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The Honolulu Advertiser
Posted on: Friday, March 3, 2006

Rain-making Kona systems behind persistent downpours

 •  Flooding paralyzes Windward O'ahu

By Jan TenBruggencate
Advertiser Science Writer

Rain-making "multiple, similar weather systems" are to blame for recent drenchings on O'ahu and Kaua'i.

While the systems are expected to be replaced by trade wind weather as the weekend approaches, forecasters expect the systems will likely be back later this month, said Jim Weyman, meteorologist in charge at the National Weather Service in Honolulu.

The weather at the western end of the Islands chain in recent weeks has been dominated by "Kona low" systems to the west of Kaua'i. The counter-clockwise flow of air around these systems draws warm, moist air to sweep over the Islands from the south.

"These systems make the air to the east and southeast of the low unstable, creating a greater chance of heavy rain showers," Weyman said.

While this unstable air flow caused much of last week's rain on both Kaua'i and O'ahu, what's driving this week's rain is different on each island.

O'ahu's rain is falling primarily along the windward side of the Ko'olau mountains. Twenty-four hour rainfall totals through midday yesterday showed the pattern: 'Ahuimanu had 4.45 inches, Punalu'u 11.33 inches and Kahuku 5.35 inches, while most leeward O'ahu areas had less than an inch.

The driving force for that rain, said Tim Craig, a forecaster for the National Weather Service, is southeast winds blowing parallel to the Ko'olau mountain range. As the moist air reaches the land, friction causes it to slow, and the steep-rising windward cliffs drive it upward. The fast-rising air then cools quickly and moisture condenses, creating heavy rainfall. New moist air continually fills in behind the old, creating a persistent downpour in the same areas.

On Kaua'i, the current rainfall pattern is tied to the convergence of wind from the south and the southeast, which moved close enough to the center of the low-pressure system to produce unstable air conditions.

The rainfall totals for the 24-hour period ending at 2:45 p.m. yesterday included: 10.55 inches at Wai'ale'ale, 5.45 inches at Kalaheo and 2.47 inches at Lihu'e Airport.

Weyman said that the Hawaiian Islands experience two or three Kona low systems each year, but that they normally pass from west to east through the Islands. The recent low systems are unusual in remaining to the west.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.

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