National Weather Service United States Department of Commerce

Monthly Precipitation Summary

State of Hawaii

Month: May 2018

Prepared: June 4, 2018

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

State: [Text data table for rain gages]

Following an eventful end to the October-April wet season, the May-September dry season started with a return to calmer weather conditions. There was a threat of significant heavy rainfall over the west half of the state in early May due to the presence of an upper atmospheric trough west of the main Hawaiian Islands. However, heavy rainfall only occurred briefly over Maui and Molokai on May 3 which produced minor flooding but no significant damage. A weak out-of-season cold front moved across the island chain on May 5 and its remnant moisture contributed to enhanced rainfall over the windward slopes on May 6 through May 8. The windward side of the Big Island picked up 3 to 7 inches of rainfall on May 7 and May 8 but there were no reports of significant damage. The windward slopes of the rest of the state received around 2 to 5 inches during this period.

The remainder of May included mostly moderate to fresh trade winds. This meant that most of the rainfall occurred over the northeast-facing windward slopes while many leeward areas remained relatively dry. Brief disruptions in the trades occurred on May 10 through 11, May 16, and May 28. The weaker large scale winds on these days allowed land and sea breezes to dominate local conditions. Along the windward slopes of the Big Island, yet another month of abundant rainfall has taken a toll on the portions of the agriculture sector. Reports from the U.S. Department of Agriculture indicated that farmers growing root crops such as sweet potato and ginger have suffered losses due to rotting from the excessive moisture. This has mainly occurred north of Hilo from Pepeekeo to Ninole.

Island of Kauai : [May 2018 map] [year-to-date map]

May rainfall totals were near to above average at many of the gages on Kauai. The U.S. Geological Survey’s (USGS) gage on Mount Waialeale recorded the highest monthly total of 26.78 inches (87 percent of average) and the highest daily total of 4.57 inches on May 18. Lower elevation leeward gages had monthly totals of 1 to 2 inches. While not excessive in the big picture, these amounts were above average for May in leeward Kauai.

Rainfall totals for 2018 through the end of May were above average at most of the gages on Kauai. Mount Waialeale had the highest year-to-date total of 233.80 inches (150 percent of average).

Island of Oahu: [May 2018 map] [year-to-date map]

Gages along the windward and interior sections of the Koolau Range had mostly near to above average rainfall totals for May. Most of the remaining gages on the island had near to below average amounts. Gages along the leeward slopes of the Waianae Range were especially dry with most totals below 20 percent of average. The Manoa Lyon Arboretum gage had the highest monthly total of 14.95 inches (148 percent of average) and the highest daily total of 3.35 inches on May 14. The arboretum’s total also registered as the highest May total at this site since 2002.

Most of the Oahu rainfall totals for 2018 through the end of May were above average. A few of the gages along the lower leeward slopes of the Waianae Range had below average totals. The USGS’ Poamoho Rain Gage No. 1 had the highest year-to-date total of 118.17 inches (125 percent of average).

Maui County: [Maui May 2018 map] [year-to-date map] [Molokai/Lanai May 2018 map] [year-to-date map]

Windward totals for May were mostly above average and leeward totals mostly below average across Maui County. The USGS’ rain gage at West Wailuaiki Stream had the highest monthly total of 29.68 inches (159 percent of average) and the highest daily total of 4.31 inches on May 7. Haiku’s 5.72 inches marked the highest May total since 1994.

Maui County rainfall totals for 2018 through the end of May were mostly near to above average. The West Wailuaiki gage had the highest year-to-date total of 140.24 inches (130 percent of average).

Island of Hawaii: [May 2018 map] [year-to-date map]

The windward slopes of the Big Island were quite wet with most of the gages posting above average May rainfall totals. In contrast, leeward areas were drier than average overall, especially in the North Kona District where several gages had monthly totals at less than 10 percent of average. The USGS’ Saddle Road Quarry gage had the highest monthly total of 31.10 inches (312 percent of average) while the highest daily total of 3.86 inches came from the Papaikou Well gage on May 7. Honokaa had its highest May total in a data record going back to 1991. Hilo Airport, Kamuela Upper, Laupahoehoe, Mountain View, Pahoa, and Piihonua had their highest May totals since 2006. On the leeward side, the Kealakekua gage logged its lowest May total since 2004.

Big Island rainfall totals were mostly near to above average for 2018 through the end of May. Isolated below average totals were located across various parts of the island. The Saddle Road Quarry gage had the highest year-to-date total of 178.72 inches (292 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