State of Hawaii

MONTH: December 2008

PREPARED: January 9, 2009

State: [Text data table for rain gages]

Like November, the month of December began with relatively stable conditions and little rainfall across the state. However, a significant shift in the weather pattern over the north Pacific resulted in wet conditions across most of the Hawaiian Islands from December 10 through the remainder of 2008. The first in a series of heavy rain events arrived in the form of a strong kona low centered several hundred miles northwest of Kauai. The associated band of heavy showers and thunderstorms arrived over Kauai late on December 10 and over Oahu early on December 11. An intense area of rain developed above the west slopes of the Waianae Range and moved northeast into central and northeast Oahu with peak rain rates in excess of 4 inches per hour. By 8 AM HST, 6-hour rainfall accumulations at several sites such as Makaha Stream, Schofield Barracks, and South Fork Kaukonahua Stream exceeded 10 inches. According to the U.S. Geological Survey (USGS), several streams in central and windward Oahu set new peak flow records during the storm. Click here for the USGS peak flow report. Flooding produced severe damage to homes, infrastructure, and farm lands with losses easily running into the range of millions of dollars. Amazingly, despite the severity of flooding, no deaths or serious injuries were reported.

A second round of serious flooding, triggered by a short wave trough, started during the night of December 12 and ended during the evening of December 13. South and west Kauai experienced serious flooding, especially in Waimea where flood waters temporarily cut off some residents in the valley. Higher elevation rain gages recorded over 10 inches in 24-hours and south Kauai gages recorded 5 to 7 inches. The shortwave trough also helped generate a severe thunderstorm with a rare tornado which moved over the Pakala area of south Kauai just before 1 PM HST on December 13. The band of heavy rainfall shifted eastward and reached Oahu during the mid-afternoon hours. Saturated ground from the December 11 rainfall quickly produced flash flooding and two boys were rescued after being swept downstream near Waianae.

Following a relatively quiet December 14, heavy showers and thunderstorms once again developed during the evening of December 15 and moved over Kauai from the southeast. A USGS gage on the South Fork of Wailua River registered a 9 ft rise in water level within a 30-minute period, prompting the issuance of a flash flood warning. No significant damages or injuries were reported as a result of this episode.

With the north Pacific still in a high amplitude configuration, a new kona low developed west of Kauai on December 18. This low formed farther away from the island chain than the previous kona low. While a few heavy showers clipped Niihau and Kauai, only minor flooding occurred.

On December 23, fresh to strong trades affected the state with showers mainly affecting east- and northeast-facing slopes. A strong upper level low moved near the Big Island from the east on Christmas Day and helped trigger repeated heavy showers over the windward slopes and flash flooding. Other than a small rockslide near Laupahoehoe and some road closures due to flooding in the Hilo area, there were no significant damages or injuries reported despite more than 10 inches of rain in 24-hours in the North and South Hilo Districts.

The upper level low weakened but lingered over the state through December 31 and maintained unstable conditions favorable for thunderstorm development. Periods of heavy rainfall over Oahu produced minor flooding problems to compound the difficulties produced by the island-wide power outage on December 26 and 27. East Maui received heavy rainfall on December 28 and 29 which produced flash flooding in the Kipahulu area. A bridge at Kalepa Gulch washed out but was quickly repaired. Lastly, to close out the year, heavy rain over north Kauai produced flash flooding in Hanalei River which forced the closure of Kuhio Highway at the Hanalei Bridge for several hours.

Island of Kauai : [December 2008 map] [Year-to-date map]

All of the gages across Kauai recorded above normal rainfall during the month of December. Though not nearly as wet as the record-breaking conditions of March 2006, it was the wettest month of 2008 at all locations island-wide. The USGS gage on Mount Waialeale recorded the state’s highest monthly total for the month with 59.21 inches (130 percent of normal) and the highest daily total of 13.75 inches on December 13. For Lihue Airport, the 19.46 inches for the month marked the third wettest December since gage establishment in 1950, surpassed only by 22.91 inches in 1968 and 22.20 inches in 1992.

