Monthly Precipitation
State of Hawaii Summary

The following summaries are excerpted from the NOAA NWS Honolulu Forecast Office's Monthly Precipitation Summaries.

October: Several significant heavy rain events across the state marked the first month of the 2004-2005 Hawaiian wet season.

The worst heavy rain event of the month occurred during the evening of October 30 over Manoa Valley in east Oahu. Intense rain falling at a peak rate of more than 5 inches per hour during one 15-minute period produced a surge of water that caused Manoa Stream to overflow its banks. The resultant flooding damaged many Manoa Valley homes and caused extensive damage to several buildings on the campus of the University of Hawaii at Manoa. Although no official damage estimates are available, the total is expected to be several millions of dollars. Fortunately, the flooding did not cause any deaths or significant injuries. For additional details, please refer to our [NWS-HFO] online Manoa Flood summary.

On October 26, another significant heavy rain event produced flash flooding over the windward slopes of the Koolau Mountains on Oahu. The areas hardest hit included the Kahuku High School campus and the Waikane and Waiahole areas. Three vehicles became stranded while attempting to cross an overflowing Waikane Stream. All vehicles were successfully recovered with no serious injuries to the vehicles' occupants. Inundation of the Kahuku High School campus damaged portions of the school's athletic complex. No damage estimates were available in time for this report.

East Kauai experienced very heavy rains during the late morning through early afternoon hours of October 15 associated with a low pressure system northwest of the island chain. Rain gages recorded over 2 inches per hour and more than 5 inches in 4 hours when an area of heavy showers moved onshore from the southeast. A group of kayakers became stranded near the Fern Grotto when a rise in water level on Wailua River made the return trip unsafe. No significant injuries were reported from this event.

Finally, a flash flood occurred during the early morning hours of October 27 when thunderstorms over the north-facing slopes of Kauai caused Hanalei River to overflow its banks. The overflow forced the closure of Kuhio Highway at the Hanalei Bridge for about 5 hours and affected taro crops in the adjacent fields.

In addition to the flash flood events, heavy rains also produced minor flooding problems on October 12 (Big Island, central and east Oahu), October 13 (Oahu, Big Island, and east Maui), October 19 (east Big Island), October 24 (east Kauai), October 25 (Oahu), and October 31 (central Oahu).

November: November saw a continuation of wet conditions experienced in October across most of the main Hawaiian Islands. Although not as intense or damaging as the October heavy rain events, the wet weather pattern required the issuance of flood advisories or flash flood warnings on 10 days during the month, keeping forecasters and emergency managers busy monitoring conditions.

The most significant heavy rain event in terms of duration and overall impacts across the state occurred during the period from November 22 through November 28 due to a persistent upper level low pressure system near the island chain. Heavy rains initiated over the southeastern flank of Mauna Loa on the Big Island during the night of November 22. Radar data indicated rain rates as high as 4 to 6 inches per hour over the slopes between Pahala and Naalehu, which strongly suggested the initiation of flash flooding. However, no significant flooding or damages were reported. Intense showers and thunderstorms also occurred over Oahu and over the Kona and Kohala areas of the Big Island on November 23, 25, 26, and 28, but no damages or injuries were reported as a result of these bouts of heavy rain.

On November 5 and 6, a slow moving, poorly defined cold front dropped over the western half of the state, bringing moderate to heavy rains to Kauai, Oahu, and Molokai. The most significant rains fell over Oahu where many gages recorded 1 to 4 inches on November 5 and another 1 to 2 inches on November 6. No significant damages occurred with this system, though the rains severely affected the afternoon rush hour commute and produced many potholes on November 5.

Finally, a strong shear line pushed across the island chain from November 13 through 15. As is usually the case with shear lines, frequent showers occurred mainly over the north through east facing slopes of the islands. Most gages along the windward slopes of Maui recorded 2 to 4 inches of rain, though the West Wailuaiki site deep in the east Maui watershed recorded 13.71 inches on November 14. Rain gages in the Kohala, Hamakua, Hilo, and Puna areas of the Big Island recorded 4 to 8 inches on November 15. No damages were reported, though two hikers required rescue on Maui when they became trapped by elevated stream levels.

Moderate trade winds dominated the weather pattern over the main Hawaiian Islands for the month of September, producing daily showers over Kauai and Oahu. However, Maui County and the Big Island experienced much less rainfall, which was somewhat unexpected under the persistent trade wind conditions.

December: After a very wet November, the more familiar trade winds started off the month of December and persisted through December 11. Trade winds became rather strong on December 3 and 4, causing some damage on the Big Island. A ridge of high pressure settled over the island chain, bringing dry conditions from December 12 through 22. And then the rains started.

A pre-frontal convergence band brought moderate to heavy showers over portions of Oahu, Molokai, and Lanai on December 22. Although no significant damages occurred, the rains created a greater than normal traffic load for Oahu commuters. The associated cold front stalled just west of Kauai. A slightly stronger cold front followed several days later with its pre-frontal convergence band producing heavy showers and thunderstorms over Kauai and Oahu on December 26 and 27. Rainfall was particularly heavy over central and west Oahu, prompting a flash flood warning for these areas. Fortunately, no significant damages or injuries were reported.

The year closed out with the island chain under threat of a strong cold front that was expected to push across the main Hawaiian Islands accompanied by strong winds, heavy rains, and strong thunderstorms. However, the low pressure system developed and strengthened farther south and west than anticipated, resulting in a delayed onset of heavy rains and a weakened weather system by the time the cold front reached Kauai on New Year's Day.

Kevin R. Kodama, Senior Service Hydrologist
National Weather Service Forecast Office, Honolulu, Hawaii