State of Hawaii

MONTH: April 2009

PREPARED: May 6, 2009

State: [Text data table for rain gages]

Late season cold front passages and the frequent presence of upper level low pressure troughs defined the April weather pattern over and near the Hawaiian Islands. However, the presence of these weather features did not translate to high monthly rainfall amounts as many locations throughout the State of Hawaii received near to below normal totals.

The first half of the month mainly included trade winds in the moderate to fresh range with daily rainfall recorded along the windward slopes of the islands. An upper level trough brought enhanced shower activity to the western half of the state on April 9 through 11 and induced a surface trough near Kauai. Heavy showers over the windward slopes of Oahu on April 10 produced minor flooding with gages recording 1- to 3-inch 24-hour totals.

A weather pattern shift in the central North Pacific during the second half of April brought conditions more like winter than spring to the Hawaiian Islands. This included two cold fronts (April 20 through 21 and April 28), snow over the summits of Mauna Kea and Mauna Loa, and weak to non-existent trade winds. The April 20 through 21 cold front produced 2 to 5 inches of rainfall in various areas of the state and minor flooding problems on Kauai, Oahu, Molokai, and Maui. Remnant moisture from the front returned to the windward slopes of Maui and the Big Island in weak trade wind flow on April 23 and 24. Unstable conditions from a passing upper level short wave helped boost rainfall totals with more than 8 inches in 24-hours recorded at West Wailuaiki in east Maui. No significant flooding problems were associated with this rainfall event. The April 28 cold front was very weak and produced little rainfall before stalling near Oahu.

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

Most of the gages on Kauai recorded below normal rainfall during the month of April. The U.S. Geological Survey (USGS) gage on Mount Waialeale posted the highest monthly total of 22.54 inches (57 percent of normal) and the highest daily total of 3.00 inches on April 9 in association with an upper level trough. The Mount Waialeale total usually leads all totals statewide but its April total was surpassed by West Wailuaiki’s (east Maui) 30.40 inches.

Most of the gage totals for Kauai remained in the below normal range for 2009 through the end of April. The 114.75 inches recorded at Mount Waialeale (78 percent of normal) far exceeded all other year-to-date totals statewide. Most of the remaining gages on Kauai had totals in the 40 to 70 percent of normal range.

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

Rain gages on the windward slopes of the Koolau Range recorded near to above normal totals during April. Most of remaining Oahu gages recorded below normal totals with many posting amounts at less than 50 percent of normal. As usual, the U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge (NWR) gage posted the highest monthly total (19.75 inches, 95 percent of normal). The Waimanalo and nearby Olomana Fire Station gages received the highest daily totals of 4.76 and 4.62 inches, respectively, on April 20 during the passage of a cold front. These totals also accounted for more than half of each site’s April rainfall accumulation.

Most of the gages on Oahu continued to show below normal rain totals for 2009 through the end of April. The Oahu Forest NWR gage led all totals island-wide at 73.51 inches (101 percent of normal) and moved up to fifth highest in the state. Most of the remaining gages had totals in the range of 40 to 70 percent of normal.

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

April rain totals across Maui County exhibited the usual wide range of conditions from 8 percent of normal at Lahainaluna to 251 percent of normal at Kahakuloa on the opposite side of the West Maui Mountains. The USGS gage at West Wailuaiki in east Maui recorded the state’s highest April total of 30.40 inches and the highest daily total of 8.06 inches on April 23. While significant, the April 23 rainfall at West Wailuaiki was spread out over several hours with most hourly totals at less than one inch. On Molokai and Lanai, dry conditions prevailed overall and most of the gages recorded monthly totals of an inch or less.

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

Most of the gages in the North Hilo, South Hilo, and Puna Districts indicated near normal totals for the month of April. Most of the remaining gages on the Big Island recorded below normal totals, many of which were below 50 percent of normal. The Laupahoehoe gage in the North Hilo District recorded the highest Big Island total of 17.85 inches (94 percent of normal) which was somewhat unusual since it’s usually a South Hilo or Puna District gage that records the highest total of the month. From April 21 through 25, the Laupahoehoe gage recorded 12.43 inches of rain during the April 21 cold front passage and as the front’s remnant moisture returned to the windward slopes in light trade winds beneath an upper level trough.

Windward Big Island gages continued to show near to above normal totals for 2009 through the end of April. Most of the leeward gages had below normal totals in the range of 40 to 70 percent of normal. The Piihonua gage continued to lead all Big Island totals with 74.41 inches (104 percent of normal) but dropped to fourth 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