Hurricane Lowell’s close passage west of Kauaʻi and Niʻihau brought damaging winds, heavy rainfall with more than 23 inches recorded on Mount Waiʻaleʻale, and dangerous surf. To understand the storm’s full impact, a University of Hawaiʻi team reconstructed Hurricane Lowell hour by hour across Kauaʻi and the state using observations from the Hawaiʻi Mesonet, a statewide network of weather stations, alongside experimental high-resolution mapping tools.

Users can visit the newly launched Hurricane Lowell website to explore interactive wind and rainfall maps, station observations and satellite storm replays.

The most striking weather signal during the storm was the wind. At Līhuʻe Airport, the highest measured hourly average wind speed reached 56 mph late on September 7. However, because �����ܲ�ʻ��’s weather stations are concentrated primarily at lower elevations, experimental high-resolution modeling was used to estimate conditions across remote terrain. The models suggest that winds over exposed high-elevation ridges were much stronger, locally exceeding 100 mph as the island’s steep topography accelerated airflow over ridges and through valleys.

Rainfall on Windward Oahu, Mount Waiʻaleʻale, Kauaʻi’s north shore

Rainfall across the western islands was also substantial throughout the event. On September 6, rainfall amounts of 5 inches or more along the Koʻolau Ridge sent flood waters down Waikāne and Waiahole Valleys on Oʻahu, forcing the closure of Kamehameha Highway.

On September 7, nearly all of Kauaʻi received more than 2 inches of rain, with many areas exceeding 5 inches. High-elevation and north shore sensors show Mount Waiʻaleʻale recorded more than 23 inches of rain on September 7, accumulating nearly 36 inches over the full three-day storm period. Meanwhile, on �����ܲ�ʻ��’s north shore, where rising river waters forced the closure of Hanalei Bridge, the Hawaiʻi Mesonet stations at Waipā and Lower Limahuli recorded more than 14 and 17 inches of rainfall, respectively, during the storm.

Coastal impacts and interactive storm replay

The report also references information from Pacific Islands Ocean Observing System (PacIOOS) on the significant coastal hazards brought by the storm. Large surf and storm surge impacted south- and west-facing shores of Kauaʻi as the hurricane tracked to the west.

To make these complex meteorological observations accessible to the public and emergency responders, the ������Ƶ team launched a dedicated Hurricane Lowell website. The site features interactive graphics that allow users to follow Lowell’s winds and rainfall through time, compare ground station observations with mapped estimates, and replay the storm using Hawaiʻi Mesonet measurements integrated with Geostationary Operational Environmental Satellite-West satellite imagery.

While long-term data shows that Hawaiʻi is getting drier overall, short, intense downpours that cause sudden, dangerous flash floods are getting stronger in the islands, according to a new study by researchers at the University of Hawaiʻi at Mānoa.

Published in the , the research from the College of Tropical Agriculture and Human Resilience (CTAHR) is the first comprehensive study to analyze extreme hourly rainfall patterns across the state over 50 years (1966–2020). The team examined decades of hourly data from 117 rain gauges across Kauaʻi, Oʻahu, Molokaʻi, Maui and Hawaiʻi Island, uncovering climate trends that traditional 24-hour daily averages often miss.

Bursts of intense rain

In steep, mountain-filled tropical watersheds like �Ჹ�ɲ���ʻ��’s, bursts of intense rain lasting one to six hours can overwhelm stream channels and storm drains before emergency alerts are even issued.

“If we want to understand extreme weather, like storms and floods, we really need to look at short-duration, high-temporal-resolution data rather than daily or monthly averages,” said Professor Yinphan Tsang of CTAHR’s Department of Natural Resources and Environmental Management and the principal investigator.

The study’s major findings include:

• Common heavy rains, happening once every two years, are generally becoming less intense across the state.
• Short, sudden bursts of rain lasting one to three hours—the severe storms that usually occur only once every 20 to 100 years—are getting significantly stronger.
• Storms lasting 6 hours or longer are weakening more noticeably than shorter one- to three-hour storms.

