2010 Precipitation Summary
State of Hawaii
MONTH: December 2010
PREPARED: January 7, 2011
A large scale blocking pattern in the north Pacific consistent with the ongoing La Nina event resulted in wet weather conditions over the Hawaiian Islands during the month of December. The significant shift in the weather pattern also produced a lower than normal frequency of trade wind days over the state. Overall, Kauai and Oahu received well above normal rainfall. Lesser amounts of rain occurred over the east half of the island chain with the windward Big Island slopes receiving below normal rainfall totals for the month.
Four main weather systems brought the majority of the rainfall to the main Hawaiian Islands. The first system was a weak cold front that reached Kauai on December 3. Heavy showers, enhanced by the mountainous terrain, produced flash flooding along Wailua River in east Kauai. Flood waters briefly inundated portions of a canoe club along the lower reach of the river.
The following week, a strong upper level trough crossed the state and triggered strong thunderstorms with intense rainfall. Rising water levels in the Hanalei River in north Kauai forced the closure of Kuhio Highway at the Hanalei Bridge for several hours during the evening of December 9. A line of intense rainfall then moved across Oahu and Maui County but fortunately moved fast enough to prevent significant flooding. Some of the thunderstorm cells within the line were strong enough to prompt the issuance of severe thunderstorm and special marine warnings.
A week of mild weather conditions settled over the state from December 12 through 18 as a large kona low developed near the Dateline far west of the main Hawaiian Islands. As the low pressure system evolved, it also shifted eastward and pulled deep tropical moisture over the state and created an unstable airmass. Prolonged moderate to occasionally heavy rainfall covered Oahu on December 19 with widespread 3 to 6 inch totals and a pocket of 8 to 10 inch totals along the east side of the island. Although streams remained within their banks, there were several reports of road damage due to significant runoff and urban flooding from poor or slow drainage issues.
Very moist and unstable conditions continued through December 27 with daily heavy rainfall events in various locations across the state. While most of these events produced just minor flooding problems, there were several cases where flash flooding resulted in property damage or road closures. On December 22, heavy rains during the afternoon produced flash flooding over the upper slopes of the South Kohala District. Flooding along Kamakoa Gulch forced emergency management officials to close Highway 190 ( Hawaii Belt Road) from the Saddle Road Junction to Waimea, and farther downstream at Puako Beach Drive. Two days later on Christmas Eve, early evening thunderstorms over the Kaneohe area of Oahu dropped 1 to 3 inches of rain which contributed to a raw sewage spill into Kaneohe Stream.
From Christmas Day through December 27, a strong upper level trough and an associated cold front developed west of the island chain. While the cold front remained west of Kauai, areas of heavy rainfall within the moderate to fresh southerly low level wind flow generated flash flooding on Kauai, Oahu, and Maui. The Maui flooding occurred late on December 25 when over 4 inches of rain over the Upcountry slopes flooded normally dry gulches along South Kihei Road. A combination of blocked culverts and high tide appeared to have enhanced the flooding problems. During the early morning hours of December 26, over 3 inches of rainfall in 6-hours affected leeward Oahu which caused flooding and road closures in the Waianae and Makaha areas. The final rainfall surge of the month occurred on December 27 when a band of heavy rainfall affected Kauai and Oahu. In the early morning hours, Hanalei River in north Kauai overflowed and forced the closing of Kuhio Highway at the Hanalei Bridge for several hours. During the early evening hours, the rain band reached west Oahu which resulted in the flooding of a freeway off-ramp near Kapolei.
The rainfall in December helped ease prolonged drought conditions across the state. For more information on the drought, please refer to the latest Drought Information Statement at: http://www.weather.gov/data/HFO/DGTHFO
All of the rain gages across Kauai reported near to well above normal totals for the month of December. The U.S. Geological Survey's Mount Waialeale gage recorded the state's highest monthly total of 40.99 inches (90 percent of normal). However, the highest daily total came from the Kapahi gage which recorded 6.30 inches during the flash flood event on December 9. This total included a maximum 15-minute accumulation of 1.03 inches and 3.30 inches recorded from 4:45 PM to 6:45 PM.
Despite the very wet conditions in December, most of the rainfall totals for 2010 across Kauai remained below normal. Rainfall late in the year was not enough to overcome deficits accumulated during earlier drought conditions. The Mount Waialeale total of 287.74 inches (68 percent of normal) was the highest 2010 total in the state but was the second lowest annual total on record for this location. The driest year at Mount Waialeale was 1993 with an annual total of 244.36 inches. Over the past several years, Mount Waialeale's running 30-year average annual rainfall total has been decreasing almost steadily, from 406 inches in 1997 to just below 384 inches in 2010. Most of the remaining gages on the island ended the year with totals in the range of 50 to 75 percent of normal.
All of the rain gages on Oahu recorded near to well above normal rainfall for the month of December. Six gages had monthly totals over 20 inches with the highest amount of 22.12 inches (451 percent of normal) posted by the Palehua gage. The highest daily total was 10.04 inches at the Maunawili gage from the heavy rain event on December 19. For several sites, especially over west and central Oahu, it was the wettest month since December 2008. The 11.73 inches recorded at Honolulu Airport marked the third wettest December on record at this location, surpassed only by 12.09 inches in 1955 and 17.29 inches in 1987.
Many of the gages on Oahu ended up with below normal rainfall totals for 2010 despite wetter than normal conditions in December. The U.S. Fish and Wildlife Service's Oahu Forest National Wildlife Refuge gage logged the highest rainfall total on the island with 179.61 inches (88 percent of normal). This was the fourth highest annual total statewide. Most of the remaining gages were in the range of 50 to 80 percent of normal.
Most of the gages in Maui County received near to above normal rainfall totals for the month of December. The USGS' Oheo Gulch gage had the highest monthly total of 14.17 inches (163 percent of normal) and the highest daily total of 7.99 inches on December 21. Normally the wettest sites, both Puu Kukui and West Wailuaiki recorded well below normal totals of 10.29 inches and 10.41 inches, respectively. These low totals were likely due to the lack of trade winds during most of the month.
Rainfall totals for 2010 were below normal at most of the gages across Maui County and largely within the range of 40 to 70 percent of normal. The USGS' Puu Kukui gage posted the highest total for 2010 with 234.57 inches (61 percent of normal). This was the second highest annual total in the island chain.
Windward gages recorded below normal rainfall totals for the month of December but most of the leeward gages had near to well above normal totals. The drier than normal windward conditions were due to the lack of trades during the month coupled with most of the significant weather systems weakening before reaching the east side of the Big Island. Mountain View's 11.18 inches (69 percent of normal) was the highest monthly total for Hawaii County. However, the highest daily total was from the Pohakuloa Training Area (PTA) Keamuku gage which registered 4.56 inches during the flash flood event on December 22.
Most of the Big Island gages finished 2010 with below normal annual rainfall totals. The USGS' rain gage at Kawainui Stream had the highest annual total of 163.84 inches (99 percent of normal). Windward totals were mainly in the range of 40 to 70 percent of normal while most of the leeward totals were below 50 percent of normal. Hilo Airport's annual total of 63.29 inches set a new record for the driest year. The previous record was 68.09 inches in 1983. Annual totals for Hilo Airport go back to 1950 with an average of more than 125 inches. Kapapala Ranch, which averages over 60 inches per year, had its second driest annual total of 19.09 inches. The record driest year was 1998 with 13.42 inches.
Data Sources: Data used in this report are largely from National Weather Service sources including climate network weather observation stations at Lďhue, 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.
Senior Service Hydrologist
NOAA/NWS Weather Forecast Office Honolulu