A digging upper trough formed west of the islands by the weekend of March 22-23, 2026. This upper trough, and the associated jet stream dynamics, were not as strong and had more progressive motion than the large and very slow-moving kona storm which plagued the islands in mid-March. Ahead of the upper trough, a compact extratropical surface low developed south of Midway on the 17th, then moved closer to Kauai by the 20th. The low eventually moved off to the northeast, leaving a trailing trough near Oahu on the 21st, which moved very slowly eastward through the rest of the island chain over the next couple of days.
The recent mid-March kona storm had left a large area of deep tropical moisture over and just south of the Big Island, as well as nearby to the south of the smaller islands. The surface low quickly pulled this moisture-rich airmass back northward over the main Hawaiian Islands. Meanwhile, the upper trough provided increasing instability and lift for clusters of heavy rainfall and thunderstorms, some with very intense rain rates. The slow-moving trough provided enough of a focus for low-level convergence to create nearly ideal conditions for flash flooding over the islands, mainly around Oahu and points eastward. While the main atmospheric features of this event were weaker than the recent kona storm preceding this event, and the rainfall less widespread, the impacts were more severe due to the already- saturated ground from the previous week requiring less rainfall to result in runoff and flooding.
Rainfall was initially focused near south Kauai and the Kauai Channel during the afternoon and evening of the 19th, before moving eastward to Oahu and intensifying later that night. Heavy rainfall ramped up quickly just after midnight on the 20th over the northern half of Oahu, peaking around 1 to 3 am HST, quickly overwhelming streams and other infrastructure. This resulted in devastating flooding in the North Shore communities of Waialua and Haleiwa, where countless homes and vehicles were inundated or swept away and many residents needing rescue. The heavy rain then shifted to east Oahu later that morning and western portions of Maui County through the evening and overnight. Maui County continued to be the focus for heavy rain through Sunday the 22nd, with several landslides and countless road closures due to flooding, before the rain eased later that night. Rainfall finally picked up over the Big Island on the 23rd, with intense thunderstorms containing rainfall rates of up to 4+ inches per hour stretching from Pahala through Hilo, though flooding impacts were mainly limited to road closures due to heavy ponding. Oahu experienced one last burst of heavy rain over the eastern end of the island during the afternoon of the 23rd, resulting in a rapid rise of the Manoa Stream which overflowed and inundated the parking lots of nearby shopping centers, area roads, and an elementary school with water and several inches of mud.
The surface and upper troughs moved east and southeast of the islands the night of the 23rd, allowing the return of trade winds. Lingering moisture and instability maintained more localized, but still heavy, showers to the northeast sides of Maui and the Big Island that lasted into the morning of the 24th. The trades brought drier and more stable air which overspread the state during the day on the 24th, finally bringing this historic event to a close.
The following are preliminary rainfall totals from the six calendar days from Thursday, March 19th through Tuesday, March 24th.