Researchers noted that rainfall trends vary widely even between neighboring areas. Because of �Ჹ�ɲ���ʻ��’s unique mountains and island terrain, rainfall intensity can increase in one neighborhood while decreasing just a few miles away.

Researchers also identified two distinct peaks when Hawaiʻi is most vulnerable to extreme hourly rain events: October through December and February through March. These heavy rains are overall driven by cold fronts, Kona storms and other winter weather systems moving through the islands. The extreme events occurring from February through March, near the end of the wet season, were unexpected and could take people by surprise. Other systems, such as tropical storms, could also bring extreme rainfall during the summer season.

Vital data for climate adaptation

The findings offer updates for emergency responders, county planners and infrastructure design.

The study was a collaborative effort at ������Ƶ Mānoa bringing together experts from CTAHR, the School of Ocean and Earth Science and Technology, the College of Social Sciences and the Water Resources Research Center. The research was primarily compiled by Maxime Gayte with co-authors including faculty members Professor Yinphan Tsang, Professor Pao-Shin Chu (Hawaiʻi State Climatologist), Professor Thomas W. Giambelluca, Jr., Researcher Yu-Fen Huang, and Researcher Hanpei Zhang.

Almost two million Olympic-size pools of water (1.3 trillion gallons) inundated Hawaiʻi August 14–16, as Hurricane Lala tracked just south of the islands. Delivering one of the state’s most significant tropical cyclone impacts in decades, the storm triggered severe flash flooding and damaging winds. Through the University of �Ჹ�ɲ���ʻ��’s Hawaiʻi Mesonet and the Hawaiʻi Climate Data Portal (HCDP), researchers captured the massive scale of this storm.

While Lala never made an official landfall, its close pass as a Category 1 hurricane brought average statewide rainfall of 11.4 inches. Hawaiʻi County received 66% of the total rainfall volume—equivalent to 1.2 million pools (833 billion gallons).

Naʻalehu flash flooding and station records

Communities such as Naʻalehu on Hawaiʻi Island experienced devastating inundation from flash flooding. Hawaiʻi Mesonet stations highlighted the extreme localized intensity of the storm: the Laupahoehoe station in Hamakua recorded a daily maximum rainfall of 32.1 inches on August 15. Almost simultaneously, the Kaiholena station, located upslope of the Naʻalehu area, recorded 24.4 inches, including a remarkable 4.4 inches of rain in a single hour, 7:30–8:30 p.m. This was the highest observed hourly rainfall rate in the state during the entire three-day event. Combined with steep mountain slopes, this intense rainfall resulted in flash flooding that severely impacted the community in Naʻalehu.

As the storm progressed, the rest of the counties experienced their peak rainfall a day later, on August 16. While their totals were lower than Hawaiʻi Island’s, they still saw substantial averages: Maui County recorded a cumulative 6.6 inches of rain, equivalent to 203,595 pools (134 million gallons). Honolulu County (Oʻahu) recorded 5.5 inches of rain, equivalent to 86,357 pools (57 million gallons), and Kauaʻi County recorded 3.1 inches, equivalent to 45,055 pools (29 million gallons).

“Storms like Lala remind us why sustained, high-quality rainfall measurement matters so much in Hawaiʻi,” said Tom Giambelluca, director of the Hawaiʻi Mesonet. “Our terrain can turn the same storm into a 2-inch event in one valley and a 40-inch event a few miles away—without a dense network like the Mesonet, we’d have no way to capture that difference or understand what actually happened on the ground.”

Strong winds and dangerous gusts

The storm also unleashed high winds, with station-derived hourly wind speeds peaking at 60 mph on Hawaiʻi Island. Strong winds were also measured on Maui (56 mph), Oʻahu (39 mph) and Kauaʻi which led to downed power lines and fallen trees across the islands. Wind gusts of 80 mph were recorded at Puʻuloa in Kohala, and up to 79 mph on Haleakalā. Researchers expect that wind gusts were even more intense in some areas that do not have a climate station.

