May 2016 Precipitation Summary

State of Hawaii

MONTH: May 2016

PREPARED: June 6, 2016

Note:  This summary uses the arithmetic mean, or average, for “normal” rainfall values.

State: [Text data table for rain gages]

May usually signals the start of the warm/dry season in the Hawaiian Islands. However, the weather pattern that arrived appeared to be more like what one would expect during the early wet season with a middle and upper level low pressure system producing several days of afternoon heavy showers and two cold frontal passages. Although weak, the two cold fronts brought enhanced rainfall to many of the north- and northeast-facing slopes across the island chain. The first front reached Kauai on May 6 and produced rainfall accumulations in the range of 4 to 8 inches over north and northwest Kauai. As often happens in this type of scenario, Hanakapiai Stream became impassable which stranded 18 hikers on the Na Pali Coast trail. The front moved slowly eastward and stalled over east Maui on May 8 and 9. The enhanced moisture within the frontal cloud band resulted in a prolonged exposure to waves of showers along the windward slopes of Haleakala, especially from Keanae to Hana. Flooding briefly closed Ulaino Road and Waikoloa Road near Hana, and rock slides affected portions of the Hana Highway on May 8. The second cold front, also weak, slowly reached Kauai on May 30 and dissipated near Maui on May 31. This front was accompanied by more stable conditions and produced just 1 to 2 inches of rain mainly over the windward slopes and no flooding problems.

Between the cold front passages the state was affected by a low pressure system which brought unstable conditions from May 24 through May 27. The unstable air mass enhanced the showers which became occasionally intense and caused minor flooding in localized spots in all 4 counties. Hanalei River on the island of Kauai almost overflowed its banks at Kuhio Highway near the Hanalei Bridge during the early morning hours of May 27 but subsided before causing any problems.

The wet conditions of May helped reduce the intensity and areal coverage of the El Nino-induced drought in several areas of the state. It turns out that an abundance of rainfall also occurred during May 1998 following the last strong El Nino which affected the Pacific. Additional details on drought conditions can be found on the U.S. Drought Monitor website at http://droughtmonitor.unl.edu and in the Drought Information Statement issued by the Weather Forecast Office Honolulu at http://www.weather.gov/data/HFO/DGTHFO.

Island of Kauai : [May 2016 map] [year-to-date map]

Most of the gages on the island of Kauai recorded near to above average rainfall totals for the month of May. The U.S. Geological Survey’s (USGS) rain gage on top of Mount Waialeale had the highest monthly total of 38.90 inches (126 percent of average). However, the highest daily total of 7.15 inches was posted by the Wainiha rain gage from the cold frontal passage on May 6. The Kokee rain gage had a monthly total of 8.95 inches (274 percent of average) which registered as the wettest May since 2002. About two-thirds of this monthly total fell on May 6.

Despite the recent wet conditions, rainfall totals for 2016 through the end of May were below average at most of the rain gages across Kauai. Mount Waialeale had the highest year-to-date total of 110.49 inches (71 percent of average). Most of the other sites had year-to-date totals in the range of 40 to 70 percent of average.

Island of Oahu: [May 2016 map] [year-to-date map]

Above average monthly rainfall totals were posted by most of the rain gages on Oahu for the month of May. The USGS’ Poamoho No. 1 rain gage had the highest monthly total of 25.52 inches (147 percent of average) and the highest daily total of 6.19 inches on May 25. The Hawaii Kai Golf Course rain gage recorded 3.78 inches (245 percent of average) which made it the wettest May in a 25-year data record. Olomana Fire Station (5.50 inches, 204 percent of average) posted its highest May rainfall total since 1997.

Most of the gages on Oahu had below average rainfall totals for 2016 through the end of May with many still at less than 50 percent of average. The Poamoho No. 1 gage had the highest year-to-date total of 60.70 inches (64 percent of average).

Maui County: [Maui May 2016 map] [year-to-date map] [Molokai/Lanai May 2016 map] [year-to-date map]

Monthly rainfall totals for the month of May were above average at most of the rain gages across Maui County. The USGS’ West Wailuaiki site recorded the highest monthly total of 58.14 inches (311 percent of average). This site also had the highest daily total of 12.64 inches on May 9 and logged a 2-day total of 24.55 inches for May 8 and 9 while the island was affected by the above-mentioned stalled cold front shower band. Rainfall records for the wettest May were broken at Kahakuloa, Pukalani, and Waikapu Country Club. In addition, Ulupalakua Ranch posted its highest May total since 1965, and Kahului Airport had its wettest May since 1987.

In spite of the unusually wet conditions during May, most of the rainfall totals for 2016 through the end of May remained in the below average range due to the sizeable deficits that accumulated early in the year. The West Wailuaiki year-to-date total of 136.37 inches (127 percent of average) was easily the highest in the state.

Island of Hawaii: [May 2016 map] [year-to-date map]

Most of the rainfall totals on the Big Island were in the near to above average range for the month of May. The USGS’ Saddle Road Quarry gage above Hilo posted the highest monthly total of 21.37 inches (214 percent of average). The highest daily total was from the USGS’ rain gage at Kawainui Stream which recorded 3.60 inches on May 8. A record-breaking May total of 10.51 inches (220 percent of average) was recorded by the Honaunau gage and the Kamuela 1 gage logged its highest May total since 1998.

Rainfall totals for 2016 through the end of May remained in the below average range at most of the rain gages on the Big Island. More than half of the total number of sites were at less than 50 percent of average. The Saddle Road Quarry gage had the highest year-to-date total of 92.57 inches (151 percent of average).

Data Sources: Data used in this report are largely from National Weather Service sources including climate network weather observation stations at Lihue, 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 Hawaii Department of Land and Natural Resources, the U.S. Geological Survey, the U.S. Bureau of Land Management, the U.S. National Park Service, the Department of Defense, the U.S. Fish and Wildlife Service, and the U.S. Department of Agriculture.  Averages come from the National Climatic Data Center (1981-2010 series) and the Rainfall Atlas of Hawaii (http://rainfall.geography.hawaii.edu/).  Data presented here are not certified and should be used for informational purposes only.

Kevin R. Kodama
Senior Service Hydrologist
NOAA/NWS Weather Forecast Office Honolulu