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September 2012 Precipitation Summary
State of Hawaii
MONTH: September 2012
PREPARED: October 3, 2012
Note: This summary uses the arithmetic mean, or average, for “normal” rainfall values.
State: [Text data table for rain gages]
Trade wind weather conditions prevailed on most of the days during the month of September with speeds mainly in the moderate to fresh range. An upper level low pressure system northwest of Kauai brought unstable conditions to the state and induced a veering of low level winds to an east-southeasterly direction on September 7 and 8. Moderate to heavy rainfall during the early morning hours of September 7 produced minor flooding problems over east Maui from Kipahulu to Haiku but no reports of damage. Under east-southeasterly winds, the Big Island blocks the large scale low level flow for the smaller islands in the chain allowing land and sea breezes to dominate local wind conditions. Afternoon sea breezes converging over the interior portions of Oahu and Lanai on September 8 generated briefly heavy showers and minor flooding of roads and small streams but no reports of significant damages.
Another bout of enhanced rainfall occurred on September 19 and 20 as a second upper level low pressure system dropped into the area from the northwest. The heaviest showers on September 19 affected the windward slopes of Oahu with 3 to 4 inches of rainfall producing minor flooding from Kaaawa to Laie. A westward-moving surface trough arrived over the state the following day and dropped another 1 to 2 inches of rainfall over the slopes of Oahu’s Koolau Range.
The days outside of these two upper level low events involved showers embedded within the trade wind flow producing rainfall over the windward slopes on an almost daily basis though daily totals were often below the long term average. One enhanced trade wind shower period involved the remnant moisture of former Hurricane Lane which dissipated in the eastern Pacific. The associated moisture area moved over the island chain on September 23 and 24 and brought increased showers over the windward slopes with maximum daily totals of 1 to 2 inches.
Dry season (May - Sep) maps: [Kauai] [Oahu] [Molokai/Lanai] [Maui] [Big Island]
Island of Kauai : [September 2012 map] [Year-to-date map]
Most of the gages on Kauai recorded below average totals for the month of September. The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 23.96 inches (79 percent of average) and the highest daily total of 3.82 inches on September 9. Lihue Airport’s 0.75 inches (35 percent of average) marked the fourth driest September since the station opened up in 1950.
Rainfall totals for 2012 through the end of September remained in the near to above average range at most locations due to the wet conditions early in the year. However, the percent of average values have been slipping overall with the passing of each drier than average month. Mount Waialeale’s 285.83 inches (98 percent of average) continued to lead all year-to-date totals statewide.
Island of Oahu: [September 2012 map] [Year-to-date map]
Most of the gages on Oahu measured near to above average rainfall totals for the month of September with Koolau sites posting the highest amounts and percent of average values. The USGS’ Poamoho 1 gage had the highest monthly total of 14.85 inches (82 percent of average) followed closely by the Manoa Lyon Arboretum total of 14.76 inches (132 percent of average). For the Manoa Lyon Arboretum site it was the wettest September since 2005. The rain gage at Kahana Stream had the highest daily total of 3.60 inches during the heavy rain event on September 19. Of this total, 3.43 inches fell within the 2-hour period from 5 AM to 7 AM.
Most of the gages on the slopes of the Koolau Range had totals in the near average range for 2012 through the end of September. Year-to-date totals for the rest of the Oahu gages fell into the below average range. The U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge gage had the highest total of 138.44 inches (86 percent of average) which was fourth highest statewide.
Maui County: [Maui September 2012 map] [Year-to-date map] [Molokai/Lanai September 2012 map] [Year-to-date map]
Windward gages had near to above average rainfall totals for the month of September. Leeward Maui rainfall was very spotty and difficult to generalize with some locations having above average totals (e.g. Kula Branch Station) relatively close to sites with below average totals (e.g. Pukalani). Leeward Molokai totals had better consistency since all fell into the below average range. The USGS gage on Puu Kukui had the state’s highest September total of 29.42 inches (133 percent of average) and Maui County’s highest daily total of 3.43 inches on September 9. Kahului Airport’s gage recorded 0.37 inches and while it may not look like much, it was the highest September total since 2000.
Most of the Maui County gages had below average rainfall totals for 2012 through the end of September. Many of the available sites had totals at less than 50 percent of average. Puu Kukui’s year-to-date total of 221.75 inches (79 percent of average) was second highest in the state.
Island of Hawaii: [September 2012 map] [Year-to-date map]
Overall, September rainfall totals across the Big Island were in the below average range. Many windward sites had almost daily measurable rainfall but below average amounts per day. For example, Hilo Airport recorded rainfall on 26 days which is above the September average of 24. However, there were only 2 days with at least 0.50 inches and no days at or above the 1-inch line. Normally, September has 6 days and 2 days at the 0.50 and 1.00 inch thresholds, respectively. The USGS’ gage at Kawainui Stream had the highest monthly total of 14.40 inches (194 percent of average) and the highest daily total of 2.57 inches on September 8.
Most of the windward Big Island gages had near to below average rainfall totals for 2012 through the end of September. Most of the leeward gages remained below 50 percent of average. The Kawainui Stream rain gage had the highest year-to-date total of 155.07 inches (144 percent of average) and remained third highest statewide.
Data Sources: Data used in this report are largely from National Weather Service sources including climate network weather observation stations at Lïhue, 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 Hawai‘i Department of Land and Natural Resources, the US Geological Survey, the US Bureau of Land Management, the US National Park Service, the Department of Defense, and the US Fish and Wildlife Service. Data presented here are not certified and should be used for information purposes only.
Kevin R. Kodama
Senior Service Hydrologist
NOAA/NWS Weather Forecast Office Honolulu