A low-pressure system is a common atmospheric feature where air rises, often leading to cloud formation and precipitation. The Kona Low is a specific, sometimes destructive, non-tropical weather system unique to the Hawaiian Islands. This phenomenon disrupts the islands’ typical weather patterns, bringing heavy rainfall and strong winds to areas that are normally sheltered.
Defining the Kona Low System
The Kona Low is classified as an extratropical or subtropical cyclone. It is distinguished from tropical systems like hurricanes because it is a cold-core low, meaning the circulation’s center is colder than the surrounding air mass at the same altitude. Kona Lows typically form northwest of the Hawaiian archipelago, and their circulation pattern reverses the usual trade wind flow.
The system is named “Kona” because it brings winds and weather to the leeward sides of the islands, which are usually sunny and dry. In Hawaiian, “kona” refers to the leeward or western side of an island, and also means a southerly or southwesterly wind. The counter-clockwise rotation of the low-pressure center, when positioned west of the islands, draws moisture-laden air from the south toward these typically dry leeward coasts.
How Kona Lows Develop
The formation of a Kona Low begins high in the atmosphere with an upper-level trough of low pressure. This system is associated with a southward plunge of cold air over the central Pacific Ocean, common during the cooler season. A deep dip in the jet stream occurs, which is a band of strong winds high in the atmosphere that guides weather systems.
This trough subsequently deepens and separates from the main flow of the jet stream, becoming a “cut-off low” that traps cold air aloft. This detachment removes the steering influence of the jet stream, allowing the system to linger for an extended period. The upper-level system then couples with a surface low-pressure area, creating a deep atmospheric vortex.
The resulting low-pressure circulation pulls in moisture from the warm waters around Hawaiʻi, fueling the system. The winds transition from the typical northeasterly trade winds to a south or southwesterly flow. This wind reversal directs torrential rain toward the leeward sides of the islands, which are usually sheltered by the mountains.
Weather Effects and Hazards
The most significant consequence of a Kona Low is widespread, heavy rainfall that can lead to flash flooding. The moisture-rich southwesterly winds interact with the steep topography of the islands, forcing the air to rise and rapidly condense water vapor. This orographic lifting effect concentrates intense precipitation over specific areas.
The typically dry leeward areas, such as the Kona coast, are particularly vulnerable because the ground is not adapted to absorb large volumes of rain. The shallower soil and sparser vegetation in these regions mean water runs off quickly, increasing the risk of flash flooding and dangerous landslides. Rainfall totals have exceeded twenty inches in some locations during past events.
Beyond heavy rain, Kona Lows generate strong, gusty winds that can cause damage. Winds associated with these systems can average 40 to 50 miles per hour, with some areas experiencing hurricane-force gusts exceeding 100 miles per hour. These winds also churn up the ocean, producing high surf and swells, particularly along the western and southwestern shores.
The cold-core nature of the system can result in unusual weather phenomena for a tropical location, including thunderstorms, hail, and blizzard conditions. High-elevation areas, such as the summits of Mauna Kea and Mauna Loa, often receive heavy snowfall and blizzard warnings during a strong Kona Low event.
Seasonal Timing and Duration
Kona Lows are seasonal cyclones that primarily affect the Hawaiian Islands during the cooler part of the year. They are most common during the late fall, winter, and spring months, generally occurring between October and April. Hawaiʻi typically experiences an average of two or three systems annually, though frequency varies significantly.
The duration of a Kona Low event is a defining characteristic and a primary concern. Since the system is cut off from the main jet stream, there is no strong atmospheric flow to push it away quickly. As a result, Kona Lows can linger near the state for several days, sometimes for a week or more, causing prolonged periods of adverse weather and continuous heavy rainfall.