State of Hawaii

MONTH: May 2009

PREPARED: June 4, 2009

State: [Text data table for rain gages]

The word “unusual” can be used to describe the weather pattern over the central North Pacific during May and especially during the latter half of the month, which was more winter-like in appearance. However, this did not translate to more rain over the Hawaiian Islands as most of the rain gages turned in below normal totals thus kicking the dry season into high gear early. Trade winds made a significant appearance on only 20 percent of the days in May which is even below the average for December (about 50 percent). The statewide rain pattern was so unusual the Kau Desert on the Big Island received more rainfall than Mount Waialeale on Kauai, normally one of the wettest spots on Earth. See individual county sections below for more details.

From May 13 through the end of the month, the trades did appear at all as an unusually persistent area of low pressure maintained a position north to northeast of the state but far enough away to prevent the occurrence of widespread heavy rainfall over the island chain. This period even featured an unusually late cold front passage on May 17 and 18. The front produced 1 to 3 inches of rain over windward Oahu and lesser amounts over the rest of the state. A surface trough of low pressure lingering near Kauai and Oahu helped trigger isolated heavy showers on both islands from May 26 through 28 but no significant flooding issues.

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

Most of the gages on Kauai turned in well below normal rainfall totals during the month of May. Normally the wettest spot in the state and one of the wettest in the world, the U.S. Geological Survey (USGS) gage on Mount Waialeale showed that in May 2009 it wasn’t even the wettest spot on the island. The 1.51 inches (4 percent of normal) recorded unofficially marked the driest May on record at this site, easily surpassed by the 5.94 inches at the Pali 2 gage in the Kau Desert on the Big Island. The gage at Anahola posted the highest monthly total on Kauai, 6.44 inches (201 percent of normal), of which 3.21 inches occurred in 1-hour on May 26. The 0.29 inches at Lihue Airport marked the driest May at this location since 0.10 inches in 2000, the current record.

Most of the gages on Kauai showed below normal totals for 2009 through the end of May with most in the 40 to 60 percent of normal range. Despite the record dry month, the 116.26 inches recorded at Mount Waialeale remained the highest year-to-date total in the state by a large margin.

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

Most of the gage totals from the island of Oahu indicated below normal rainfall with the exception of east Oahu which received above normal rainfall largely due to the frontal passage on May 17. The USGS’ Poamoho No. 2 gage recorded Oahu’s highest total of 5.63 inches but this registered at only 27 percent of normal. The Kamehame gage had the highest percent of normal value at 319 percent (4.46 inches). Kamehame also had the highest daily total of 2.75 inches on May 17 during the previously mentioned frontal passage.

Most of the Oahu rain gages continued to indicate below normal totals for 2009 through the end of May. The U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge (NWR) gage led all totals island-wide at 78.23 inches (87 percent of normal) and moved up to fourth in the state. Most of the remaining totals were in the range of 40 to 70 percent of normal.

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

Most of the gages across Maui County recorded below normal totals with more than half of the available sites below 50 percent of normal. The USGS gage at West Wailuaiki posted the highest monthly total of 3.39 inches though this was only 19 percent of normal. Puu Kukui, normally the second wettest spot in the state, showed only 1.98 inches (6 percent of normal). Kahului Airport’s 0.01 inches marked the driest May since the 0.00 inch total recorded in 2003.

Most of the Maui County gage totals remained in the below normal range for 2009 through the end of May. The 95.02 inches recorded at West Wailuaiki (102 percent of normal) continued to lead all totals county wide and was ranked second in the state.

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

Similar to the rest of the state, the Big Island totals largely painted a drier than normal picture with only isolated above normal totals. Considered to be an unusual occurrence, the Pali 2 gage recorded the highest monthly total of 5.94 inches (540 percent of normal). This is also noteworthy since the Pali 2 gage is located in an area called the Kau Desert and because the total surpassed the combined totals from Mount Waialeale and Puu Kukui, normally the two wettest spots in the state.

Windward Big Island gages continued to show near to above normal totals for 2009 through the end of May due to high rainfall totals early in the year. Most of the leeward gages had below normal totals in the range of 40 to 70 percent of normal. The Piihonua gage maintained its lead among all Big Island totals with 76.48 inches (88 percent of normal) but slipped to fifth 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