October 2016 Precipitation Summary

State of Hawaii

MONTH: October 2016

PREPARED: November 9, 2016

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

State: [Text data table for rain gages]

The start of the 2016 – 2017 Hawaiian Islands wet season featured trade winds during all of October, which were unseasonably persistent for this time of year.  On average, trade winds occur on roughly two-thirds of the days in October across the island chain. Trade wind intensities were mainly in the moderate to fresh range, though stronger high pressure to the north of the state cranked up intensities into the fresh to strong range over the last 10 days of the month. The wet season runs through the end of April.

Generally speaking, windward areas received frequent rainfall while leeward areas were abnormally dry. Heavy rainfall occurred over the Big Island and Maui when remnant moisture from former tropical cyclone Ulika moved across both islands as an upper level low pressure system passed overhead. The resultant unstable conditions produced significant rainfall over the windward slopes that started on October 1 and continued through October 5. Several windward Big Island gages recorded over 10 inches of rainfall during the 5-day period with upper Hilo sites logging 15 to 20 inches. Alenaio Stream in Hilo overflowed into the Bayfront soccer fields by design, and nearby Kamehameha Avenue was closed due to widespread standing water. Aside from these poor drainage areas, there were no other areas of significant damage. The Hilo area certainly can handle a lot of rain. On the leeward side of the Big Island, afternoon heating triggered brief heavy showers over the South Kohala District on October 4. Runoff overflowed a small drainage near Waikoloa Village and partially closed Waikoloa Road at the intersection with Pua Melia Street.

Atmospheric conditions stabilized once the upper level low pressure system and the deep tropical moisture exited the area near the main Hawaiian Islands. While showers continued daily over the windward slopes, lower intensities and coverage resulted in substantially smaller daily totals. On October 10, a weak cold front, the first of the wet season, reached the island chain. This front managed to provide only a small enhancement in rainfall with less than half an inch registering on the rain gage network. Another weak cold front moved over the state on October 26. This front was a bit more active and a few sites logged 1 to 2 inches from the event.

Island of Kauai : [October 2016 map] [year-to-date map]

Persistent trade winds generated ample rainfall along the windward slopes of Kauai which resulted in near to above average monthly totals for gages in the area. Meanwhile, leeward locations, which normally see an uptick in rainfall during this time of year, remained dry. Many of the leeward gages posted monthly totals below 40 percent of average. The highest monthly total came from the U.S. Geological Survey’s (USGS) Mount Waialeale gage with 27.09 inches (80 percent of average). This was the highest October total at this site since 2005. Mount Waialeale also had Kauai’s highest daily total of 3.50 inches on October 3. On the south side of the island, the Kalaheo rain gage had its lowest monthly total on record, 0.70 inches, or just 13 percent of average.

Many of the gages across Kauai continued to have near to above average rainfall totals for 2016 through the end of October. Recent dryness has dropped leeward totals farther into the below average range. Mount Waialeale had the highest year-to-date total of 306.20 inches (94 percent of average) and remained as the wettest spot in the state so far this year.

Island of Oahu: [October 2016 map] [year-to-date map]

Gages along the interior slopes of the Koolau Range had near to above average monthly rainfall totals for October. The rest of the gages on Oahu had below average totals, many of which were less than 50 percent of average. Manoa Lyon Arboretum had the highest monthly total of 16.12 inches (126 percent of average) and the highest daily total of 3.46 inches on October 4. These wet conditions were in contrast to Hakipuu Mauka, which had its lowest October total on record, and Waimanalo, which had its driest October since 1995.

Rainfall totals at most of the rain gages on Oahu were in the near to above average range for 2016 through the end of October. The exceptions were mostly from the leeward slopes of the Waianae Range. The USGS’ Poamoho No. 1 Gage had the highest year-to-date total of 160.10 inches (87 percent of average).

Maui County: [Maui October 2016 map] [year-to-date map] [Molokai/Lanai October 2016 map] [year-to-date map]

Rain gages across Maui County reported a wide range of rainfall conditions with near to above average October totals along the windward slopes and below average totals over the remaining areas. The USGS’ West Wailuaiki gage had the highest monthly total of 32.07 inches (187 percent of average). Puu Kukui, another USGS site, had the highest daily total of 3.67 inches on October 21. Kula Branch Station’s 0.05 inches marked the driest October at this location since 1996. Just a month ago, this site had its wettest September on record.

Most of the gages across Maui County had near to above average rainfall for 2016 through the end of October. The West Wailuaiki gage had the highest year-to-date total of 303.27 inches (160 percent of average) and was only slightly less than Mount Waialeale’s total.

Island of Hawaii: [October 2016 map] [year-to-date map]

October rainfall totals were above average at most of the Big Island’s windward gages. The USGS’ Saddle Road Quarry rain gage above Hilo recorded an amazing 57.81 inches (539 percent of average), far surpassing all other monthly totals across the state. This gage logged 19.46 inches during the first 5 days of the month with the peak daily total of 7.23 inches occurring on October 2. Despite the high monthly total, there were no 1-hour totals at or above an inch. In contrast to the wet windward conditions, most of the leeward sites posted below average totals. Although there was a flash flood event that affected Waikoloa Village, the source rainfall occurred upslope and the gage at Waikoloa only registered 0.07 inches (8 percent of average) the entire month.

Windward Big Island locations had near to above average rainfall for 2016 through the end of October. Recent dryness has dropped some of the leeward year-to-date totals into the below average range. The Saddle Road Quarry gage had the highest 2016 total of 260.66 inches (227 percent of average) which was the third highest in the state.

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