropped just over 16 inches of rain in 12 hours at the Kealakomo rain gage in the lower Puna District. The radar image below shows the band of heavy rainfall at 9 PM HST on April 7, 2026 over the lower slopes of the Puna District and Kaʻū District of the Big Island. On the morning of April 8th, the band shifted offshore, but Geostationary Lightning Mapper (GLM) data showed that thunderstorms were still quite active over the coastal waters (image loop below).

Same as the above image, but over the Big Island at 9 PM HST, April 7, 2026. The red and orange areas have the most intense rainfall.

NOAA/NESDIS visible image loop centered on the main Hawaiian Islands with Geostationary Lightning Mapper (GLM) data overlaid. The loop covers the period from 6:31 AM to 8:56 AM HST, April 8, 2026. For more information on the GLM data, please click here.

On April 9th, the focus of heavy rainfall shifted to the opposite end of the state with Kauaʻi taking its turn. Cells of heavy rain within a large cloud band moved over the island from the south during the afternoon hours. Luckily, it appeared that the highest totals were confined to the upper slopes of Kauaʻi so runoff was mainly confined to the streams and rivers. The main flooding impact appeared to be the usual closure of Kūhiō Highway near the Hanalei River Bridge. This occurred at 5:37 PM HST, according to the Kauaʻi Emergency Management Agency (personal communication). The MRMS radar image below shows cells of heavy rain moving onshore from the south of Kauaʻi. The cells were moving fast enough to prevent significant flooding over the lower elevations with infrastructure. A hydrograph from the U.S. Geological Survey’s gage at Hanalei River (below) shows the peak in flow occurred at 5:20 PM HST. This gage is upslope from the Hanalei Bridge and any flood peak would take around 45 to 60 minutes to reach Kūhiō Highway.

Same as the above image, but over Kauaʻi at 3:30 PM HST, April 9, 2026.

Hydrograph of water level data from the U.S. Geological Survey’s gage at Hanalei River centered on April 9, 2026. The image is from NOAA’s National Water Prediction Service (NWPS).

The main rain band that affected Kauaʻi on April 9th shifted eastward and impacted Oʻahu and Maui County on April 10th. A loop of GOES-18 infrared (IR) satellite images (below) from the morning of April 10th shows the band over the mid-section of the state. The highest clouds (orange colors) are over Maui County, but the bulk of the rainfall is actually occurring in the band just west of the orange colors with the green colors bubbling up over and north of Oʻahu. The highest rainfall occurred over the North Shore area of Oʻahu, roughly the same area as the March 20th flood event. This can be seen in the MRMS Pass 2 (radar and rain gage) estimate covering the 12-hour period from 7 AM to 7 PM HST on April 10th.

NOAA/NESDIS color enhanced infrared image loop centered on the main Hawaiian Islands. The loop covers the period from 3:50 AM to 8:40 AM HST, April 10, 2026.  In this enhancement curve, blues, greens, yellows, and oranges show increasingly colder and higher cloud tops. Lighter gray shades are low clouds, and the darker gray areas are the ocean or land areas.

Image of the MRMS Pass 2 precipitation estimate (radar and rain gage) covering the 12-hour period ending at 7 PM HST, April 10, 2026.  The image is from the MRMS  website and it is cropped for visual clarity.

This is just my opinion, and maybe I’m just not remembering correctly, but it seems having two storms in a row with the highest event rainfall occurring over the North Shore of Oʻahu is unusual. I’ve included below an image of MRMS Pass 2 data from a kona low rain band that hit Oʻahu on December 6, 2021. In this case, it’s the rainfall estimate covering the 12-hour period ending at midnight on December 7th. I’m not focusing on the amounts here, but the orientation of the maximum rainfall band, which is mainly north-south over the island. This is the type of rainfall distribution I would normally expect from significant wet season storm systems over Oʻahu, and not two in a row focusing over the North Shore. Again, it’s just my perception and I haven’t looked into it in any detail so I may be completely off base. Anyway, I digress…

Same as above, but for the 12-hour period ending at midnight HST, December 7, 2021.

After dumping on Oʻahu, the rainfall shifted east to Maui County on the afternoon and evening of April 10th. Fortunately, the intensity and total amounts were not nearly as high as what occurred in the March storms, which limited flooding impacts. Rainfall across the state was largely suppressed on April 11th, allowing areas to dry out a bit. It was feared that an upper tropospheric disturbance would tap into moisture focused along a surface trough over Oʻahu on April 12th and trigger a final round of heavy rainfall. This was indicated by many of the global  and high-resolution mesoscale weather models. Luckily, the model projections proved to be overbaked and amounts were much less than expected, sparing residents from another round of flooding impacts.

