State of Hawaii

MONTH: December 2011

PREPARED: January 9, 2012

State: [Text data table for rain gages]

In the lower levels of the atmosphere, the trade winds were unusually persistent considering the time of year with breakdowns on only 2 days during the month (December 1 and 31). Normally, trade winds occur with an average frequency of just over 50 percent during December. In addition, anomalous low pressure in the middle and upper levels of the atmosphere from December 10 through 22 near the main Hawaiian Islands resulted in unstable conditions which produced enhanced rainfall over the windward slopes. Several gages on the windward side of the Big Island recorded more than 30 inches of rainfall during December making it the wettest month since March 2009. Fortunately the rainfall activity was well distributed during each day and resulted in only minor flooding problems and a few rockslides along the Hamakua Coast.

On the smaller islands, some of the rainfall crossed over into the leeward sections resulting in some flooding problems. The most significant flooding occurred on Oahu during the early morning hours of December 13 as an area of thunderstorms with heavy rainfall swept across the Koolau Range. With the ground already saturated and stream levels elevated from a brief but heavy round of rain earlier that night, this second round of heavy rainfall quickly produced significant runoff and flash flooding in Manoa, Kalihi, Nuuanu, and Halawa Streams. The flooding in Manoa Stream briefly inundated an adjacent property with about one foot of water and mud at about 3 AM HST. Mud-laden runoff also poured onto the H-1 Freeway and closed the westbound lanes for several hours.

While showers soaked windward areas, most of the leeward areas remained dry especially on the Big Island and Maui. Several locations received less than a quarter inch of rainfall in what should have been one of the wettest months of the year. For more information on the ongoing leeward drought conditions, please refer to the latest Drought Information Statement at: http://www.weather.gov/data/HFO/DGTHFO

Island of Kauai : [December 2011 map] [Year-to-date map]

Persistent trade winds produced near to above normal rainfall totals at windward locations but well below normal totals at leeward sites. The monthly total of 29.92 inches (99 percent of normal) from the U.S. Geological Survey (USGS) gage on Mount Waialeale was the highest amount on Kauai but was surpassed by several totals from Maui and the Big Island. Waialeale also had Kauai’s highest daily total of 3.79 inches on December 13. In contrast, the Waimea Heights gage upslope from Waimea town received just 0.15 inches (4 percent of normal).

Most of the gages on Kauai closed out 2011 with near to above normal rainfall totals. The Mount Waialeale total of 365.85 inches (93 percent of normal) was the state’s highest annual rainfall total and the highest annual total at this location since 2006. However, the 10-, 20-, and 30-year running averages set new records for lowest values at 352.74, 354.58, and 382.22 inches, respectively.

Island of Oahu: [December 2011 map] [Year-to-date map]

Many of the gages on the slopes of the Koolau Range posted near to above normal totals for the month of December. The U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge (NWR) gage had the highest monthly total of 22.41 inches (119 percent of normal). The Manoa Lyon Arboretum gage recorded 12.17 inches during a 6-day period from December 10 through 15. This includes the island’s highest daily total of 5.06 inches on December 13. Most of the leeward gages had below normal totals.

Most of the gages on Oahu ended 2011 with rainfall totals in the near to above normal range. The Oahu Forest NWR gage had the highest annual total of 192.51 inches (87 percent of normal) and was the third highest statewide.

Maui County: [Maui December 2011 map] [Year-to-date map] [Molokai/Lanai December 2011 map] [Year-to-date map]

Most of the available gages across Maui County recorded below normal rainfall for the month of December. The USGS’ Puu Kukui gage reported the state’s highest monthly total of 47.29 inches (159 percent of normal) and Maui County’s highest daily total of 7.46 inches on December 14. The lack of windward Maui gages due to data logger problems or funding cuts resulted in a gap in windward real-time data coverage. However, a volunteer Cooperative Observer at Honomaele (west of Hana Airport) reported 16.35 inches for December (156 percent of normal). The Kamalo gage on southeastern Molokai reported an above normal monthly total of 4.17 inches.

Most of the gages in Maui County finished the year with below normal rainfall totals. The Honomaele gage mentioned previously ended up with 104.99 inches (81 percent of normal). Puu Kukui had the highest annual total of 226.57 inches (62 percent of normal) which was second highest in the state.

Island of Hawaii: [December 2011 map] [Year-to-date map]

Windward areas of the Big Island received well above normal totals for the month of December. The highest monthly total was 34.09 inches (266 percent of normal) at the USGS’ Saddle Road Quarry gage while the highest daily total was 8.32 inches at Laupahoehoe on December 11. The wettest portion of the month occurred during the period from December 19 through 24. Notable 6-day totals included 18.14 inches at Saddle Road Quarry, 15.24 inches at Glenwood, and 13.38 inches at Waiakea Uka. While the 6-day totals were impressive, the hourly totals were less than an inch which helped prevent serious flooding problems. As wet conditions were occurring along the windward slopes, most leeward areas were very dry, especially in the North Kona District where gages recorded totals at less than 20 percent of normal.

Most of the gages across the Big Island ended 2011 with below normal totals with most leeward sites at less than 50 percent of normal. Several windward gages pushed up into the near normal range following the wet December conditions. The highest annual total was from the Glenwood gage which received 149.27 inches (64 percent of normal) during the year.

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