National Weather Service United States Department of Commerce

Monthly Precipitation Summary

State of Hawaii

Month: January 2018

Prepared: February 7, 2018

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

State: [Text data table for rain gages]

The new year started with generally dry conditions within a pattern of moderate to fresh trade winds. On January 12, a large low pressure system in the North Pacific pushed the ridge of high pressure over the state from its normal position well north of the islands. This resulted in a light wind pattern over the main Hawaiian Islands through January 15 with land and sea breezes dominating local wind conditions and very little rainfall. Trade winds resumed on January 16 and became rather strong for a couple of days starting on January 18. An upper level trough of low pressure moved over the state on January 21 which destabilized the underlying airmass moving in moderate to fresh trade winds. This helped produce enhanced showers, mainly along the windward slopes of the state through January 27. The highest rainfall totals occurred along the slopes of the North Hilo, South Hilo, and Puna Districts with the U.S. Geological Survey’s (USGS) Saddle Road Quarry gage logging 13.91 inches from January 20 through 22, and 27.50 inches from January 18 through 26. Aside from minor flooding on some flood prone roads there were no reports of significant property damage. On January 26, two visiting hikers were swept away by rising water levels in Wailuku River. One was rescued but the other has not been found.

The last couple of days in the month had the island chain well within the westerlies and trade winds shoved south of the state. A dynamic, unstable weather pattern prevailed with heavy rainfall and other forms of severe weather which continued into early February. On January 31, a weak cold front stalled near Oahu and served as the focal region for strong thunderstorm development. These thunderstorms contained intense rainfall but were moving rapidly which helped limit rainfall accumulations and mitigated significant flooding problems.

Island of Kauai : [January 2018 map]

Very dry conditions during the first half of January resulted in below average monthly totals at most of the rain gages on Kauai. Wetter conditions during the second half of the month were not enough to overcome the initial rainfall deficits and more than half of the gages ended up with monthly totals below 50 percent of the January average. The USGS’ gage on Mount Waialeale had the highest monthly total of 25.17 inches (102 percent of average) and the highest daily total of 6.46 inches on January 24. The gages at Anahola and Kokee posted their lowest January totals since 1995 and 2001, respectively.

Island of Oahu: [January 2018 map]

Like Kauai, dry conditions on Oahu quickly built up significant rainfall deficits during the first half of the month. Rainfall during the second half of the month provided some recovery but was not enough to prevent more than half of the sites ending the month with rainfall totals at less than 50 percent of average. The USGS’ Poamoho Rain Gage No. 1 had the highest monthly total of 11.54 inches (63 percent of average). The highest daily total was 3.65 inches at Manoa Lyon Arboretum on January 20.

Maui County: [Maui January 2018 map] [Molokai/Lanai January 2018 map]

Most of the gages across Maui County reported below average totals for the month of January with several totals at less than 10 percent of average. The USGS’ gage on Puu Kukui had the highest monthly total of 16.76 inches (54 percent of average) and the highest daily total of 5.52 inches on January 23. Ulupalakua Ranch had its lowest January total since 1945. Significant, though less extreme, Waikapu Country Club and Kahului Airport had their lowest January totals since 1993 and 2001, respectively.

Island of Hawaii: [January 2018 map]

While many sites across the state were experiencing dry conditions, the windward slopes of the Big Island were drenched during the second half of the month. Near to above average rainfall totals were recorded from Laupahoehoe in the North Hilo District to Keaumo in the Puna District. The USGS’ Saddle Road Quarry gage had the highest monthly total of 30.92 inches (296 percent of average) and the highest daily total of 5.74 inches on January 21. Hilo Airport and Pahoa had their highest January totals since 2002, and Mountain View has not recorded this much rain since 2004. The rest of the Big Island paints a different picture with nearly all remaining sites posting below average monthly totals. Driest conditions were in the North and South Kona Districts where all of the gages posted totals at less than 20 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