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April 2013 Precipitation Summary
State of Hawaii
MONTH: April 2013
PREPARED: May 3, 2013 – updated on May 22 to add wet season data
Note: This summary uses the arithmetic mean, or average, for “normal” rainfall values.
State: [Text data table for rain gages]
The month of April continued the March pattern of below normal trade wind frequency with just over a third of the days having large-scale low level winds from the northeast through east directions. On average, the trades blow across the island chain roughly 70 percent of the days in April.
Instead of trade winds, the west half of the state received two weak cold front passages which served as reminders that April is still very much within Hawaii’s wet season. The first cold front began affecting Kauai on April 13 as unstable conditions produced strong thunderstorms over the coastal waters and periods of moderate to heavy rainfall over many areas of the island. The areas of rainfall moved fast enough to prevent significant flooding problems. The front itself reached Kauai on April 14, moved slowly over Oahu, and then dissipated near Molokai on April 15. About an inch of rain fell on Oahu during the frontal passage which caused only minor flooding problems.
The second front affected the west half of the state about a week later. Including the passage of the pre-frontal rain bands and the frontal cloud band, about 1 to 2 inches of rain fell on Kauai and Oahu from April 21 through 23 as the front moved slowly eastward before dissipating near the Kaiwi Channel between Oahu and Molokai. Interestingly, it was the remnant of the front that caused the most problems as it moved back over Oahu in the returning trade winds on April 24. Heavy rainfall occurred over the central Koolau Range during the early nighttime hours with an intense rain core remaining nearly stationary over Waikane Valley. A U.S. Geological Survey (USGS) gage on Waikane Stream measured a water level rise of nearly 7 feet in an hour, and flood waters forced the closure of Kamehameha Highway near the Waikane Stream Bridge for several hours. Fortunately there were no significant injuries reported though several vehicles became stranded in the rapidly rising waters.
The lack of trade winds and the inability of the weak cold fronts to push across the entire state resulted in very dry conditions over most of Maui and the Big Island. April is usually one of the wettest months of the year for the windward slopes of both islands because there is normally an abundance of rain shower activity embedded within the trade wind flow.
The end of April also marked the end of the 2012 – 2013 Hawaiian wet season, or hooilo. Based on an average of rankings from 8 long term sites across the state, this wet season ended up as the driest in the past 30 years. The highest single-site ranking was only 20th at Lihue Airport. Ulupalakua Ranch in Upcountry Maui had its driest wet season not just in the past 30 years but in its entire record going back to 1925. To some this may not seem accurate, especially considering the occasional rainy conditions during March and April for residents in the west half of the state. However, recall that October and November 2012 were both very dry with 19 sites setting new records for the driest October and 24 sites having the driest November ever. Drought coverage actually increased in the state for the first two months of the wet season. October and November usually have the highest frequencies of flash flood events during the year but no flash flood events occurred in the Hawaiian Islands until late December. A few heavy rain events occurred during the latter half of the wet season but areas of extreme drought managed to persist through a fifth consecutive wet season on the Big Island and a second consecutive wet season on Maui.
October 2012 - April 2013 wet season rainfall maps: [Kauai] [Oahu] [Molokai/Lanai] [Maui] [Big Island]
Island of Kauai : [April 2013 map] [year-to-date map]
The two late-season cold fronts produced above average rainfall totals for gages in west and south Kauai, but the lack of trade wind showers resulted in below average totals for the gages on the east-facing slopes. Lihue Airport (3.05 inches, 136 percent of average) had its wettest April since 2000 and the Hanapepe gage (3.63 inches, 189 percent of average) recorded its highest April total since 2004. Despite receiving only 45 percent of the April average rainfall, the USGS gage on Mount Waialeale still posted the highest monthly total in the state with 16.81 inches. This total marked the driest April at this site since 1992.
Most of the gages on Kauai have observed near average rainfall totals for 2013 through the end of April. The USGS’ Mount Waialeale gage had the highest year-to-date total of 98.88 inches (79 percent of average) and passed Maui’s Puu Kukui for the highest total statewide.
Island of Oahu: [April 2013 map] [year-to-date map]
Almost all of the west Oahu gages recorded above average monthly rainfall totals while gages on the east-facing slopes mainly had near average totals. The USGS’ Poamoho Rain Gage No. 1 posted the highest monthly total of 16.72 inches (76 percent of average) while the U.S. Fish and Wildlife Service’s Oahu Forest National Wildlife Refuge (NWR) gage had the highest daily total of 6.85 inches on April 24. West Oahu gages in Waianae and Lualualei recorded their highest April rainfall totals ever in records dating back to the early 1990s.
Most of the gages on Oahu had near average rainfall totals for 2013 through the end of April. The Oahu Forest NWR gage had the highest year-to-date total of 76.64 inches (105 percent of average) which was the fourth highest total in the state.
Maui County: [Maui April 2013 map] [year-to-date map] [Molokai/Lanai April 2013 map] [year-to-date map]
Very dry conditions across Maui County during April. All of the gages on the island of Maui had monthly totals below 30 percent of average with leeward Maui totals at less than 10 percent of average. The lack of trade winds and the inability of the cold fronts to reach Maui resulted in Puu Kukui and Hana Airport both having their lowest April totals on record.
The dryness over the past two months has pushed most of the Maui County rainfall totals into the below average range for 2013 through the end of April. Puu Kukui’s year-to-date total of 96.80 inches (72 percent of average) was the highest in the county and was second highest in the state.
Island of Hawaii: [April 2013 map] [year-to-date map]
Most of the gages on the Big Island recorded below average rainfall totals for the month of April. The USGS gage on Kawainui Stream had the highest monthly total of 5.16 inches (33 percent of average). This location also recorded the highest daily total of 1.61 inches on two days, April 8 and 18. The gages at Waiakea Uka, Piihonua, and Pahoa set new records for the driest April, and the gages at Hilo Airport and Mountain View had their second driest April on record. The Hilo Airport monthly total of 2.97 inches missed the all-time April record set in 1962 by only 0.04 inches. This difference is on the scale of a passing shower lasting a couple of minutes.
Despite the dryness of March and April, several windward Big Island gages still maintained near average totals for 2013 through the end of April. This was mainly due to the very wet conditions in February. The rest of the sites across the island had below average year-to-date totals. The Waiakea Uka total of 50.25 inches (75 percent of average) remained as the highest total on the island but dropped one rank to seventh 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