In April 2018, a series of

thunderstorms A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...

produced record-breaking rainfall on the

Hawaiian Islands The Hawaiian Islands ( haw, Nā Mokupuni o Hawai‘i) are an archipelago of eight major islands, several atolls, and numerous smaller islets in the North Pacific Ocean, extending some from the island of Hawaii in the south to northernmost ...

Kauaʻi Kauai, () anglicized as Kauai ( ), is geologically the second-oldest of the main Hawaiian Islands (after Niʻihau). With an area of 562.3 square miles (1,456.4 km2), it is the fourth-largest of these islands and the 21st largest island ...

and

Oahu Oahu () ( Hawaiian: ''Oʻahu'' ()), also known as "The Gathering Place", is the third-largest of the Hawaiian Islands. It is home to roughly one million people—over two-thirds of the population of the U.S. state of Hawaii. The island of O� ...

. An

upper-level low A cold-core low, also known as an upper level low or cold-core cyclone, is a cyclone aloft which has an associated cold pool of air residing at high altitude within the Earth's troposphere, without a frontal structure. It is a low pressure syst ...

moved across the area on April 13, generating a

mesoscale convective system A mesoscale convective system (MCS) is a complex of thunderstorms that becomes organized on a scale larger than the individual thunderstorms but smaller than extratropical cyclones, and normally persists for several hours or more. A mesoscale con ...

that moved over eastern Oahu, producing localized heavy rainfall that reached . The heaviest rainfall occurred on northern Kauaʻi. There, a

rain gauge A rain gauge (also known as udometer, pluvia metior, pluviometer, ombrometer, and hyetometer) is an instrument used by meteorologists and hydrologists to gather and measure the amount of liquid precipitation over a predefined area, over a period ...

owned by the Waipā Foundation, just west of Hanalei, recorded of rainfall in the 24 hours between 12:45 p.m. on April 14 and 15. This was the greatest 24-hour rainfall total on record in the United States, surpassing the previous record of in

Alvin, Texas Alvin is a city in the U.S. state of Texas within the Houston-The Woodlands-Sugar Land metropolitan area and Brazoria County. As of the 2020 U.S. Census, the city population was 27,098. Alvin's claim to fame is Baseball Hall of Famer Nolan Rya ...

on July 25–26, 1979, set during Tropical Storm Claudette. Through the event, the same gauge recorded a total of of rain. The heavy rainfall produced

flash flooding A flash flood is a rapid flooding of low-lying areas: washes, rivers, dry lakes and depressions. It may be caused by heavy rain associated with a severe thunderstorm, hurricane, or tropical storm, or by meltwater from ice or snow flowing o ...

and landslides that covered roads and washed away several vacant houses. The floods damaged or destroyed 532 houses. Public damage totaled $19.7 million and overall damage was estimated at over $125 million. Damage on Kauai was the worst from a natural disaster since

Hurricane Iniki Hurricane Iniki ( ; Hawaiian: ''iniki'' meaning "strong and piercing wind") was the most powerful hurricane to strike Hawaii in recorded history. Forming on September 5, 1992, during the strong 1990–1995 El Niño, Iniki was one of eleven Centr ...

in 1992. Landslides and flooding covered parts of Kuhio Highway.

Meteorological history

On April 11, 2018, an

was located west of the Hawaiian island of Kauaʻi. The low moved toward Hawaii and drew deep tropical moisture from the south. The low eventually opened into a

trough Trough may refer to: In science * Trough (geology), a long depression less steep than a trench * Trough (meteorology), an elongated region of low atmospheric pressure * Trough (physics), the lowest point on a wave * Trough level (medicine), th ...

, which crossed over the Hawaiian islands on April 13. That day, a

(MCS) developed northwest of

Molokai Molokai , or Molokai (), is the fifth most populated of the eight major islands that make up the Hawaiian Islands archipelago in the middle of the Pacific Ocean. It is 38 by 10 miles (61 by 16 km) at its greatest length and width with a us ...

and intensified while moving westward through the

trade winds The trade winds or easterlies are the permanent east-to-west prevailing winds that flow in the Earth's equatorial region. The trade winds blow mainly from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisp ...

. The MCS moved over eastern

, dissipating after moving ashore. Another upper-level low developed on April 14, fueling additional rainfall through April 16, which caused record rainfall on

. The first of a series of thunderstorms moved over northern Kauaʻi around midnight on April 14. After a five-hour break in rainfall, another set of thunderstorms hit northern Kauaʻi around midnight on April 15 and lasted about eight hours. The third set of thunderstorms began around 10 AM on April 15 and continued until around 7 PM.

