May 2017 Precipitation Summary

Monthly Precipitation Summary

State of Hawaii

Month: May 2017

Prepared: June 14, 2017

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

State: [Text data table for rain gages]

Lingering wet conditions from a dissipating late season kona low marked the start of the Hawaiian Islands dry season, which runs from May through September. The enhanced rainfall produced minor flooding problems on the Big Island and Maui on May 1. By May 2, the low pressure system dissipated southwest of Kauai and was replaced by moderate to fresh trade winds that persisted through May 7.

On May 8, a new kona low developed several hundred miles northwest of Kauai which caused low level winds to veer to a southeasterly direction. The local air mass became sufficiently unstable to support enhanced showers along the windward slopes of the Koolau Range that resulted in minor flooding problems. As the low drifted westward and dissipated, trade winds resumed by May 10 at mainly moderate to fresh intensities and continued unabated through May 25. During this period, an upper level trough near the state helped enhance shower activity embedded within the trade wind flow. These showers produced daily totals of 1 to 4 inches along the windward slopes of the Big Island on May 14 and 15 which resulted in soggy conditions but otherwise no significant flooding issues. A late season cold front moved north of the main Hawaiian Islands on May 26 and produced a light wind pattern that allowed land and sea breezes to dominate local conditions. Briefly heavy afternoon showers affected the Hilo and Puna areas of the Big Island on May 28, and the Upcountry area of Maui on May 29, but did not cause significant flooding problems. Trade winds resumed on May 30 and continued through the end of the month.

Island of Kauai : [May 2017 map] [year-to-date map]

Most of the rainfall totals for the month of May across Kauai were in the near to above average range. The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 33.70 inches (109 percent of average) which made it the wettest month of the year so far despite being outside of the usual wet season. This location also recorded more than an inch of rain on 10 days in May including the island’s highest daily total of 5.78 inches on May 12.

Over half of the rain gages on Kauai had near average rainfall totals for 2017 through the end of May. Mount Waialeale had the highest year-to-date total of 105.97 inches (68 percent of average).

Island of Oahu: [May 2017 map] [year-to-date map]

May rainfall totals on Oahu were mostly in the near to below average range with an inconsistent distribution pattern. For example, the Waianae Kawiwi gage recorded a monthly total of 1.42 inches which was 139 percent of average. In contrast, Makua Range a few miles northwest and Kalaeloa Airport to the southeast recorded totals that checked in at only 21 and 55 percent of average, respectively. The Manoa Lyon Arboretum gage had the highest monthly total of 11.53 inches (114 percent of average). The USGS’ Poamoho No. 1 Gage had the highest daily total of 3.51 inches on May 8. The Kahuku gage had its lowest May rainfall total since 2009.

Rainfall totals for 2017 through the end of May were in the near average range at most of the gages across Oahu. The USGS’ Poamoho No. 1 Gage had the highest year-to-date total of 67.66 inches (72 percent of average).

Maui County: [Maui May 2017 map] [year-to-date map] [Molokai/Lanai May 2017 map] [year-to-date map]

Below average totals were posted by most of the gages across Maui County. Totals from the windward sites were significantly below average despite the presence of trade wind conditions during most of the month. The USGS’ gage on top of Puu Kukui had the highest monthly total of 14.49 inches but this was just 52 percent of the long term May average. This site also recorded the highest daily total of 3.37 inches on May 14. The USGS’ gage at West Wailuaiki Stream logged its lowest May total since 2009.

Most of the rainfall totals for 2017 through the end of May were in the near to above average range across Maui County. The USGS’ gage at West Wailuaiki Stream had the highest year-to-date total of 72.51 inches (67 percent of average).

Island of Hawaii: [May 2017 map] [year-to-date map]

May rainfall totals were in the near to below average range at most of the Big Island gages. The highest monthly total of 13.25 inches (91 percent of average) was from the Waiakea Uka gage. The highest daily total was 3.61 inches from the Waiakea Experiment Station on May 15. The Kamuela gage posted its lowest May total (0.31 inches, 8 percent of average) in a data record going back to 1991.

Below average rainfall totals for 2017 through the end of May have been recorded by most of the gages on the Big Island. The USGS’ gage at Kawainui Stream had the highest available year-to-date total of 55.83 inches (82 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