September 2017 Precipitation Summary

Monthly Precipitation Summary

State of Hawaii

Month: September 2017

Prepared: October 4, 2017

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

State: [Text data table for rain gages]

September is usually a month of transition from the persistent trade winds of the dry season to the more variable conditions of the wet season as the trade winds become more frequently disrupted by nearby or approaching low pressure systems in the westerlies. So after three months of uninterrupted trades, on September 1 through September 4 a low pressure system in the North Pacific pushed the low level subtropical ridge of high pressure unseasonably far south to a position just north of the main Hawaiian Islands. The resultant large scale light wind conditions allowed land and sea breezes to dominate local conditions and switched the island chain to a more active afternoon rainfall pattern with suppressed rainfall during the nighttime and morning hours. A few of the afternoon showers were briefly heavy along the Kona slopes of the Big Island on September 1 but did not cause any significant flooding problems. A weak cold front associated with the low pressure system eventually reached Kauai on September 6 before retreating back toward the west as the trade winds returned on September 7. Note that an early September cold front reaching the main Hawaiian Islands is not common and marked the earliest arrival in at least the last 18 years. The average arrival date of the wet season’s first cold front in that 18-year time span is October 19 while the latest was December 4 in 2000.

The September 7 return to trades lasted only two days as a light wind pattern once again settled over the island chain on September 9. On September 12, the low level ridge shifted southward over the state and stayed there through September 14. This type of pattern is normally not seen until mid- to late-December. The afternoon showers reached levels strong enough to produce nuisance flooding on September 9 (Kona slopes), September 11 (windward Oahu), and September 14 (Kona and Kauai).

Trade winds returned in earnest on September 16 and maintained moderate to fresh intensity levels until September 23 when it dropped to light to moderate levels. The light trades on September 24 opened a window for heavy afternoon showers to develop along the slopes of the South Kohala District on the Big Island. These showers did not result in any reports of flood damage. Low level winds switched to a southerly direction on the last day of the month as a low pressure system developed to the north of the state.

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

Light wind conditions and a stable atmosphere on many days during the month is usually not conducive for abundant rainfall. As a result, most of the gages on the island of Kauai logged below average rainfall totals for the month of September. The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 16.77 inches (55 percent of average) and the highest daily total of 3.90 inches on September 9. Lihue Airport was on pace to have its driest September on record but significant rainfall on the last day of the month broke that trend line. In the end, Lihue Airport’s 0.54 inches (25 percent of average) missed the record of 0.44 inches set in 2007 and came in as the second lowest September total. The gages at Hanapepe and Omao did break records for the lowest September total in a data log going back to 1993 and 1995, respectively.

Rainfall totals for 2017 through the end of September were in the near to below average range at most of the gages across Kauai. Mount Waialeale had the highest year-to-date total of 209.27 inches (72 percent of average).

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

Following a relatively wet August, most of the gages on Oahu posted below average rainfall totals for the month of September. The Manoa Lyon Arboretum gage had the highest monthly total of 7.02 inches (63 percent of average) while the Ahuimanu Loop gage had the highest daily total of 3.13 inches on September 10. Records for the driest September were broken at Hawaii Kai Golf Course, Kamehame, and Waiawa Correction Facility. The Kahuku gage recorded its lowest September total since 2001.

Most of the gages on Oahu had rainfall totals for 2017 through the end of September in the near to below average range. The USGS’ Poamoho No. 1 Gage had the highest year-to-date total of 107.12 inches (65 percent of average).

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

September rainfall totals across Maui County registered in the below average range for most of the gages. The USGS’ rain gage at West Wailuaiki Stream had the highest monthly total of 7.97 inches (56 percent of average) and the highest daily total of 1.59 inches on September 9. While there were no long term records broken, the Kahakuloa gage posted its lowest September total since 2002. Note that there are a couple of new data points showing up on the Maui rainfall map. One is for the Maalaea Bay weather station and the other is at the Kealia Pond National Wildlife Refuge. Both sites will usually show low rainfall totals, especially during the summer months.

Despite the drier conditions recently, most of the rainfall totals across Maui County for 2017 through the end of September remained in the near to above average range. The USGS’ rain gage at West Wailuaiki Stream had the highest year-to-date total of 128.83 inches (75 percent of average).

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

Monthly rainfall totals were in the near to above average range at most of the gages in the North and South Kona Districts of the Big Island. Totals from the rest of the gages on the island were mostly in the below average range. The Kealakekua gage had the highest monthly total of 11.30 inches (161 percent of average) while the Waiaha Stream rain gage had the highest daily total of 3.69 inches on September 2. The gage at Laupahoehoe (0.92 inches, 11 percent of average) posted its lowest September total on record. The previous record was 1.61 inches set in 2004.

Most of the gages on the Big Island had rainfall totals for 2017 through the end of September in the below average range. The USGS’ rain gage at Kawainui Stream had the highest year-to-date total of 103.55 inches (96 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