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Click on the icons below to learn more about the hazards detailed in the 2020 Maui HMP.
A hurricane is a large, rotating storm with high speed winds. Hurricanes form over warm, tropical waters when warm, moist air rises, creating an area of low pressure. Surrounding areas of high pressure push toward the low pressure, then also warm and rise. As warmed air continues to rise, surrounding air swirls around the low-pressure area to take its place. As the warm air rises it cools and forms clouds. The storm system spins and grows as it continues to be fed by warm ocean water. As the storm spins faster, a low-pressure eye forms in the center of the storm. The eye is clear and calm, but is surrounded by the eyewall, which contain the storm’s strongest winds and heaviest rains. As a hurricane makes landfall, it weakens as its supply of warm ocean water is cut off. The figure below shows a cross-section of a hurricane.
Hurricanes and tropical storms (also referred to as tropical cyclones or typhoons), can be damaging and potentially deadly in the Hawaiian Islands. These storms can bring excessive amounts of rain, strong and damaging winds, storm surge, high waves, erosion along shorelines, and tidal and coastal flooding. In the Northern Hemisphere, hurricanes typically form in three regions of the world: the tropical Atlantic, the eastern tropical Pacific, and the western tropical Pacific. In the Southern Hemisphere, hurricanes form in the western tropical Pacific and Indian Ocean. Hawaiʻi lies in the Central Pacific, which, on average, experiences four to five hurricanes every year.
The three major threats caused by hurricanes include storm surge, high winds, and flooding associated with heavy rainfall.
In the United States, forecast centers classify hurricanes and similar, but weaker storms in the following categories according to their maximum sustained winds:
Tropical Depression: A weak tropical cyclone with a surface circulation including one or more closed isobars (lines or curves of constant pressure) and highest sustained winds (measured over one minute or more) of less than 38 mph. Tropical depressions are assigned a number denoting their chronological order of formation in a given year.
Tropical Storm: A tropical cyclone with highest sustained winds between 39 and 73 mph.
Hurricane (or Typhoon west of 180º longitude): A tropical cyclone with highest sustained winds greater than 74 mph.
Kona Storms: Kona storms, also referred to as Kona lows, are subtropical cyclones that occur in the north central Pacific during the cool season. The Hawaiian word “kona” means leeward and refers to the southwesterly winds associated with the storm that replace the usually dominant trade winds in Hawaiʻi that blow from the northeast. Unlike hurricanes, Kona storms form over cool waters, and typically occur from October through April in Hawaiʻi. Kona storms originate from a southward dip in the jet stream (a band of strong winds high in the atmosphere that drive weather systems) that sinks and breaks off, leaving behind a low-pressure circulation that is cut off from the main core of the jet. These lows can linger for several days. Swirling winds and clouds form around this low-pressure system in a manner similar to that of a hurricane.
The impacts of Kona storms are similar to those of a hurricane or tropical storm, including heavy rains, hailstorms, high winds, large surf, waterspouts, and severe thunderstorms. Heavy rains associated with Kona storms can cause flooding and landslides. Kona storms can also bring blizzard conditions to high elevations.
All of Maui County is susceptible to hurricanes and tropical storms. However, areas impacted by specific storms will depend on hurricane storm track and location. Maui County can be impacted by hurricanes that do not make a direct hit (e.g. the eye does not make landfall in the county), as damaging winds and bands of rain can impact locations one hundred miles or more from the eye of the storm, depending on storm size.
Further, coastal areas are often the hardest hit during a hurricane. They are often the recipient of a hurricane’s strongest winds and rainfall before it weakens upon tracking inland. In addition, coastal areas have the potential to be impacted by storm surge, one of the most deadly and damaging impacts on hurricanes. On the Island of Maui, Kahului (one of the county’s most urbanized cities), coastal areas of Wailuku (the county seat), and the tourist destination of Kīhei are some of the most populated areas within potential storm surge locations. On the Island of Molokaʻi, Kaunakakai, the island’s most populous town, is largely within estimated storm surge areas.
