Kona low

From Wikipedia, the free encyclopedia

Kona lows are deep cyclones that form during the cool season of the central Pacific ocean. They are mainly cold core cyclones, which places them in the extratropical cyclone rather than the subtropical cyclone category. Hawaii typically experiences two to three annually which can impact the state for a week or more. Kona is a Polynesian term for leeward, since traditional tradewinds in Hawaii slacken during the presence of one of these cyclones. Among their hazards are heavy rain, hailstorms, flash floods and their associated landslides, high winds which result in large surf and swells, waterspouts, and severe weather.

Contents 1 Origin of term 2 Character of the cyclones 3 Frequency and effects 3.1 Kona winds 4 References

[edit] Origin of term

Kona is a Polynesian term for leeward. This was done to explain the change in wind direction for the Hawaiian Islands from easterly to southerly when this type of cyclone is present. For many slopes of the mountains in Hawaii, the southerly direction is on the leeward side of where the normal upslope winds and rains occur during their traditional tradewind regime.^[1]

[edit] Character of the cyclones

Once termed as subtropical cyclones, a definition change in the term during the early 1970's makes categorization of the systems not straightforward. The differentiation between subtropical cyclones and Kona lows are that Kona lows are typically cold core, making them extratropical cyclones.^[2] However, they generally share the subtropical cyclone characteristic of losing their associated weather fronts with time since the cyclones tend to retrograde slowly.

[edit] Frequency and effects

Hawaii typically experiences two to three annually^[3] between October and April.^[4] The cyclone events for Hawaii can be long lived, affecting the state for a week or more.^[5] Kona lows produce a wide range of weather hazards for Hawaii. Among them are heavy rain, hailstorms, flash floods and their associated landslides, high winds which result in large surf and swells, waterspouts, and severe weather.^[4]

[edit] Kona winds

Kona winds are associated with cyclones with central pressures below 1000 hPa which pass within 500 miles/820 km northwest of the Hawaiian Islands. Strong Kona winds usually last for a day. Considerable damage is caused to boats lying southwest of the islands. On land, trees are uprooted and roofs can be blown off homes. When passing through mountain gaps and over mountains, downsloped winds gust over 100 mph/165 km/h which causes much of the destruction.^[6]

The most powerful Kona Storm in the last fifty years struck the Hawaiian islands between January 8-11, 1980. The low pressure primarily responsible for the severe conditions had a baromic pressure of 975mb on January 8th while passing north of the state, one of the lowest pressure readings ever recorded in Hawaiian waters as a result of an extratropical storm. ^[7] The storm caused severe weather across the entire state, forcing the closure of all airports. Rainfall from the storm was extremely heavy, surpassing twenty inches in many locations - particularly on the Big Island and Maui. High surf from the storm caused extensive damage, particularly along the Kona coast of the Big Island, where the towns of Kailua-Kona and Puako received extensive damage.^[8] Winds from the storm averaged around 40-50mph out on the open ocean, but varied tremendously on land due to unique geographic features. Some areas experienced wind gusts well over hurricane force - including a low altitude gust of 106 mph (171 km/h) on Lahaina Roads, Maui. ^[9] Haiku, on the windward coast of Maui, exerienced a wind gust of 75 mph (121 km/h). ^[10] On the high mountain summits of the Big Island and Maui, winds of over 110 mph (180 km/h) were recorded. Damage from the storm was estimated at between $25-35 million (in 1980 dollars). ^[11]

[edit] References