Island of Kauai Inches Kilohana (USGS) 5.16 N Wailua Ditch (USGS) 4.22 Mount Waialeale (USGS) 4.00 Lihue Airport (ASOS) 3.61 Waiahi Rain Gage (USGS) 3.42 Hanamaulu (UHM) 3.20 Poipu (HSOIS) 3.08 Kalaheo (12008) 2.90 Wailua (12005) 2.86 Omao (12004) 2.84 Lihue Var. Stn. (12006) 2.81 Lawai NTBG (UHM) 2.68 Kapahi (12003) 2.58 Island of Oahu Inches Kaala (UHM) 25.21 Punaluu Stream (USGS) 18.06 Moanalua RG (USGS) 18.04 Hakipuu Mauka (13004) 18.01 Tunnel RG (USGS) 17.94 Schofield East (RAWS) 17.92 Kahana (USGS) 17.13 Maunawili (13008) 17.00 Schofield Barracks (RAWS) 16.18 St. Stephens (13006) 16.03 Manoa Lyon Arboretum (3050) 15.95 Kalawahine (UHM) 15.50 Lyon (UHM) 15.11 Kamananui Stream (USGS) 14.29 Luluku (13016) 13.46 Pupukea Road (USGS) 12.91 Kalahee Ridge (UHM) 12.90 Kuaokala (RAWS) 11.93 Heeia NERR (NOAA/NOS) 11.72 Dillingham (RAWS) 11.65 Waianae Valley (RAWS) 11.27 Oahu Forest NWR (USFWS) 11.10 Niu Valley (13001) 11.07 Nuuanu Res No. 1 (UHM) 10.92 Napuumaia (UHM) 10.88 Palehua (RAWS) 10.87 Kaluanui (UHM) 10.37 Olomana Fire Station (13009) 9.90 Waiahole (USGS) 9.33 Palolo Fire Station (13010) 9.03 Poamoho RG 1 (USGS) 8.96 Kii (RAWS) 8.70 Hawaii Kai Golf Crse (13015) 8.69 Bellows AFS (HSOIS) 8.40 Moanalua (13003) 7.79 Waipio (3010) 7.79 Ahuimanu Loop (13005) 5.30 Waianae (13014) 5.21 Waianae Boat Harbor (HSOIS) 4.98 Waihee Pump (13002) 4.34 Kahuku Training Area (RAWS) 4.00 Honouliuli (RAWS) 3.73 Honolulu Airport (ASOS) 3.43 Punaluu Pump (13013) 3.39 Kalaeloa Airport (ASOS) 2.71 Island of Molokai Inches Honolimaloo (UHM) 22.31 Keopukaloa (UHM) 18.64 Puu Alii (RAWS) 18.64 Kamalo (14013) 13.99 Makapulapai (RAWS) 12.48 Kaunakakai Mauka (14004) 9.85 Molokai Airport (ASOS) 8.57 Island of Lanai Inches Lanai City (14012) 13.37 Lanai 1 (RAWS) 8.46 Lanai Airport (ASOS) 7.05 Island of Maui Inches Puu Kukui (USGS) 19.61 Hanaula (UHM) 18.48 West Wailuaiki (USGS) 11.03 Wailuku (14007) 10.48 Honolua (UHM) 10.06 Lahaina WTP (UHM) 9.55 Kahakuloa (14002) 9.10 Lahainaluna (14011) 8.79 Waikamoi Treeline (UHM) 7.81 Olowalu (UHM) 7.58 Waikapu Country Club (14005) 6.92 EMI Baseyard (UHM) 6.56 Haiku (14001) 6.38 Piiholo (UHM) 6.22 Hana Airport (HSOIS) 5.62 Summit (UHM) 4.37 Kahului Airport (ASOS) 4.25 Kula 1 (RAWS) 3.94 Na Kula 3.58 Park HQ (UHM) 3.21 Kepuni (USGS) 3.05 Ulupalakua (14003) 2.83 Pukalani (14006) 2.67 Nene Nest (UHM) 2.39 Kealia Pond (USFWS) 2.34 Island of Hawaii Inches Laupahoehoe (UHM) 8.89 Pahala (15004) 8.11 Honolii Stream (USGS) 5.81 Kapapala (RAWS) 5.59 Piihonua (UHM) 5.47 Waipunalei (UHM) 5.11 Kapapala Ranch (15003) 5.06 Pahoa (UHM) 4.37 Saddle Quarry (USGS) 4.33 Mountain View (15014) 4.17 Honokaa (15010) 3.91 Pali 2 (RAWS) 3.71 Kehena (UHM) 3.29 IPIF (UHM) 3.20 Kawainui Stream (USGS) 3.05 Kamuela (15005) 2.89 Spencer (UHM) 2.85 Kahua Ranch (15006) 2.67 Puu Mali (RAWS) 2.51 Kona Hema (UHM) 2.46 Laupahoehoe PD (15001) 2.42 Pahoa (15015) 2.41 Hakalau (RAWS) 2.41 Kulaimano (UHM) 2.34 Kahuku Ranch (RAWS) 2.18 Nahuku (UHM) 2.01
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