September 2015 Precipitation Summary

State of Hawaii

MONTH: September 2015

PREPARED: October 6, 2015

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

State: [Text data table for rain gages]

August was wet over many areas of the state. For some areas, September was even wetter. This is not surprising since the island chain has been surrounded by warm waters 1 to 3 degrees Celsius above average. These anomalously warm waters have provided the fuel to produce more active showers and thunderstorms than would normally be expected. In addition to the warm ocean conditions, the circulations of tropical cyclones Ignacio and Jimena north of the island chain helped draw in a deep tropical airmass and bands of enhanced rainfall. One of these bands arrived over the southern coast of Oahu on September 3 and produced thunderstorms and torrential rainfall over downtown Honolulu with rates of 3 to 4 inches per hour. Several roads became impassable and businesses and properties in low-lying areas were significantly inundated by flood waters.

The period from September 4 through 6 involved unstable conditions within light to moderate low level winds from the southeast. Under this pattern, the east-facing slopes of the Koolau Range on Oahu become susceptible to persistent heavy rainfall due to the interaction of the moist southeasterlies with the northwest to southeast oriented mountain range. On the evening of September 6, this scenario generated periods of heavy rainfall from Kaneohe to Hauula with the most intense rain cores occurring near Punaluu and along the slopes above Waiahole and Waikane. Flash flooding in Waikane Stream put Kamehameha Highway under more than a foot of water resulting in the closing of that main road for several hours and inundated several nearby properties. This same scenario played out once again on September 11 as the remnant of tropical cyclone Jimena passed north of Kauai which shifted the low level winds to the southeast once again. Due to the saturated soils from the earlier heavy rain event, flash flooding in Waikane Stream quickly put Kamehameha Highway under impassable depths of water for a few hours.

In addition to heavy rainfall on Oahu, the first week of September also included the first of several heavy rain events over the Kona slopes of the Big Island. On September 4, showers and thunderstorms developed during the afternoon hours with the strongest and most persistent rain cores occurring in the North and South Kona Districts from Holualoa to Honaunau. Waiaha Stream, normally a dry gulch, overflowed its banks along Hualalai Road and Kuakini Highway just southeast of Onioni St. Several videos posted on the internet also indicated severe runoff that was obviously causing property damage. Afternoon heavy rainfall repeated itself on several days the following week, though flooding may have been worse due to the saturated ground conditions from earlier rainfall. In addition to repeat flooding in Waiaha Stream, significant flooding also occurred along low-lying spots on Alii Drive and in Kainaliu, Kealakekua, and Honaunau. Rainfall totals in the Kona area from September 2 through 7 ranged from 4 to over 8 inches, while totals during the period from September 11 through 18 came in at 4 to just under 10 inches. By the end of September 18, several of the Kona rain gages had posted totals that broke all-time September records with more than a week to go in the month. In addition to flooding impacts in Kona, heavy rainfall in the Kau District also resulted in flooding on Highway 11 near Kawa Flats on September 16 and 17. This is a known problem spot during heavy rain events and has a significant impact in that it cuts the main route around the Big Island for several hours until the flooding subsides.

Island of Kauai : [September 2015 map] [year-to-date map]

All of the rain gages on Kauai recorded near to above average totals for the month of September. The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 66.68 inches (220 percent of average) and the highest daily total of 9.23 inches on September 4. This was the most rain in a month at this location since March 2006 (94.30 inches) and broke the record for September rainfall. While no other long term records were broken, Anahola and Lihue Airport had their wettest September since 1996, and Kokee, Wainiha, Hanalei, Wailua, and Omao had their highest September totals since 2005.

All of the rain gages on Kauai recorded near to above average totals for the month of September. The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 66.68 inches (220 percent of average) and the highest daily total of 9.23 inches on September 4. This was the most rain in a month at this location since March 2006 (94.30 inches) and broke the record for September rainfall. While no other long term records were broken, Anahola and Lihue Airport had their wettest September since 1996, and Kokee, Wainiha, Hanalei, Wailua, and Omao had their highest September totals since 2005.

Rainfall totals for 2015 through the end of September were in the near to above average range at most of the gages on Kauai. Mount Waialeale had the state’s highest year-to-date total of 265.16 inches (91 percent of average).

Island of Oahu: [September 2015 map][year-to-date map]

All of the gages on Oahu posted above average monthly totals. In fact, all locations with long term data indicated record-breaking September rainfall. These sites were Kahuku, Honolulu Airport, Aloha Tower, Punaluu Pump, Waihee Pump, Luluku, Olomana Fire Station, Waimanalo, Nuuanu Upper, Niu Valley, Kamehame, Hawaii Kai Golf Course, Waiawa CF, Palisades, Waipio, Poamoho Experiment Farm, Waianae Kawiwi, and Kunia. In the case of Aloha Tower, the September 2015 total was more than three times greater than the previous record. The highest monthly total of 51.37 inches (288 percent of average) was from the U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge (NWR) gage while the highest daily total was from the USGS’ Moanalua gage with 9.37 inches on September 11.

Most of the rainfall totals for 2015 through the end of September were in the near to above average range. The Oahu Forest NWR gage had the highest year-to-date total of 179.26 inches (111 percent of average).

Maui County: [Maui September 2015 map] [year-to-date map] [Molokai/Lanai September 2015 map] [year-to-date map]

Nearly all of the rain gages in Maui County posted near to above average rainfall for September. The USGS’ Puu Kukui gage had the highest monthly total of 42.36 inches (191 percent of average) and the highest daily total of 5.75 inches on September 15. Monthly totals from Kahakuloa and Kula Branch Station broke September rainfall records and Pukalani had its highest rainfall total since 1992.

Rainfall totals for 2015 through the end of September were in the near to above average range at most of the sites in Maui County. The highest available year-to-date total was from the National Park Service’s Puu Alii gage with 139.25 inches (187 percent of average).

Island of Hawaii: [September 2015 map] [year-to-date map]

Almost all of the rain gages on the Big Island recorded above average rainfall totals for the month of September. The USGS’ Saddle Road Quarry gage had the highest monthly total of 49.79 inches (508 percent of average), including 15.11 inches for the 2-day period of September 20 through 21. The highest single-day total was from the Mountain View gage with 7.84 inches on September 20. Rain gages at Honaunau on the Kona slopes and Hilo Airport reported record-breaking September rainfall. Observers for manually read Cooperative Program rain gages at Kainaliu and Kealakekua also indicated record monthly totals at their sites.

Rainfall totals for 2015 through the end of September remained in the near to above average range at most of the gages on the Big Island. After a very wet two-month period, the USGS’ Saddle Road Quarry gage passed the Kawainui Stream rain gage with the highest year-to-date total of 170.71 inches (164 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