The image below shows the MRMS radar and rain gage rainfall estimate covering the 6-day period from April 7th through April 12th. The highest amounts occurred early in the event over the east and southeast sections of the Big Island associated with the band of intense thunderstorms. The large swath of rainfall over the west half of the state mainly comes from the April 9th and 10th activity.

MRMS 6-day accumulated precipitation covering April 7th through April 12th, 2026. The data in this graphic were downloaded 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 from each county across the state in inches. As I’ve done 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. Since the totals come directly from the RRAHFO product, the HCDP archive, and the WRCC RAWS archive, they have not been examined for errors. Locator maps for each of the gages are included below the table.

Station	Apr 7	Apr 8	Apr 9	Apr 10	Apr 11	Apr 12	6-day total
Kauaʻi
Waialeale	0.51	0.09	15.00	2.42	0.00	0.48	18.50
Mōhihi Crossing	0.40	0.05	8.75	3.01	0.00	0.58	12.79
Waiakoali	0.44	0.05	8.83	2.72	0.01	0.52	12.57
Puʻu Lua	0.45	0.06	9.07	2.40	0.01	0.47	12.46
Waiʻalae	0.39	0.03	8.19	3.10	0.01	0.52	12.24
Oʻahu
Kuaokalā	0.18	0.03	0.08	7.78	0.05	0.67	8.79
Kaʻala	0.33	0.22	0.58	6.75	0.15	0.70	8.73
Kamananui	0.18	0.10	0.27	7.20	0.11	0.46	8.32
Mililani	0.61	0.34	0.26	5.14	0.00	0.46	6.81
Schofield Barracks	0.21	0.42	0.67	4.75	0.02	0.43	6.50
Maui County
Kuiki	0.86	2.24	0.28	11.40	2.09	0.45	17.32
Hāna Airport	7.93	5.85	0.05	1.15	0.94	0.65	16.57
Kahikinui	0.16	1.70	0.34	7.65	2.30	0.34	12.49
Pōhaku Pālaha	0.24	1.17	0.24	6.52	0.70	0.22	9.09
Haleakalā Summit	0.00	0.66	0.35	5.50	0.12	0.32	6.95
Big Island
Kealakomo	5.18	11.46	0.02	1.39	0.64	0.00	18.69
Pali 2	9.41	1.03	0.12	4.18	1.15	0.06	15.95
Pāhoa Beacon	10.66	1.31	0.05	1.39	0.46	0.01	13.88
Pāhoa (UHM)	7.75	2.36	0.04	1.07	0.33	0.02	11.57
Kapāpala RAWS	5.59	0.68	0.10	2.98	1.37	0.05	10.77

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.

The table below shows the peak rainfall amounts (in inches) for different durations over the April 7th through April 12th event. Return periods (in years) are in the rows below the rainfall amounts. The data for sites with * after the station names are from the Hawaiʻi Mesonet. The Hāna Airport totals are from the 15-minute data available on the HADS website. Data for the rest of the sites are from the Western Region Climate Center’s RAWS data archive. Return periods for each site are from NOAA Atlas 14.

With the exception of the Kealakomo station on the Big Island and Kuaokalā on the west end of Oʻahu, rainfall amounts over the various durations were much less than the March storms. This is consistent with the lower level of flooding impacts from the April event. Thank goodness!

Station	15-min	1-hr	3-hr	6-hr	12-hr	24-hr
Oʻahu
Kuaokalā	N/A	1.50	4.02	6.87	7.54	7.75
		2-5	25	100-200	25-50	10-25
Kamananui	N/A	1.18	3.22	4.97	6.54	7.21
		< 2	2-5	5-10	5-10	2-5
Maui
Kuiki *	0.46	1.28	2.82	4.50	8.89	13.08
	< 2	< 2	< 2	2-5	5-10	10-25
Hāna Airport	0.94	2.76	5.38	5.94	7.86	8.41
	2-5	5-10	10	10-25	5-10	5-10
Kahikinui *	N/A	0.88	2.40	3.99	6.24	9.48
		< 2	2-5	2-5	5-10	10-25
Haleakalā Summit *	0.19	0.60	1.54	2.65	3.77	5.51
	< 2	< 2	< 2	2	2-5	2-5
Pōhaku Pālaha *	0.32	0.79	1.95	3.12	4.89	7.08
	< 2	< 2	< 2	< 2	< 2	< 2
Big Island
Kealakomo	N/A	2.53	6.22	11.94	16.05	16.56
		10-25	100-200	500-1000	200-500	50-100
Pali 2	N/A	2.06	5.38	8.35	9.63	10.37
		2-5	10-25	10-25	5-10	2-5

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