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NOAA > NWS > WFO HFO Home Page > Hydrology > May 2013 Precipitation Summary
May 2013 Precipitation Summary
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State of Hawaii

MONTH: May 2013

PREPARED: June 5, 2013

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

State: [Text data table for rain gages]

A weather pattern which looked more like conditions found during the heart of the wet season produced a below average number of trade wind days for May and above average rainfall totals for many locations across the state.  Just over half of the days in the month involved trade winds whereas an average frequency would be just over 75 percent.

The most significant rain event occurred during the last week of the month when a surface trough and upper level low pressure system combined to produce unstable conditions over the main Hawaiian Islands.  Highly localized and persistent heavy rainfall developed during the early morning hours of May 28 along Oahu’s Koolau Mountain Range and continued for about 24-hours.  Several gages in the central Koolaus recorded 15 to 20 inches of rainfall during the event and streams draining both the windward and leeward slopes rose considerably.  Only one, Kalihi Stream, was reported to have overflowed its banks within private property due to clogged culverts.  It was near this location that four people swimming within the stream were swept away but were fortunately rescued with relatively minor injuries.

Other notable weather events during May involved two unseasonably late cold front systems which affected the island chain on May 5 and 6, and from May 18 through 20.  The early May cold front dropped 1 to 3 inches of rain on Kauai, Oahu, and Maui County but did not cause any flooding problems.  The second cold front was part of a complex low pressure system far to the north of the state.  Rainfall associated with the cold front itself did not amount to much, but the unstable conditions over the island chain prior to the front’s arrival helped generate heavy afternoon rainfall over the leeward portions of the Big Island and Oahu on May 18 and 19.  The Big Island rainfall mainly focused on the South Kohala District and caused minor flooding problems near the Saddle Road Junction and near the 8 to 10 mile markers on the Kohala Mountain Road.  Rather than causing problems, this late season rainfall brought some welcome relief to an area that has been plagued by persistent drought conditions for many months.

A final rainfall event worth noting occurred from May 11 through 13 as a result of remnant moisture from a dissipated cold front embedded within the trade winds.  The enhanced rainfall mainly affected the windward slopes of the state with the highest amounts in the range of 4 to 6 inches recorded on Maui and Oahu.  One of the rainfall surges during the afternoon of May 12 pushed Waikane Stream out of its banks and covered Kamehameha Highway with several inches of water.  Outside of this impact there were no other reports of significant damage or injuries.

Island of Kauai : [May 2013 map] [year-to-date map]

Most of the rain gages on the island of Kauai posted near to above average rainfall totals for the month of May.  The main exceptions were along the lower elevations of the southeastern and southern slopes from Lihue to Waimea where the monthly totals checked in at less than 50 percent of average.  The U.S. Geological Survey’s (USGS) Mount Waialeale gage had the highest monthly total of 34.41 inches (112 percent of average) and the highest daily total of 12.69 inches on May 13 associated with the remnant cold front moisture in the trades.

A majority of the gages on Kauai have recorded near average rainfall totals for 2013 through the end of May.  Mount Waialeale had the highest total of 133.29 inches (86 percent of average) and retained its rank as the highest total statewide.

Island of Oahu: [May 2013 map] [year-to-date map]

Most of the gages across Oahu reported above average monthly rainfall totals.  Several sites had more than double the long term May average.  The USGS’ Poamoho Rain Gage 1 recorded the state’s highest monthly total of 41.08 inches (237 percent of average), which also marked its wettest May on record.  Record high monthly rainfall totals also came from the gages at Wilson Tunnel (27.04 inches, 347 percent of average) and Luluku (18.13 inches, 312 percent of average).   

The highest daily total was from the USGS’ Moanalua Rain Gage which logged 21.22 inches for May 28 during the Koolau heavy rain event.  A more detailed breakdown showed hourly accumulations of 1 to 2 inches with a peak value of 3.62 inches, and a maximum 6-hour total of 12.36 inches.  While these are impressive rainfall numbers, it appears that the rainfall was distributed sufficiently over time and mainly confined to the upper elevations which helped prevent Moanalua Stream from overflowing its banks.  In contrast, the December 7, 2003 flash flood event had intense rainfall covering the entire Moanalua Stream basin which produced a much greater stream flow response and widespread damage to the Mapunapuna industrial area.

Most of the gages on Oahu had near to above average rainfall totals for 2013 through the end of May.  The U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge gage had the highest year-to-date total of 105.14 inches (117 percent of average) and moved up to the third highest total in the state.

Maui County: [Maui May 2013 map] [year-to-date map] [Molokai/Lanai May 2013 map] [year-to-date map]

After a very dry April, most of the Maui County gages recorded above average rainfall totals for the month of May.  The USGS’ Puu Kukui gage had the highest monthly total of 20.93 inches (75 percent of average) and the highest daily total of 4.96 inches on May 12.  Hana Airport’s 13.36 inches (239 percent of average) marked this site’s wettest May since 1993 and only the fifth time since 1951 where this site’s May total exceeded 10 inches.  Kahakuloa (8.46 inches, 224 percent of average) had its wettest May on record, and Wailuku and Kula Branch Station had their wettest May since 1992 and 2004, respectively.  Interestingly, Ulupalakua Ranch finished the first month of the 2013 dry season with 3.07 inches (166 percent of average) which is just over three-fourths of the total recorded at this site during the entire wet season from October 2012 through April 2013.

The wet conditions in May brought a few site totals back into the near average range for 2013 through the end of May though most remain firmly below average.  The Puu Kukui gage has recorded 117.73 inches (73 percent of average) so far in 2013 which is second highest in the state.

Island of Hawaii: [May 2013 map] [year-to-date map]

Most of the gages on the Big Island, even those on the leeward side, reported near to above average monthly rainfall totals for May.  The Waikii gage in the upper slope of the South Kohala District recorded its highest May total on record.  Other notable totals came from gages at Pahala (4.67 inches, 216 percent of average) and Kahua Ranch (5.87 inches, 191 percent of average) which had their highest May totals since 2002 and 1998, respectively.  The PTA Keamuku gage had the highest daily total of 3.20 inches on May 18.  Of this total, 2.25 inches occurred between 4 PM and 5 PM HST. 

Rainfall totals for 2013 through the end of May were in the near to below average range across the Big Island.  The Waiakea Uka gage had the highest year-to-date total of 63.40 inches (77 percent of average) but fell two notches to ninth 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 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