“One of the biggest challenges with wind is that we simply don’t have stations everywhere, especially at higher elevations and in remote terrain,” said Matt Lucas, HCDP product development lead. “Newly developed high-resolution wind speed maps let us estimate conditions in the gaps between stations, especially in extreme terrain where winds can be accelerated 2–4 times by landscape effects. This means disaster responders, property owners, and land managers aren’t left guessing about what happened in places where we didn’t have a station.”

The Hawaiʻi Mesonet, a statewide network of state-of-the-art weather stations, provides critical weather information across the islands giving a birds eye view into the data behind extreme weather events. Using observations from the Hawaiʻi Mesonet and digital maps produced on the HCDP maps, the team has released a new interactive website documenting the weather impacts of Hurricane Lala. ������Ƶ the HCDP Extreme Event Page to explore new interactive rainfall and wind speed maps as well as station data from the event. End users can sort information by county and for each day of the storm.

University of Hawaiʻi at Mānoa researchers revealed that Waikīkī is facing a fundamental shift in flood hazards as sea levels rise—transitioning from a flooding that is driven primarily by rainfall to events increasingly dominated by tidal processes. The team identified two key pathways that will become more significant with sea-level rise, both of which will increase public exposure to sewage-contaminated waters. The study was published in .

“Our findings make clear that current flood management strategies for Waikīkī are incomplete,” said Kayla Yamamoto, climate modeling analyst at the in the ������Ƶ Mānoa (SOEST). “Most planning focuses on surface damage and economic loss from storms, but largely ignores the contamination dimension. Our results show that contaminated flooding will become more frequent, more extensive, and eventually a daily occurrence rather than a storm-driven one. There are currently no effective management strategies in place to address this.”

Simulating future scenarios

The team used an open-source, physics-based flood model to simulate how multiple flood sources interact in Waikīkī. The team used an advanced flood model that, unlike previous models, integrates all sources of flooding—rain, tides, underground water behavior, and storm drains—to provide a single, complete view of the hazard

“What we found is that during extreme rainfall like we’ve been experiencing, high tides and elevated water levels in the Ala Wai can combine to create conditions where contaminated water flows back into low-lying streets and sidewalks,” said Shellie Habel, study co-author and coastal geologist with the Coastal Research Collaborative and . “As sea level rises, it will take less extreme rainfall and tides to cause similar flooding in the future.”

The two key pathways they identified were: storm drain backflow, where polluted water from the Ala Wai Canal is forced into streets and public spaces in Waikīkī through drainage systems, and groundwater emergence, which brings sewage and other contaminants from aging and leaking sewage infrastructure to the surface.

The model simulations show that storm drain backflow is projected to occur even when there is no rainfall:

• 1 foot of sea-level rise: Storm drain backflow occurs during extreme tides, even without rain.
• 2 feet of sea-level rise: Storm drain backflow occurs during moderate daily tidal conditions.
• 4 feet of sea-level rise: Groundwater emergence (bringing sewage to the surface) begins to occur without rain.

Researchers compared their model simulations against tide gauges, canal water level sensors, groundwater monitoring wells, and photographs of street-level flooding during three real recent storm events, including a major 50-year Kona storm in December 2021, a moderate storm in April 2023, and a five-year Kona storm in May 2024.

Implications for Waikīkī, beyond

The Ala Wai Canal is one of the most polluted waterways in Hawaiʻi, containing sewage, heavy metals and pathogens such as Vibrio and MRSA. Exposure to these waters is a documented risk, with MRSA infections linked to Hawaiʻi waters already contributing to an estimated 200 deaths per year in the state. Because Waikīkī is a primary economic engine where residents and visitors are in constant contact with coastal waters, the anticipated flooding represents a growing public health and environmental crisis.