Impact

The Honolulu

National Weather Service The National Weather Service (NWS) is an agency of the United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weather-related products to organizations and the public for the ...

issued various

flash flood watch A flash flood watch (SAME code: FFA; also referred as a "green box" by meteorologists) is severe weather watch product of the National Weather Service that is issued when conditions are favorable for flash flooding in flood-prone areas, usually w ...

es and

warning Warning may refer to: Signal * Precautionary statement * Warning sign * Warning system * Warning (traffic stop), issued by a police officer in lieu of a citation following a traffic stop Books * A Warning (book), ''A Warning'' (book), a 2019 ...

s. The trough began dropping heavy rainfall over Hawaii on April 13. Floods damaged or destroyed 532 houses. The flooding knocked down trees and power lines, eroded many bridges and roads, and washed away crops, beehives, and livestock. Overall damage was estimated at over $125 million. Damage to public buildings was estimated at $19.7 million. Despite the damage, there were no deaths or injuries.

Oahu

Across eastern Oahu, the mesoscale convective system produced rainfall rates of about per hour. Over a 15-minute period, a rain gauge in Niu Valley recorded of precipitation, which has a 4–10% annual

return period A return period, also known as a recurrence interval or repeat interval, is an average time or an estimated average time between events such as earthquakes, floods, landslides, or river discharge flows to occur. It is a statistical measurement typ ...

. The same station recorded of rainfall over a two-hour period, which has a 0.5–1% annual return period. The rainfall on Oahu was largely limited to the eastern one-third of the island. A station at

Daniel K. Inouye International Airport Daniel K. Inouye International Airport , also known as Honolulu International Airport, is the main airport of Oahu, Hawaii. At Waimanalo Stream north of Niu Valley, the rains caused the waters to rise in 30 minutes, and in an hour. During the rain event, Wailupe Gulch reported an rise in floodwaters. Flooding entered cars and hundreds of houses on Oahu. The floods also caused significant

runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, a stock market ...

, leaving a layer of debris on roads. A portion of

Kalanianaole Highway Route 72, commonly known as Kalanianaole Highway, is the main highway serving southeast Oahu. Beginning at the eastern terminus of Interstate H-1 (H-1), the east–west highway travels along the southeastern shore of Oahu through vario ...

was covered with nearly of water, closing a stretch of the highway from

Aina Haina Aina may refer to: * Aina (given name) Places * Aina (Crete), a town of ancient Crete * Aïna River, a river in Cameroon and Gabon * Aina, Iran, a village in Kermanshah Province, Iran Art and entertainment * ''Aina'' (1977 film), a 1977 Paki ...

Makapuʻu Makapuu is the extreme eastern end of the Island of Oahu in the Hawaiian Islands, comprising the remnant of a ridge that rises 647 feet (197 m) above the sea. The cliff at Makapuu Point forms the eastern tip and is the site of a prominen ...

. A mudslide covered a portion of the same highway near

Kailua Kailua () is a census-designated place (CDP) in Honolulu County, Hawaii, United States. It lies in the Koolaupoko District of the island of Oahu on the windward coast at Kailua Bay. It is in the judicial district and the ahupua'a named Ko'ol ...

. Downed trees closed one block of Maunaloa Avenue in

Kaimuki Kaimukī is a residential neighborhood in Honolulu, Hawaii, United States. History In the 19th century the area was a farm of King Kalākaua, where ostriches roamed wild over the mountain side. It later became the site of a carnation farm for ...

Kauaʻi

On Kauaʻi, the rainfall event occurred at low elevations near the northern coast. This was unusual, as previous rain events on the island usually occurred over higher elevations. The heaviest rainfall was early on April 15. A

owned by the Waipā Foundation, just west of Hanalei on Kauaʻi's northern coast, recorded of rainfall in the 24 hours ending at 12:45 p.m. local time. The total greatly surpassed the Hawaiʻi 24-hour rainfall record of set on January 25, 1956, at

Kīlauea Kīlauea ( , ) is an active shield volcano in the Hawaiian Islands. Located along the southeastern shore of the Big Island of Hawaii, the volcano is between 210,000 and 280,000 years old and emerged above sea level about 100,000 years ago. Hi ...

. The total also broke the national 24-hour rainfall record of set during Tropical Storm Claudette on July 25–26, 1979 in

, a record confirmed in December 2018. The Waipā rain gauge also recorded over 15 minutes, over an hour, over six hours, over 12 hours, and over 48 hours. Elsewhere on northern Kauaʻi, a rain gauge in Wainiha recorded , and a gauge in Hanalei recorded before it failed. Minimal rainfall occurred on the southern and western coasts of Kauaʻi. The intense rainfall washed out several river gauges. Along the

Hanalei River The Hanalei River on the island of Kauai in Hawaii flows north from the eastern slopes of Mount Waialeale for U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map accessed April 28, 2011 until it reache ...

, the

United States Geological Survey The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...

