A developing upper low northwest of the state, in combination with tropical moisture, induced periods of heavy showers and thunderstorms, and generated historic flash flooding conditions over the Garden Isle of Kauai.  An apparent 24-hour rainfall total of 49.69 inches, ending at 1245 HST April 15th, was recorded at an automated rain gauge
in Waipa, Kauai, about a mile west of Hanalei.  If this total is certified, it will be a new 24-hour rainfall record for the United States, beating the old record of about 43 inches in Alvin, TX, on July 25-26, 1979.
The deluge, mainly over northern Kauai but also affecting East Oahu, damaged or destroyed farms and various structures, including 532 homes; downed trees and power lines; flooded homes, businesses and vehicles; and closed and damaged numerous roadways with water and debris flows, with highway and road repairs estimated at $35 million.  There were apparently no significant injuries, but material losses will be extremely exorbitant, with public property damages alone estimated at $19.7 million.  Hawaii's state legislature already approved, as of April 25th, $100 million in relief aid for flood-ravaged communities on Kauai and Oahu.  (The aid package was later increased to $125 million.).

Debris Flow in HONOLULU, HAWAII
From 13-APR-18 21:35:00 to 13-APR-18 21:35:00 (HST-10)
Location: WAIMANALO
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: County Official

Mud slide (debris flow) caused a partial road blockage at Kalanianaole Highway and Keolu Drive near Kailua on the windward side of Oahu.

Heavy Rain in HAWAII, HAWAII
From 11-APR-18 11:10:00 to 11-APR-18 14:05:00 (HST-10)
Location: HAWI
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Heavy Rain in HONOLULU, HAWAII
From 15-APR-18 04:24:00 to 15-APR-18 05:19:00 (HST-10)
Location: KAILUA
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Heavy Rain in HAWAII, HAWAII
From 15-APR-18 17:13:00 to 15-APR-18 21:00:00 (HST-10)
Location: PAAUILO
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Heavy Rain in HAWAII, HAWAII
From 13-APR-18 00:36:00 to 13-APR-18 02:12:00 (HST-10)
Location: KONA VILLAGE RESORT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Heavy Rain in HAWAII, HAWAII
From 13-APR-18 13:43:00 to 13-APR-18 19:43:00 (HST-10)
Location: PUUANAHULU
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Flash Flood in KAUAI, HAWAII
From 14-APR-18 05:30:00 to 14-APR-18 12:56:00 (HST-10)
Location: HANALEI
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Law Enforcement

Kuhio Highway was closed near Haena Beach Park and the Maniniholo Dry Cave due to deep water flowing over the roadway.

Debris Flow in KAUAI, HAWAII
From 14-APR-18 14:00:00 to 14-APR-18 14:00:00 (HST-10)
Location: HANALEI
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Three landslides (debris flows) closed Kuhio Highway between Wainiha and Waikoko in northern Kauai.

Flash Flood in HONOLULU, HAWAII
From 13-APR-18 20:42:00 to 13-APR-18 23:40:00 (HST-10)
Location: AINA HAINA
Flood cause: Heavy Rain
Property damage: 2.20M
Crop damage: 0.00K
Reported by: County Official

Traffic stalled in all lanes of Kalanianaole Highway in East Oahu near Niu Valley due to 10-12 inches of standing water on the roadway, and the water was still rising.  Moments later, the roadway was closed from Aina Haina to Makapuu due to flooding.

Flash Flood in KAUAI, HAWAII
From 14-APR-18 19:00:00 to 16-APR-18 01:40:00 (HST-10)
Location: HANALEI
Flood cause: Heavy Rain
Property damage: 17.60M
Crop damage: 0.00K
Reported by: Emergency Manager

In northern Kauai, Kuhio Highway was closed in Hanalei near the Hanalei Bridge due to flooding as of 1900 HST.  Two homes were destroyed on Wainiha Powerhouse Road in Wainiha as of 2000 HST, and at the same time the roadway itself was washed out.  At 0130 HST on the 15th, multiple people became trapped in their homes between Hanalei and Haena.  Around 0400 HST, guests in the Hanalei Colony Resort in Haena had been moved to the second floor due to rising water.  And at 0400 HST, water as high as 8 feet was covering Kuhio Highway in Hanalei and other areas, including submerging many vehicles.  Small hail was reported by a member of the public at 0600 HST in Wailua in the eastern part of Kauai.

Flash Flood in KAUAI, HAWAII
From 14-APR-18 14:34:00 to 14-APR-18 17:15:00 (HST-10)
Location: HANALEI
Flood cause: Heavy Rain
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Emergency Manager

Kuhio Highway was once again closed due to flooding near Haena Beach Park.

Heavy Rain in HAWAII, HAWAII
From 14-APR-18 15:11:00 to 14-APR-18 19:13:00 (HST-10)
Location: UPOLU AIRPORT
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Heavy Rain in HONOLULU, HAWAII
From 14-APR-18 22:37:00 to 15-APR-18 01:18:00 (HST-10)
Location: KANEOHE STATION
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Heavy Rain in HAWAII, HAWAII
From 15-APR-18 01:50:00 to 15-APR-18 10:03:00 (HST-10)
Location: KAHANA
Property damage: 0.00K
Crop damage: 0.00K
Reported by: Official NWS Observations

Thunderstorm Wind in HONOLULU, HAWAII
From 13-APR-18 21:05:00 to 13-APR-18 21:05:00 (HST-10)
Location: WAIALAE
Magnitude: 39.00 EG
Property damage: 10.00K
Crop damage: 0.00K
Reported by: County Official

Maunaloa Avenue in Kaimuki in East Oahu was closed between 8th and 9th Streets because of downed power lines.