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May 2014 Precipitation Summary
State of Hawaii
MONTH: May 2014
PREPARED: June 5, 2014
Note: This summary uses the arithmetic mean, or average, for “normal” rainfall values.
State: [Text data table for rain gages]
While April produced nearly uninterrupted trade winds, the month of May was noteworthy for its lack of trades during a time of the year when the trade wind frequency should be more than 70 percent. The month started with moderate to fresh trades. However, a persistent area of low pressure far to the north and northwest of the state pushed the subtropical high pressure area to the east of its normal position and weakened its associated pressure ridge. This caused a shift in the low level winds to a southeasterly direction near the island chain starting on May 2 and lasting through May 10. In these situations, the Big Island’s mass prevents large scale southeasterlies from reaching the smaller islands which enables land and sea breezes to dominate local circulations. Rainfall also has a higher likelihood of occurring in the interior sections during the afternoon hours instead of mainly favoring the windward slopes.
On May 11, a late season shear line brought a brief but welcome change in weather conditions as it moved across Kauai and Oahu, then stalled and dissipated near the Big Island on May 13. The shear line brought back trade wind conditions with some spots along the northeast-facing slopes receiving 1 to 2 inches of rain. No significant flooding occurred during this event.
Southeasterly low level winds returned on May 15 and persisted through May 25 bringing a humid air mass along with land and sea breezes on the smaller islands. A strong upper level low pressure system dropped into the area northwest of the main Hawaiian Islands on May 23 which generated unstable conditions favorable for heavy rainfall. Daily totals in the range of 1 to 3 inches occurred on Kauai but there were no reports of significant flooding. The most intense rainfall took place over Oahu on May 25 from thunderstorms triggered by afternoon heating and enhanced by the upper level low. Several Oahu gages registered 2 to 4 inches, most of which occurred within a 3-hour period from 1 PM to 4 PM HST. The gage at Pacific Palisades recorded the highest rainfall intensity of 1.17 inches in the 15-minute period from 1:45 to 2:00 PM HST. Stream gages showed rapid increases in water levels and there were several reports of significant road ponding but there were no reports of road closures or property damage. Trade winds finally returned on May 26, Memorial Day, and persisted at moderate to fresh levels through the remainder of the month. Afternoon thunderstorms flared up over the Kona slopes of the Big Island on May 26 as the upper level low lingered above the trade winds to the southwest of the state, but the heavy rainfall produced only minor flooding issues.
Island of Kauai : [May 2014 map] [year-to-date map]
Most of the gages on Kauai recorded above average rainfall. The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 17.10 inches (56 percent of average) and the highest daily total of 3.27 inches on May 10. Some of the Kauai sites may not see this much rain again in May for quite some time. For example, the gages at Mana (6.42 inches, 738 percent of average) and Puu Opae (9.70 inches, 591 percent of average) posted their highest May totals on record. However, the data set for both sites goes back only to 2010, which is a small sample size. Data from 1905 to 2000 for a discontinued gage near the current Mana site provides a picture of the distribution of monthly rainfall totals over the long period of time. In its nearly 100-year record, the old Mana gage posted a May total more than 6.00 inches only twice, with a record value of 6.63 inches in 1980. So it appears safe to say that the 6.42 inches at the current Mana gage is rather exceptional.
Rainfall totals for 2014 through the end of May were near to above average at most of the gages across Kauai. The Mount Waialeale gage (95.66 inches, 61 percent of average) finally passed the USGS’ Kilohana gage (94.97 inches, 115 percent of average) for the highest year-to-date rainfall total on Kauai. This total was fourth highest in the state.
Island of Oahu: [May 2014 map] [year-to-date map]
All of the gages on Oahu posted near to above average May rainfall totals. The U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge (NWR) gage showed the highest monthly total of 22.57 inches (131 percent of average) while the Pacific Palisades gage had the highest daily total of 4.48 inches on May 25. In addition to the previously mentioned 1.17 inches in 15 minutes, the Palisades gage recorded a one-hour total of 1.73 inches between 2 PM and 3 PM HST and 3.05 inches between 1 PM and 3 PM HST during the May 25 event. The Waiawa CF gage had a higher one-hour total of 1.85 inches during the same 2 PM to 3 PM HST period.
Honolulu Airport’s 3.35 inches registered as the wettest May since 1978. Waipio, Kunia, and Lualualei posted their highest May totals in a data record going back to 1991.
Most of the gages on Oahu had rainfall totals for 2014 through the end of May in the near to above average range. Wet May conditions have eliminated the rainfall deficits that were starting to build in April. The Oahu Forest NWR gage had the highest year-to-date total of 105.39 inches (117 percent of average) and moved up to second highest in the state.
Maui County: [Maui May 2014 map] [year-to-date map] [Molokai/Lanai May 2014 map] [year-to-date map]
A majority of the gages in Maui County reported near to above average monthly rainfall for the month of May. Lower elevation gages along the south- and west-facing slopes had below average totals. The USGS’ gage on Puu Kukui had the highest monthly total of 16.59 inches (60 percent of average) and the highest daily total of 5.18 inches on May 2. Molokai Airport’s total of 4.51 inches was the second highest May total on record for this location. Pukalani’s total of 3.28 inches and Waikapu Country Club’s 2.62 inches marked the highest May total in records going back to 1992 and 1993, respectively.
All of the available Maui County rainfall totals were in the near to above average range for 2014 through the end of May. Puu Kukui’s 149.22 inches (92 percent of average) was the highest year-to-date total in the state.
Island of Hawaii: [May 2014 map] [year-to-date map]
After the wet conditions in April, windward locations dried out in May with near to below average monthly totals. In contrast, many leeward spots indicated wetter than average conditions for this time of year. The highest monthly total came from the U.S. Department of Agriculture’s Island Dairy gage with 12.48 inches (117 percent of average). The highest daily total of 5.24 inches came from the USGS’ rain gage at Kawainui Stream on May 1. Following its driest April since 1996, the Kealakekua gage (9.76 inches, 182 percent of average) had its wettest May in a data record going back to 1991.
Most of the gages on the Big Island had 2014 rainfall totals through the end of May in the near to above average range. The USGS’ rain gage at Kawainui Stream had the Big Island’s highest year-to-date total of 96.72 inches (142 percent of average) but dropped down to 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 and Ua Net networks 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