The wet December conditions helped nudge several gage totals into the near normal range for 2008. Breaking down the distribution, available gages along the north and northeast slopes finished the year in the near to below normal range while most of the gages in the east- and south-facing slopes ended up in the near to above normal range. As expected, Mount Waialeale’s 351.66 inches (83 percent of normal) topped all totals statewide for 2008.

Island of Oahu: [December 2008 map] [Year-to-date map]

All of the gages on Oahu recorded near to well above normal rainfall for the month of December in what was by far the wettest month of 2008. More than half of the gages received more than double the normal rainfall for the month and most sites recorded the highest monthly totals since March 2006. The Poamoho 2 rain gage, operated by the USGS, had the highest monthly total of 40.10 inches (160 percent of normal). However, the South Fork Kaukonahua gage recorded the highest daily total of 14.16 inches during the severe flash flooding on December 11. A breakdown of the daily total reveals some notable details, such as 9.28 inches in 3 hours (4:30 to 730 AM HST) and 4.26 inches in 1 hour (5:30 to 6:30 AM HST). Honolulu Airport’s 7.58 inches (261 percent of normal) for the month stands as the wettest December since the record-setting 17.29 inches in 1987.

Rain totals for 2008 ended up in the near to below normal range at most locations on Oahu with most totals in the range of 60 to 90 percent of normal. The U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge gage finished the year with the highest Oahu total of 169.73 inches (83 percent of normal), which ranked third highest in the state.

Maui County: [Maui December 2008 map] [Year-to-date map] [Molokai/Lanai December 2008 map] [Year-to-date map]

Rainfall totals across Maui County showed an interesting level of consistency as all locations recorded near to above normal rainfall. Heavy rainfall over east Maui during the latter half of the month helped the Kaupo Gap rain gage record the highest county-wide monthly total of 34.48 inches (257 percent of normal). The highest daily total of 7.69 inches occurred at the USGS’ West Wailuaiki gage on December 27.

Rain totals for 2008 ended up in the below normal range at most of the Maui County gages. The Puu Kukui gage finished the year at 210.22 inches which was second highest in the state but only 54 percent of normal. The USGS gage at Oheo Gulch had the highest percent of normal value at 118 percent.

Island of Hawaii: [December 2008 map] [Year-to-date map]

Most of the gages on the Big Island recorded above normal rainfall for the month of December. The exceptions were in the drought-plagued districts of Hamakua, North Kohala, and South Kohala which once again received below normal rainfall. The South Hilo District rain gages had the highest monthly totals on the island due in large part to the heavy rainfall from the December 25 and 26 flood event. Hilo Airport led all gage totals with 30.38 inches (289 percent of normal) but Waiakea Uka had the highest daily total of 13.37 inches on December 26. While 30.38 inches is regarded as a very high monthly total, even by Hilo standards, December was not the wettest month of the year at this location. The wettest month for 2008 at Hilo Airport was February with 39.08 inches, of which 29.72 inches occurred in the first 5 days of the month.

Big Island rain totals for 2008 were mostly in the below normal range. The South Hilo and Puna District gages had near normal totals. The Waiakea Uka gage recorded the most rainfall in 2008 at 145.36 inches (81 percent of normal), or sixth highest in the state. For Hilo Airport, it was the first time since 1990 in which two months (February and December) during the year had totals higher than 30 inches.

Data Sources: Data used in this report are largely from National Weather Service sources including climate network weather observation stations at Lïhue, Honolulu, Kahului, and Hilo, the Hydronet state network of automated rain gages, and selected Cooperative Observer sites. Additional data come from automated rain gages operated by the State of Hawai‘i Department of Land and Natural Resources, the US Geological Survey, the US Bureau of Land Management, the US National Park Service, the Department of Defense, and the US Fish and Wildlife Service. Data presented here are not certified and should be used for information purposes only.

Kevin R. Kodama
Senior Service Hydrologist
NOAA/NWS Weather Forecast Office Honolulu