September 2016 Precipitation Summary

State of Hawaii

MONTH: September 2016

PREPARED: October 5, 2016

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

State: [Text data table for rain gages]

Abundant moisture from the deep tropics helped fuel another month of wet conditions for many areas of the state during what is normally part of the dry season. September started with the passage of Hurricane Lester to the north of the main Hawaiian Islands. Lester was the second of a tropical cyclone double feature, following the close passage of Hurricane Madeline just to the southeast of the Big Island in late August. While there were no reports of strong winds, the trailing rain band from Lester produced heavy showers and minor flooding over the leeward Kohala slopes of the Big Island and portions of east Maui on September 3.

The first full week of the month brought settled weather conditions with moderate to fresh trade winds and a needed break from the tropical cyclone activity. Rainfall occurred daily, mainly along the windward slopes, but amounts were generally small.

Weather during the week of September 11 to September 17 was anything but settled. On September 11, a weak tropical disturbance brought a large area of deep tropical moisture over the main Hawaiian Islands. As this occurred, an upper level low pressure system moved over the state from the northwest. The combination of these features produced a very unstable air mass primed for thunderstorms and intense rainfall with Maui County taking the brunt of the activity. The main event within this period of inclement weather occurred during the evening of September 13 on the island of Maui. After two days of wet conditions which primed the watersheds in the West Maui Mountains, a strong thunderstorm dropped over 5 inches of rainfall in a 2-hour period on top of Puu Kukui. While this amount alone is not too impressive for one of the wettest spots in the country, it was part of 10 inches in a day and 15 inches in a 3-day period that resulted in a massive flood wave that traveled down Wailuku River (formerly Iao Stream). The flooding caused significant damage to Iao Valley State Park, Kepaniwai Heritage Gardens County Park, and several homes in Iao Valley above Wailuku town. Early estimates of the damages were at least $15 million and several area residents were rescued during the event when the river overflowed its banks and inundated their homes. As the rainfall on Puu Kukui drained in several directions off the West Maui Mountains, flash flooding also occurred in Maalaea, Ukumehame, Launiupoko, and Kahoma Stream (in Lahaina). Several landslides along Honoapiilani Highway just west of Maalaea closed the road for several hours and stranded travelers caught between slides. Heavy rainfall earlier in the day over east Maui also caused flash flooding and landslides along the Hana Highway. On September 14, another round of thunderstorms fired up, but this time over the southern slopes of Haleakala during the early afternoon hours. Flash flooding overflowed Manawainui Gulch which briefly closed Piilani Highway.

Other areas affected by the mid-September heavy rain event included the Puna District of the Big Island, where South Kopua Road was briefly closed by flooding on the night of September 14, and the Koolau Range of Oahu, which had some minor flooding along the windward slopes on September 15. Weather conditions slowly stabilized late in the week with drier conditions in moderate trade winds finally returning on September 17.

The rest of the month produced mainly moderate to occasionally fresh trade winds. Trailing deep tropical moisture from the remnants of tropical cyclone Orlene passing north of the island chain produced moderate to heavy rainfall and minor flooding along the windward slopes of Haleakala on September 23. The last week in the month saw the formation of Tropical Storm Ulika far to the southeast of the Big Island. This tropical cyclone dissipated far to the east of Hilo on September 30, but its remnant moisture would play a role in heavy rainfall production early in October.

With all of the heavy rainfall across portions of the state, drought conditions actually worsened over leeward Kauai. Reports from the island indicated worsening pasture conditions and drier than normal brush over the southern and southwestern slopes. Similarly, heavy rainfall over portions of Maui appears to have missed the existing drought area near Kihei and Wailea and agriculture officials indicated no improvement in pasture conditions.

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

Monthly rainfall totals from most of the gages on the windward side of Kauai were in the near average range. As mentioned above, leeward sites had mostly below average totals with several below 50 percent of the September long term average. In fact, the Lihue Airport total of 0.64 inches (30 percent of average) was the lowest September total since 2007. The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 32.18 inches (106 percent of average) and the highest daily total of 4.30 inches on September 17.

Rainfall totals for 2016 through the end of September were in the near to above average range at most of the rain gages on Kauai. Mount Waialeale had the highest year-to-date total of 279.11 inches (95 percent of average), which was the highest total in the state.

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

For the second year in a row, Oahu ended up with above average September rainfall at most of the gages on the island. The gages at Honolulu Airport, Aloha Tower, Kunia, Moanalua, Nuuanu Upper, and Kamehame recorded their second wettest September ever, second only to September 2015’s record-breaking totals. The Manoa Lyon Arboretum gage posted the highest monthly total of 22.24 inches (199 percent of average). The USGS’ Moanalua Rain Gage had the highest daily total of 4.50 inches on September 13.

Most of the rain gages on Oahu had rainfall totals for 2016 through the end of September in the near to above average range. The exceptions were mainly from the leeward slopes of the Waianae Range with a couple of Waianae coastal sites at less than 40 percent of the year-to-date average. The USGS’ Poamoho No. 1 Gage had the highest year-to-date total of 151.89 inches (92 percent of average).

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

Nearly all of the gages across Maui County posted above average rainfall totals for the month of September. The USGS’ West Wailuaiki gage had the highest monthly total of 31.26 inches (220 percent of average). Another USGS site, Puu Kukui, had the highest daily total of 10.01 inches on September 13 which helped produce the massive flash flood in Wailuku River. A more detailed breakdown of the Puu Kukui total during the flash flood event shows a peak 15-minute rainfall of 1.25 inches (7:00 to 7:15 PM HST) and a peak 1-hour total of 4.15 inches (6:30 to 7:30 PM HST). Gages at Haiku, Kahakuloa, Hana Airport, Pukalani, and Waikapu Country Club recorded their highest September totals on record. Kula Branch Station had its second wettest September on record and Kahului Airport had its highest September total since 1992.

Rainfall totals across Maui County for 2016 through the end of September were in the near to above average range at most of the gages. The West Wailuaiki gage had the highest year-to-date total of 271.20 inches (158 percent of average) which was the second highest total in the state.

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

Most of the windward gages on the Big Island had near to above average rainfall totals for September. The USGS’ Saddle Road Quarry gage had the highest monthly total of 26.06 inches (266 percent of average). The highest daily total was 4.90 inches from the Glenwood gage during the flash flood event on September 14. In contrast, many of the leeward gages had below average monthly totals with Kealakekua having its lowest September total since 2006.

Big Island rainfall totals for 2016 through the end of September were in the near to above average range at most locations. The Saddle Road Quarry gage had the highest year-to-date total of 202.85 inches (195 percent of average). This total 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