Kona storms also have the potential to impact the entire county. Cloudy and wet conditions are likely to impact the typically dry and sunny western and southwestern sides of the islands. Volcano peaks and high elevations may experience blizzard-like conditions.
While Maui County has only received one direct hit from a hurricane or tropical storm in recent history (Tropical Storm Olivia), many historic hurricanes have tracked close enough to the county to impact one or more of its islands. Official hurricane tracking began in 1950. However, one notable hurricane before this reporting period began has been described as impacting Maui County; in 1874, storm Die Deutsche Seewarte III destroyed 23 homes Molokaʻi and several others on Lānaʻi. Since 1950, 18 hurricanes and tropical storms have had a significant impact on one or islands in Maui County. Additional information about these events can be found in the HMP Hazard Profile for Hurricanes.
As demonstrated by previous occurrences, powerful hurricanes, tropical storms, and Kona storms occur in Maui County. Category 5 hurricanes as assigned via the Saffir Simpson scale are the most severe hurricanes. However, the severity of hurricanes and tropical storms can also be determined by damage amounts. In Maui County, the most damaging hurricane to impact the county was Hurricane Lane in 2018, which caused approximately $6.5 million in damages and resulted in a presidential disaster declaration. Hurricane Lane was initially forecasted to strike Maui with Category 5 hurricane force winds. While the storm still brought tremendous damage, it is an example that stronger events are possible.
However, more intense hurricane events have also been experienced on neighboring islands. For instance, Kauaʻi County received the brunt of Hurricane Iwa in 1982, which produced an estimated $234 million in damage. Similarly, Hurricane Iniki was a Category 4 hurricane that hit the County of Kauaʻi in September 1992, causing almost $2 billion in damages. Events such as these are possible in Maui County. In addition, hurricane and tropical storm frequency and intensity are projected to increase in the central Pacific region under climate warming scenarios. Similarly, projected sea level rise combined with storm surges could have impacts from which coastal communities are unable to fully recover.
Future Probability
Previous occurrences have indicted 24 events (18 hurricane/tropical storms and six Kona storms) in 70 years. This indicates an event impacts Maui County approximately every four years. However, it is likely that many Kona storm events occurring within Maui County have gone unreported. Hurricane and tropical storm frequency in Maui County is projected to increase along with warming trends. It should be noted that hurricane and tropical storm frequency in Maui County is highly variable based on ENSO patterns. Hurricanes are much more likely to impact Hawaiʻi during El Niño years. For instance, in 2015, which was an El Niño year, the central Pacific region experienced 15 named storms (eight hurricanes and five major hurricanes), making 2015 the most active season since 1970. While it is unknown exactly how the timing and intensity of ENSO patterns will change in the future, climate model results indicate a doubling of El Niño and La Niña extremes in the 21st century when compared to the 20th century.
All current and future buildings, infrastructure, and populations in Maui County are considered at risk to hurricanes, tropical storms, and Kona storms. These storms result in damaging winds, heavy rains and floods, and storm surges. Further, these storms can cause cascading impacts; for instance, storm surges may cause severe coastal erosion, heavy rains may prompt landslides, or, in the case of Hurricane Lane, gusty winds may fan wildfires in unsaturated parts of the county. Specific impacts are described below:
Buildings and Infrastructure: All current and future buildings and infrastructure, including critical facilities are considered at risk to hurricanes, tropical storms, and Kona storms. Damaging winds and heavy rains can impact coastal and inland areas of Maui County, and storm surges can impact coastal areas. Strong sustained winds and even stronger wind gusts associated with these storms can blow shingles, siding, awnings, and even roofs off buildings. Falling trees and tree limbs can damage structures. Objects picked up by wind can be hurled through the air, damaging structures and breaking windows when contact is made. In some cases, structures can be blown off foundations.
Further, heavy rains can cause riverine and flash flooding, which can inundate structures. Heavy rains can also seep through roofs and walls, especially ones that have been damaged by winds, causing water damage. Water damage can lead to structural damage, wood rot, and mold growth.
