State of Hawaii

MONTH: October 2009

PREPARED: November 4, 2009

State: [Text data table for rain gages]

After persisting through most of September, the trade winds were absent over the State of Hawaii on more than half of the days in October. On October 4 and 5, an upper level trough combined with moist low level southeasterly flow brought unstable conditions and occasional heavy rainfall to the west half of the island chain. Several locations over windward Oahu and portions of east Kauai received 1 to 2 inches of rainfall with isolated 3 to 5 inch totals. The heavy rains prompted the issuance of flash flood warnings for both islands but fortunately there were no reports of significant impacts or injuries. Following this heavy rain episode, a weak shear line moved across the state on October 10 and 11 and dropped 1 to 3 inches of rain with isolated higher amounts along the windward slopes.

During mid-October, a large area of low pressure in the north Pacific pushed the subtropical ridge southward to a position near the Hawaiian Islands. This resulted in light winds regionally with land and sea breezes dominating wind circulations locally and low rainfall totals overall. Moderate to occasionally fresh trades returned on October 17 and lasted until October 20. The trade wind-generating subtropical high pressure system subsequently shifted northeastward putting the state within an area of large scale southeast winds near the surface. In addition to the vog from Kilauea volcano and an increase in humidity accompanying the shift in the winds, a brief but very heavy downpour on October 25 produced flash flooding along the windward coast of Oahu. The flooding was most significant at Waikane Stream where it overflowed and forced the closure of Kamehameha Highway for several hours. A few properties near the stream also received some flood damage. Also occurring during this period was the formation of Tropical Depression 03-C southeast of the Big Island on October 19. This tropical cyclone eventually developed into Hurricane Neki and turned northward toward the Papahanaumokuakea Marine National Monument. Neki crossed the monument near French Frigate Shoals on October 23 and remained sufficiently distant from the main Hawaiian Islands to not cause any significant impacts other than some high clouds.

Low level winds remained from the south through southeast until October 29 when a weak cold front, the first of the 2009-2010 wet season, reached Kauai and deposited up to an inch of rain. The last 2 days of October brought moderate trade winds back to the state, much to the relief of local residents.

Island of Kauai : [October 2009 map] [Year-to-date map]

Most of the rain gages across the island of Kauai recorded below normal rainfall during the month of October. A few sites along the north- and east-facing slopes of the island, such as Wainiha and Wailua, posted above normal totals largely due to significant rainfall on October 4 (upper level trough) and October 10 (shear line). One of the heavy showers affecting Wailua dropped 1.19 inches in a 15-minute period just after midnight on October 5. The U.S. Geological Survey’s (USGS) gage on the summit of Mount Waialeale recorded 20.91 inches (69 percent of normal), which was the highest monthly total on the island and second highest in the state. This gage also recorded the highest daily total of 4.92 inches during the October 4 heavy rain event. Most of the leeward sites indicated totals at less than 50 percent of normal.

Most of the available gages on Kauai have received below normal rainfall for 2009 through the end of October. Mount Waialeale’s 244.06 inches (71 percent of normal) continued to lead all statewide totals by a wide margin.

Island of Oahu: [October 2009 map] [Year-to-date map]

After a very dry September, most of the gages on the windward side of Oahu recorded above normal rainfall during October. The U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge (NWR) gage measured 27.29 inches during the month (161 percent of normal), which beat Kauai’s Mount Waialeale for wettest spot statewide. This site also received the highest daily total of 6.48 inches during the shear line passage on October 11. Leeward gages mostly recorded below normal totals including several sites with less than 50 percent of normal rainfall.

Most of the gage totals on Oahu remained in the below normal range for 2009 through the end of October. The main exceptions were from the windward Koolau area from Hakipuu to Waimanalo which have received near to above normal amounts of rainfall so far this year. The Oahu Forest NWR total of 166.12 inches (98 percent of normal) was the highest on the island and fourth highest statewide.

Maui County: [Maui October 2009 map] [Year-to-date map] [Molokai/Lanai October 2009 map] [Year-to-date map]

It was “feast or famine” rainfall-wise once again across Maui County with central and Upcountry Maui showing less than 10 percent of normal rainfall and some spots in east Haleakala at greater than 100 percent of normal. The USGS gage at West Wailuaiki recorded the highest monthly total of 18.40 inches (120 percent of normal) while the Puu Kukui gage recorded the highest daily total of 7.62 inches on October 3. At Kula Branch Station in Upcountry Maui, the 0.20 inches recorded in the past two months marked the driest September-October on record since data collection began in 1978.

Most of the Maui County gages have recorded below normal rain totals for 2009 through the end of October. The West Wailuaiki gage had the highest county-wide total of 176.05 inches (106 percent of normal) among the available gages. The remaining totals were mainly in the 50 to 80 percent of normal range.

Island of Hawaii: [October 2009 map] [Year-to-date map]

Most of the windward Big Island gages reported near to above normal rainfall during October while leeward gages once again indicated below normal totals. The Pahoa gage recorded the highest monthly total of 12.77 inches (111 percent of normal). It is a bit unusual for Pahoa to record the highest monthly total since higher elevation sites such as Mountain View or Glenwood usually receive more rainfall. Most of the leeward gages recorded less than 50 percent of normal rainfall as drought conditions continue to affect these areas.

Most of the windward Big Island gages showed near to above normal totals for 2009 through the end of October. Leeward gages remained mostly in the range of 40 to 70 percent of normal. Glenwood’s 130.87 inches (101 percent of normal) led all Big Island totals and was fifth highest statewide.

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