Many coastal cities around the world rely on estuarine waterways to drain their stormwater, and face the same combination of aging infrastructure, rising seas and contaminated waters.

“Our modeling framework is transferable, and we hope this study serves as a wake-up call to modernize stormwater and wastewater infrastructure, integrate contamination risk into coastal flood planning, and build early warning systems before these thresholds are crossed,” Yamamoto said.

More than 2 trillion gallons of water—enough to fill 3 million Olympic-sized swimming pools—inundated �Ჹ�ɲ���ʻ�� in March. The accumulated rainfall over 14 days reached as high as 3,000% of normal historical levels for this time of year, culminating in a destructive “rain bomb” over ��ʻ�����. Through the University of �Ჹ�ɲ���ʻ��’s �Ჹ�ɲ���ʻ�� Mesonet and the �Ჹ�ɲ���ʻ�� Climate Data Portal, researchers captured the scale of these back-to-back Kona low systems, mapping localized threats and providing crucial data on the state’s severe flooding.

Between March 1 and March 23, statewide rainfall averaged 18.25 inches—more than 2.6 times the standard March average of 6.85 inches.

While the first storm (March 10–16) brought hurricane-force wind gusts of 135.4 mph to �Ჹ�ɲ���ʻ�� Island and up to 62 inches of rain to Maui, the second Kona storm between March 19 and 23 triggered a new wave of devastation. The second storm dumped up to 61 inches of rain in localized areas, producing destructive floods across eastern and central �ѴDZ��ǰ첹ʻ��, West Maui and ��ʻ�����.

Communities such as Waialua and Haleiwa on ��ʻ�����’s north shore experienced devastating inundation. �Ჹ�ɲ���ʻ�� Mesonet stations highlighted the widespread intensity of the storm: the ����ʻ������ station recorded the island’s highest two-day rainfall of roughly 22 inches, including 19.67 inches in a 24-hour period beginning the evening of March 19. Almost simultaneously, the nearby �����������ʻ�� Ridge station above Waimea Valley recorded 9.75 inches in 24 hours.

����ԴDz�, Palolo flash floods

As the two-week rainy period neared its end, an intense, localized atmospheric event struck ����ԴDz� and Palolo valleys on March 23. Honolulu Mayor Rick Blangiardi described it as a “classic rain bomb,” heavy rain caused by a stationary storm cell. This “rain bomb,” over ����ԴDz� and Palolo valleys, dropped 2 to 4 inches of rain per hour.

Six �Ჹ�ɲ���ʻ�� Mesonet stations in the Nuʻuanu-����ԴDz� area recorded between 3.5 and 6.5 inches of rain, the majority of which fell within a three-hour window. Falling on already saturated ground, this turned streams into raging torrents and triggered significant flash floods.

The �Ჹ�ɲ���ʻ�� Mesonet, a statewide network of state-of-the-art weather stations, is proving to be a critical source of weather information, especially valuable during extreme events.

“We are building the mesonet to serve multiple purposes, including research, resource management, support for farmers and ranchers, and others,” said Thomas Giambelluca, �Ჹ�ɲ���ʻ�� Mesonet project lead, and former director of the ������Ƶ Water Resources Research Center. “But, providing data when and where it is most needed before and during extreme events like floods and wildfires, might be its most important purpose. Mesonet data will make us better prepared for future events by improving weather forecasts and enabling emergency managers to plan for and respond to extremes.”

For University of Hawaiʻi at Mānoa PhD student Brian Gorberg, the devastating Kona low storms weren’t just an academic research topic—they were a recurring nightmare that inundated his home three times in less than four weeks. Renting the bottom bedroom of a home in Waialua on Oʻahu’s north shore, Gorberg witnessed the final and most severe flood send an 8-foot wall of water through his place.

After experiencing two recent floods that ruined his belongings, Gorberg was seeking refuge at a friend’s house down the street when the third flood hit. The flash flooding was catastrophic, acting like a “dam spillway” through his neighborhood. The water destroyed his Jeep Patriot given to him by his dad, caused a neighboring house to spin off its foundation, and trapped another neighbor in neck-deep water.

