October 2017 Precipitation Summary

Monthly Precipitation Summary

State of Hawaii

Month: October 2017

Prepared: November 8, 2017

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

State: [Text data table for rain gages]

The North Pacific weather pattern that affected the State of Hawaii proved to be rather unsettled with conditions at times resembling mid-winter as opposed to early fall. As a result, the 2017 – 2018 wet season kicked off with most areas of the state having wetter than average conditions. The month started with a cold front approaching the island chain but it stalled on October 2 before reaching Kauai. Pre-frontal shower bands within the moist southerly low level flow brought briefly heavy showers to portions of Kauai and Oahu but did not produce significant flooding problems. This was followed by a weak cold front on October 6 which managed to reach the main Hawaiian Islands but produced only a modest increase in rainfall activity.

During the next week a trough of low pressure embedded within a large area of deep tropical moisture moved over the state from the east. The unstable air mass over the area supported periods of heavy rainfall over the windward slopes from October 12 through October 14. Isolated rainfall totals reached 3 to 6 inches but there were no reports of significant flooding problems. After the trough exited the area it was replaced by a large high pressure system centered far to the north of the state. This placed the island chain within a relatively tight atmospheric surface pressure gradient which produced strong trade winds from October 16 through October 20. In this pattern numerous showers were carried over windward slopes in the trade wind flow or developed in situ along the slopes. Rainfall intensities were generally light to moderate and were moving along too fast to result in significant accumulations on any specific spot.

On October 22, the large scale weather pattern affecting the Hawaiian Islands shifted from stable trade winds to more unsettled conditions with a strong low pressure system and cold front approaching Kauai from the northwest. The cold front swept over the state on October 23 and 24 producing heavy rainfall not just along the front itself but also in the moist southerly winds ahead of the main frontal rain band. The speed of the front over Kauai and Oahu was fast enough to prevent significant rainfall accumulations and flooding problems. However, the front’s eastward progress slowed as it pushed through Maui County resulting in higher rainfall totals. Runoff forced the closure of the Haleakala Highway and the Hana Highway junction, and a landslide closed the Piilani Highway at Kalepa Gulch. Also accompanying the front over Maui was a severe thunderstorm which moved onshore near Kihei and Maalaea. Frequent lightning and strong outflow winds from the thunderstorm toppled trees and utility poles in central Maui and contributed to an island-wide power outage that lasted most of the day. The front stalled near the Big Island on October 24 but remained active with widespread rainfall and some embedded thunderstorms. Flooding closed several roads, including Kuakini Highway near Kailua-Kona, Wood Valley Road near Pahala, and several roads in Hilo. Weather conditions slowly improved late on October 24 but remained wetter than usual through October 25 as the frontal rain band dissipated just east of the Big Island.

The month concluded with the development of a kona low far northwest of the main Hawaiian Islands. A peripheral rain band reached Kauai during the night of October 31 and produced heavy rainfall which caused Hanalei River to overflow and forced the closure of Kuhio Highway near the Hanalei Bridge for a few hours. A subsequent batch of heavier showers moved over Kauai during the morning of November 1 and produced an even larger flood wave in Hanalei River and several other rivers and streams on the island. Additional details on the November 1 phase of the event will be provided in the November summary.

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

An active weather pattern during the month of October produced above average rainfall totals at most of the gages on Kauai. The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 35.19 inches (104 percent of average) and the highest daily total of 5.17 inches during the flash flood event on October 31. The monthly total at this gage marked the wettest October since 2005. New records were set for the highest October rainfall totals at the Anahola and Kapahi gages. The data archive for these sites goes back to 1991 and 1993, respectively. Rainfall totals from the Omao, Hanapepe, and Wainiha gages registered as the wettest October since 2004.

The wet October conditions pushed rainfall totals for 2017 through the end of October into the near average range at most of the gages on Kauai. Mount Waialeale had the highest year-to-date total of 244.46 inches (75 percent of average).

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

October rainfall totals were in the near to above average range at most of the gages on Oahu. The USGS’ Moanalua Rain Gage had the highest monthly total of 12.38 inches (113 percent of average). The highest daily total came from the Waihee Pump gage which recorded 3.37 inches on October 14. Of the few below average totals, most were from gages along the windward slopes of the Koolau Range from Kahana to Kahuku. On the southeast side of the Koolaus, the Maunawili gage posted its highest October total since 2006.

Most of the gages on Oahu had rainfall totals for 2017 through the end of October in the near average range. The USGS’ Poamoho No. 1 Gage had the highest year-to-date total of 118.95 inches (64 percent of average).

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

Above average rainfall totals were logged by most of the rain gages across Maui County for the month of October. At most of the sites, a large fraction of the monthly totals were associated with the strong cold front passage on October 24. The USGS’ Puu Kukui rain gage had the highest Maui County monthly total of 26.94 inches (103 percent of average), which was the highest October total since 2004.  Puu Kukui also had the highest daily total of 5.05 inches on October 24. Records for the highest October rainfall totals were broken at Mahinahina and Kahakuloa. Kaunakakai Mauka had its wettest October since 2002, and Kula Branch Station, Ulupalakua Ranch, Waikapu Country Club, and Kahului Airport had their highest October totals since 2006.

Almost all of the Maui County totals for 2017 through the end of October were in the near to above average range. The USGS’ rain gage at West Wailuaiki Stream had the highest year-to-date total of 145.24 inches (77 percent of average).

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

Most of the rain gages on the Big Island recorded above average rainfall totals for the month of October. The few near to below average rainfall totals were mostly from the Hamakua and North Hilo Districts. The Glenwood gage had the highest monthly total of 23.36 inches (135 percent of average). The highest daily total was 6.03 inches posted by the USGS’ Saddle Road Quarry gage. Records for the highest October rainfall totals were broken at the Kahua Ranch, Kealakekua, and Honaunau gages. Kapapala Ranch posted its highest October total since 2006.

Although most of the Big Island had a wet month, rainfall totals for 2017 through the end of October remained in the near to below average range at most of the gage locations due to the severity of previous dryness, especially over the summer. The USGS’ rain gage at Kawainui Stream had the highest year-to-date total of 117.57 inches (103 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