Kīhei-Mākena has the greatest amount of infrastructure exposure, with up to 805 buildings (valued at $2 billion) in storm surge areas, whereas Molokaʻi has the greatest proportion of structures potentially at- risk, with 21% of its buildings (and 22% of total building value) in the Category 4 storm surge area.
Severe winds associated with these storms can cause damage to communications infrastructure and utility poles, and above ground power lines can be blown down. When utility lines are damaged by hurricanes, functionality of water, sewer, gas, and electricity systems may be compromised. Surges may cause saltwater intrusion into drinking water sources or overwhelm storm sewer systems, which can lead to urban flooding, especially in combination with heavy rainfall. Storm surge may also severely erode or undermine coastal highways and/or bridges. Heavy rains may trigger landslides that block or damage roadways. For instance, during Hurricane Lane, Hāna Highway was blocked in several places due to landslides. Because of their geographic extent, roads and utility lines can have a greater chance of being impacted by hurricane events. Further, if any section of a road or utility line is damaged or flooded during a hurricane, the entire system may be impacted.
Health and Safety: Hurricanes, tropical storms, and Kona storms present serious risks to life safety. Winds, heavy rainfall, and storm surge present risk to drowning, structure collapse, and being struck with fallen trees or wind- driven objects. Powerful surges can turn floating debris into projectiles that can cause injuries, deaths, and property damage. Fires can also ignite from broken gas lines, ruptured tanks, or wildfires fanned by strong winds.189 Blizzard conditions from Kona storms may trap unprepared hikers at high elevations, such as those at Haleakalā National Park. All populations within Maui County, including residents and visitors, are considered at risk to hurricanes, tropical storms, and Kona storms.
Hurricanes and tropical storms can be closely monitored and tracked. Therefore, accurate warnings are possible within days of an event, and watches may be available up to a week prior to an event as the storm system forms at sea. Precautions must be taken to ensure early alert and warning of vulnerable populations including the elderly, people with disabilities, children and families, and those with language or communication barriers.
Public health impacts from hurricanes, tropical storms, and Kona storms are similar to those of coastal flooding and tsunamis. Aside of direct threats to life and property, hurricanes can result in the contamination of drinking water through saltwater intrusion, inundation of hazardous materials facilities, inundation of wastewater treatment plants, and broken utility or sewer lines. Flood water may contain these hazardous materials and be dangerous to traverse. In estuaries and wetlands, saltwater intrusion endangers the public health, kills vegetation, and can send wildfire fleeing from flooded areas. Further, flooded structures or those with water damage from heavy rainfall may quickly develop mold or wood rot, presenting serious health risks to inhabitants if not treated properly.
Economic Impacts: Hurricanes, tropical storms, and Kona storms can have wide-reaching economic impacts. Direct economic impacts may result from destroyed buildings and infrastructure, including power utility poles and lines, potable water and wastewater facilities, bridges, and roadways. The force of storm surges may destroy piers, breakwaters, and seawalls. Ships, marinas, and fishing fleets may be battered, sunk, moored, or stranded, impacting the fishing and tourism industries. Confined harbors, especially in combination with storm tides, are vulnerable to surges. Business interruptions can occur, and damages or changes to eroded beaches may impact tourism for periods long after the storm.
Climate Change Impacts: Hurricane and tropical storm frequency and intensity are projected to increase in the central Pacific region under climate warming scenarios. While research is still unclear, this phenomenon may be driven by warmer ocean waters, which feed tropical cyclone development. In addition, hurricane and tropical storm frequency in Maui County is highly variable based on ENSO patterns, as hurricanes are much more likely to impact Hawaiʻi during El Niño years. While it is unknown exactly how the timing and intensity of ENSO patterns will change in the future, climate model results indicate a doubling of El Niño and La Niña extremes in the 21st century when compared to the 20th century. Similarly, projected sea level rise combined with storm surges could mean hurricanes impact areas further and further inland over time, and could have impacts from which coastal communities are unable to fully recover. Coral reefs provide protection against storm surges by forming an offshore barrier that works to dissipate wave energy.
Impacts on Kona storms due to climate change are unknown but, similar to hurricanes, are likely to increase in frequency and intensity.