(USGS) estimated that floodwaters rose , representing a peak flow rate of 32,700 cubic feet per second (ft³/s, 926 m³/s) before the station failed. The Wainiha River reported a record flow rate during the event. The rain event caused the worst natural disaster in Kauai since

in 1992. The sudden rains caused

landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, ...

s, mostly in isolated areas, as well as

, causing power outages in Hanalei and Wainiha. Rising waters washed out four vacant homes and entered dozens of others. The rising waters forced guests at the Hanalei Colony Resort to move to the second floor. The floodwaters reached deep near Hanalei, closing parts of Kuhio Highway. Landslides occurred at 15 locations along the highway, leaving residents in Wainiha and Haena without regional road access, disrupting relief efforts. Three bridges along the route were damaged, which delayed repairs, leaving the Kuhio Highway closed as of November 2018. Residents used a detour on a dirt road. The flooding forced the temporary closure of three beaches – Kalapaki Beach, Koloa Landing, and Moloaa Bay – as well as two parks,

Nā Pali Coast State Park Nā Pali Coast State Park is a state park in the U.S. state of Hawaii, located in the center of the rugged northwest side of Kauai, the second-oldest inhabited Hawaiian island. The Nā Pali coast itself extends southwest from Kee Beach al ...

and

Haʻena State Park Hāena State Park is a state park on the north shore of the Hawaiian island of Kauai. It is often called the "end of the road" and marks the endpoint of the Kuhio Highway. The park provides access to beaches, trails, and several ancient Hawai ...

. Wainiha and Haena residents were urged to limit their water use due to contaminated water supply.

Aftermath and records

Kauai Mayor Bernard Carvalho Jr. declared a state of emergency for portions of Kauai, which effectively restricted tourist operations in the Hanalei area. He extended the emergency four more times, effective through January 4, 2019. After the floods, Hawaiian Governor

David Ige David Yutaka Ige (; born January 15, 1957) is an American politician and engineer who served as the eighth governor of Hawaii from 2014 to 2022. A Democrat, he served in the Hawaii State Senate from 1995 to 2014 and the Hawaii House of Repres ...

also declared a State of Emergency. On April 18, the Hawaii legislature approved $125 million in disaster funding, including $100 million for Kauaʻi. On May 8, President

Donald Trump Donald John Trump (born June 14, 1946) is an American politician, media personality, and businessman who served as the 45th president of the United States from 2017 to 2021. Trump graduated from the Wharton School of the University of ...

declared a federal disaster area for Kauaʻi and

Honolulu Honolulu (; ) is the capital and largest city of the U.S. state of Hawaii, which is in the Pacific Ocean. It is an unincorporated county seat of the consolidated City and County of Honolulu, situated along the southeast coast of the islan ...

counties. Initially, the declaration was limited to federal reimbursement for repairs to public infrastructure, but on June 27 it was expanded to include individual assistance. Ultimately, the

Federal Emergency Management Agency The Federal Emergency Management Agency (FEMA) is an agency of the United States Department of Homeland Security (DHS), initially created under President Jimmy Carter by Presidential Reorganization Plan No. 3 of 1978 and implemented by two Exe ...

(FEMA) approved 250 individual assistance applications, totaling about $1.5 million, with another $2.6 million in public assistance grants. FEMA opened three disaster recovery centers, which provided resources for the application process; these centers closed by July 20. On November 30, Carvalho granted $500,000 to six nonprofits for counselors, meals, emergency supplies, and rebuilding parts of the

Limahuli Garden and Preserve The Limahuli Garden and Preserve is a and on the north shore of Kauai island, Hawaii. It is one of the five gardens of the non-profit National Tropical Botanical Garden. Description Limahuli lies within a tropical valley covering three dist ...

. Local and national emergency crews rescued people from their flooded homes by helicopter, boat, and bus, including 475 by helicopter; more than 40% of those rescued by helicopter were tourists. The Hawaii Department of Transportation ran convoys throughout the day along the damaged portions of Kuhio Highway while it was being repaired. The cost of reconstructing the highway was estimated at around $100 million; much of it was covered by the federal government. In August 2018, Hurricane Lane dropped heavy rainfall in the same area, causing further flooding damage to houses and the Kuhio Highway. The Hanalei post office, damaged by the floods, reopened on July 30. A

Red Cross The International Red Cross and Red Crescent Movement is a Humanitarianism, humanitarian movement with approximately 97 million Volunteering, volunteers, members and staff worldwide. It was founded to protect human life and health, to ensure re ...

shelter in Kilauea housed 13 people after the event. Some of the displaced stayed with family or friends. The Hanalei Colony Resort evacuated its guests on April 16 and remained closed through October. It temporarily housed National Guardsmen and families displaced by the floods. At least two dozen students used the resort as a temporary school. The resort's staff won the Excellence in Community Service Award at the 29th Annual Na Po‘e Pa‘ahana Awards, a ceremony for excellence in the hospitality industry. On August 2, 2018, National Weather Service Senior Hydrologist Kevin Kodama inspected and calibrated record-breaking rain gauge at Waipā Garden, confirming that the instrument was functioning normally. On December 4, 2018, the National Climate Extremes Committee verified that the accumulation was the highest 24-hour rainfall total in the U.S.

Notes

References

{{Reflist 2018 meteorology Natural disasters in Hawaii April 2018 events in the United States Hawaii floods 2010s floods in Oceania 2010s floods in the United States Hawaii floods