“I got woken up by a giant wave,” Gorberg said. “I assumed the worst because I study hydrology. I assumed the dam failed and I knew I had to get out of the watershed. So, I drove all the way to the evacuation site. I actually ran up the hill because I didn’t believe the evacuation site was safe enough.”

Flooding expertise proved critical

Gorberg’s academic background proved critical during the floods. As an student in the and working with the under advisor Chris Shuler, his expertise allowed him to understand the mechanics of the disaster. During the floods, Gorberg spent hours walking through his neighborhood trying to warn residents ahead of the rising waters.

Gorberg’s current research work is to create high-end, reproducible flood models and stage height maps to better protect local communities. Working alongside undergraduate students Chiara Duyn, Megan Wong and Anne Dominique, Gorberg created a that provides residents with transparent, scenario-based flood information.

While the app and Gorberg’s maps currently focus on Windward ��ʻ����� watersheds such as Kāneʻohe, the underlying flood models are designed to be reproducible, and Gorberg aims to eventually expand this mapping to any flood zone across Hawaiʻi. The flood maps simulate exact inundation levels when local streams breach their banks at incremental heights—such as 12, 13 or 14 feet—allowing communities to visualize their risk and prepare for an approaching storm.

Reflecting on the tools he is building, Gorberg emphasized the urgent need for better preparedness in Hawaiʻi.

“Every stream gauge in the mainland has these maps. It’s not fair that Hawaiʻi doesn’t have it, especially considering these events,“ Gorberg said. ”And because there are graduate students like myself who have created this… that’s like the missing link in Hawaiʻi that would honestly solve this event, or would have been different if we had those maps.”

In the wake of historic flooding on Oʻahu’s North Shore, University of Hawaiʻi at ����ԴDz� (JABSOM) students continue to provide care where it’s urgently needed. On March 23, JABSOM’s Houseless Outreach and Medical Education (H.O.M.E.) Project—a student-run initiative that provides free healthcare to unsheltered and underserved communities—set up an on-site clinic at Waialua Elementary, treating 25 patients with a team of 16 student volunteers under the guidance of faculty.

“We’ve been seeing a wide range of patients. We’ve seen a fair amount with wounds and infections, probably from the storm water,” said Jill Omori, H.O.M.E. Project director.

Other patients sought care for back pain, joint pain and eye infections, conditions that can worsen when access to routine care is disrupted.

“We also saw people with GI (gastrointestinal) problems because of the contaminated water, but also just regular medical issues like hypertension or diabetes that still need to be controlled,” Omori said.

Student dedication and rural health disparities

The clinic weaves hands-on care into the JABSOM curriculum, but Omori said the majority of the students today came on their own time.

“A lot of times, the students come down to the North Shore and do service projects here and there,” Omori said. “But something like this really hits home for them. It’s really nice for them to be able to give back to the community this way.”

Second-year medical student Michael Ajimura saw that impact firsthand.

“A lot of people weren’t able to get care because of the past few days of flooding, as well as those who were injured helping out or because of the flooding,” Ajimura said. “Just being able to help them has been really rewarding. When we say that we care for the community, it’s everyone. Being out here is fulfilling, and it’s something a lot of the students are more than willing to do.”

The response also highlighted the broader issue of rural health in Hawaiʻi, which is exacerbated during a disaster.

“Rural health doesn’t just mean the neighbor islands,” Omori said. “Waialua is a great example of a rural community right here on Oʻahu. They have some of the same problems that other rural communities have, even without storms. I think it just emphasizes some of the rural health disparities and the need for more equitable care.”

As recovery continues on the North Shore, JABSOM’s H.O.M.E. Project plans to hold a pop-up clinic every day during the week of March 23. They will either be at Waialua District Park or at Haleiwa Beach Park. .

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