National Weather Service United States Department of Commerce

Monthly Precipitation Summary

State of Hawaii

Month: December 2017

Prepared: January 9, 2018

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

State: [Text data table for rain gages]

Just like November, the month of December started with a continuation of a heavy rain event from the previous month. In this case it was an upper level trough tapping into a deep layer of tropical moisture which started generating heavy rain on November 27 and continued into December 3. The December portion of the event produced some daily totals in the 2 to 4 inch range, mainly on the windward slopes of the Big Island, but no significant flooding problems. Please refer to the November 2017 summary for more details on this heavy rain event (http://www.prh.noaa.gov/hnl/hydro/pages/nov17sum.php ).

Weather conditions stabilized on December 4 as the upper level trough weakened and moved away from the state. This also marked a shift in the regional weather pattern with very few trade wind days for the remainder of the month. A series of weak cold fronts moved across the main Hawaiian Islands on December 5, 9-10, and 12-13. In all three cases there were a few locations that received 1 to 2 inches of rainfall but most areas remained rather dry.

On December 19 a stronger cold front reached Kauai and Oahu and deposited 1 to 2 inches of rainfall. The cold front rain band became better aligned with significant upper level support on December 20 which increased the intensity and coverage of rainfall as it moved over Maui County. Portions of Maui received 6 to almost 9 inches of rainfall which caused flooding problems in several areas. On the windward side of the West Maui Mountains, Kahekili Highway was closed near mile marker 7 due to flooding. To the east, landslides forced the closure of the Hana Highway near Kaupakulua Road for several hours. Significant flooding also affected numerous roads in Kahului and Wailuku for most of the day. As the frontal rain band reached the Big Island, heavy rainfall developed over the lower elevations of the South Hilo and Puna Districts which caused localized property damage and road flooding in downtown Hilo.

Another cold front with significant upper level support approached the island chain on December 26. This time the heaviest rainfall occurred within pre-frontal rain bands in southerly low level winds. Oahu took the brunt of the rainfall during this event with several gages recording 4 to 7 inches of rainfall mostly during the afternoon and early evening hours. Localized urban flooding occurred in Honolulu near Kapahulu Avenue. During the night of December 26, upper level support lifted to the north resulting in much lighter rainfall when the rain bands moved eastward into Maui County.

Island of Kauai : [December 2017 map] [2017 total map]

Kauai missed the bulk of the significant rain events affecting the island chain in December. As a result, most of the rain gages on the island posted monthly totals below 50 percent of average. The highest monthly total of 12.98 inches (76 percent of average) came from the U.S. Geological Survey’s (USGS) Kilohana rain gage rather than the usual Mount Waialeale gage. This is not surprising in a weather pattern that featured several weak cold fronts which were too shallow to push significant amounts of shower bearing clouds over the center of the island. The Kilohana gage also posted the highest daily total of 1.87 inches on December 26.

Most of the rain gages on Kauai finished 2017 with annual totals in the near average range. Mount Waialeale had the highest 2017 total of 308.64 inches (78 percent of average). The 30-year running average for Mount Waialeale dropped for a third consecutive year and has decreased in 18 out of the last 21 years.

Island of Oahu: [December 2017 map] [2017 total map]

Largely due to the December 26 heavy rain event, most of the gages along the southeastern half of Oahu posted monthly totals in the near to above average range. Totals from most of the gages on the northwestern half were in the below average range. The Maunawili gage had the highest monthly total of 15.05 inches (158 percent of average) and the highest daily total of 7.06 inches on December 26. The Palolo Fire Station gage logged its highest December total since 2008.

Oahu rainfall totals for 2017 ended up in the near average range at most of the gages. The USGS’ Poamoho Rain Gage No. 1 had the highest 2017 total of 149.65 inches (66 percent of average). Honolulu Airport had its wettest year since 2006.

Maui County: [Maui December 2017 map] [2017 total map] [Molokai/Lanai December 2017 map] [2017 total map]

Heavy rainfall on December 20 helped push most of the monthly totals across Maui County into the above average range. The USGS’ Puu Kukui gage had the highest monthly total of 28.50 inches (96 percent of average). In a rare occurrence, the highest daily total of 8.69 inches on December 20 came from the Wailuku gage. The highest daily totals usually come from the USGS’ rain gages at Puu Kukui or West Wailuaiki Stream. A more noteworthy statistic from that day was the 6.40 inches recorded at Kahului Airport which broke the site record for the highest single day rainfall total in a period of data going back to 1954. The previous record was 5.82 inches observed on December 21, 1955. The record rainfall helped make it the wettest December at Kahului Airport since 1996. Pukalani and Waikapu Country Club both posted their highest December totals since 2007.

Rainfall totals for 2017 were in the near to above average range at most of the gages across Maui County. The highest available total for the year was from the USGS’ rain gage at West Wailuaiki Stream with 208.67 inches (93 percent of average).

Island of Hawaii: [December 2017 map] [2017 total map]

Most of the rainfall totals for the month of December were in the below average range. The USGS’ rain gage at Kawainui Stream had the highest monthly total of 15.95 inches (119 percent of average). The highest daily total was 4.61 inches on December 1 from the USGS’ Saddle Road Quarry gage. The Pali 2 gage recorded its lowest December total since 2006.

Overall, Big Island rainfall totals for 2017 were in the near to below average range. The USGS’ rain gage at Kawainui Stream had the highest available total for 2017 at 173.54 inches (128 percent of average). Breaking it down by region, windward and Kona slope totals were mainly near average. Kau and interior Big Island totals were mostly below 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