August 2017 Precipitation Summary

Monthly Precipitation Summary

State of Hawaii

Month: August 2017

Prepared: September 6, 2017

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

State: [Text data table for rain gages]

The trade winds have been uninterrupted since June with August wind speeds mainly in the moderate range. On August 2, the trades slackened a bit which allowed early morning land breezes over the windward slopes of Oahu to interact with the incoming trade wind flow. This interaction resulted in locally heavy showers that produced totals in the range of 1 to 4 inches.

Later in the month, a large area of deep tropical moisture moved over the main Hawaiian Islands from the east. On August 20 and 21, an upper level low pressure system northwest of the state tapped into this moist air mass and triggered episodes of heavy rainfall over portions of the state. The initial round of heavy rain occurred during the afternoon hours of August 20 over the South Hilo and Puna Districts as well as the Kona slopes. Gages recorded rainfall totals as high as 1 to 2 inches but there were no reports of significant flooding problems. Late that night, thunderstorms developed over Kauai on the opposite end of the state. The thunderstorms did not last long but dumped enough rain over the Hanalei River basin to produce a flash flood of sufficient magnitude to briefly close Kuhio Highway a little after midnight. After this round of rainfall subsided on Kauai, a new episode of heavy rain flared up over the slopes of the Koolau Range on Oahu just before sunrise on August 21. Rainfall totals up to around an inch did not result in significant impacts other than slowing the morning commuter traffic. The rainfall over Oahu subsided by late morning but flared up when the shower area reached Kauai. Adding complexity to the situation, strong thunderstorms north of Kauai pushed an outflow boundary southward which reached the Garden Isle in the early afternoon. The combined shower area and outflow boundary increased the intensity and coverage of heavy rainfall and promptly generated flash flooding over the north- and east-facing slopes of the island. Hanalei River once again overflowed its banks and closed Kuhio Highway for the second time in a day. Landslides occurred near Kalihiwai, and several drivers were stranded on Loop Road in upper Wailua due to the inundation of low water crossings at tributaries to Wailua River. Rain gages on Kauai indicated event totals of around 2 to just over 4 inches. Stable trade wind conditions resumed after the upper low moved westward away from the island chain on August 22.

The last rainfall event of significance occurred on August 29 as a weak surface trough of low pressure moved over Oahu coincident with daytime heating. Intense thunderstorms, with cloud tops reaching 45,000 to 50,000 feet, developed over Honolulu and dropped more than 3 inches of rain in an hour and over 5 inches for the event during the peak of the afternoon rush hour. Fortunately there were no reports of significant flood impacts beyond the delays in commuter travel times.

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

Despite the late August heavy rain event, most of the rain gages on Kauai posted near to below average rainfall totals for the month. The driest conditions were along the leeward slopes with low elevation sites west of Hanapepe recording less than a quarter of an inch all month. The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 25.16 inches (72 percent of average). For a second consecutive month, the gage at Wainiha had the highest daily total, recording 4.14 inches during the flash flood event on August 21.

Kauai rainfall totals for 2017 through the end of August were in the near average range at most of the gages. Mount Waialeale had the highest year-to-date total of 192.50 inches (73 percent of average).

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

While most of the gages on Oahu recorded near to above average rainfall totals for the month of August, several leeward sites in need of a boost in rainfall remained in the below average range. The Manoa Lyon Arboretum gage logged the highest monthly total of 14.14 inches (109 percent of average) while the USGS’ Moanalua Rain Gage No. 1 had the highest daily total of 5.17 inches during the afternoon heavy rain event on August 29. An interesting footnote to the Oahu report is that Honolulu Airport’s 1.85 inches marked the third consecutive August with more than an inch of rainfall. Recording more than an inch of rain in August for three consecutive years has never happened before in a data record going back to 1946. The August average rainfall at Honolulu Airport is 0.56 inches.

Most of the gages on Oahu had rainfall totals for 2017 through the end of August in the near average range. The USGS’ Poamoho No. 1 Gage had the highest year-to-date total of 100.13 inches (68 percent of average).

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

Gages across Maui County reported a wide range of conditions, from an above average 1.27 inches (153 percent of average) at Lanai 1, to a near average 17.65 inches (104 percent of average) at the USGS’ West Wailuaiki gage, to 0.02 inches (3 percent of average) at Molokai Airport. A little more than half of the monthly totals were in the below average range. The West Wailuaiki total was the highest in Maui County. This site also had the highest daily total of 3.62 inches on August 21. The Molokai Airport’s 0.02 inches broke the record for the lowest August rainfall total at this location, beating the previous record of 0.04 inches set in 1984.

Rainfall totals for 2017 through the end of August were in the near to above average range at most of the gages across Maui County. The USGS’ rain gage at West Wailuaiki Stream had the highest year-to-date total of 120.86 inches (77 percent of average).

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

Big Island rainfall totals for the month of August were in the below average range at most of the gages. The USGS’ Saddle Road Quarry gage had the highest monthly total of 23.29 inches (188 percent of average). However, the highest daily total came from the Piihonua gage with 4.23 inches recorded on August 21. While the South Hilo District and northern Puna District had wet conditions, gages on the lower Hamakua slopes and most of the leeward areas reported below average totals. Among these sites, the gage at Honokaa reported its lowest August total since 1996. Interior sections received some spotty afternoon showers which helped push a few of the totals into the near to above average range.

Most of the gages on the Big Island had rainfall totals for 2017 through the end of August in the below average range. The USGS’ rain gage at Kawainui Stream had the highest year-to-date total of 98.24 inches (98 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