National Weather Service United States Department of Commerce

Monthly Precipitation Summary

State of Hawaii

Month: April 2018

Prepared: May 4, 2018

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

State: [Text data table for rain gages]

April is the last month of the wet season for the Hawaiian Islands. Usually, heavy rain event frequency drops off and rainfall totals decrease. Not this year. The most significant events of the entire wet season, both in terms of hydrologic magnitude and impacts to the public, occurred in mid-April 2018. The events were set up by a large scale environment consisting of an upper level low pressure system which tapped into enhanced moisture embedded within the low level trade winds. These conditions produced torrential rainfall and severe flooding over portions of Kauai and Oahu from late Friday, April 13 through Sunday, April 15, 2018. The rainfall on Oahu came from a mesoscale convective system (MCS) that was moving westward toward Oahu in the trades. Prior to making landfall, the MCS appears to have merged with a leeside convective cloud line streaming off of northwest Molokai and intensified. The area of intense rainfall with rates greater than 5 inches per hour moved across east Oahu between 7:00 PM and 9:00 PM HST on April 13. Although the affected area was localized, the intensity of the rainfall produced tremendous amounts of runoff which produced substantial damage in Waimanalo along the windward slopes of the Koolau Range, and from Aina Haina to Hawaii Kai along the leeward slopes. The MCS dissipated south of Oahu shortly after moving offshore.

The Kauai event began shortly before midnight on Saturday, April 14 when the initial thunderstorm cells moved over northern side of the island. Several waves of thunderstorms through the morning hours resulted in the closure of Kuhio Highway at Haena and at the Hanalei Bridge. The Saturday morning heavy rainfall was not sustained since the cells moved across the area instead of anchoring in one spot. By 8 AM HST, the highway at the Hanalei Bridge reopened as conditions settled down. However, shortly after noon the first of three episodes of intense rainfall initiated over north Kauai. Unlike most heavy rain events which affect the island, this round of rainfall was centered along the lower elevations and coastal sections instead of over the upper slopes. Around this time several landslides between Waikoko and Wainiha closed Kuhio Highway. As rainfall continued through the afternoon, severe flooding washed out the Wainiha Powerhouse Road and destroyed two unoccupied homes. The first intense rainfall episode ended at around 7 PM HST as rainfall eased over north Kauai.

The second intense rainfall episode started around midnight on April 15 with thunderstorms firing up over the north-facing slopes of Kauai. Rainfall rates greater than 5 inches per hour were recorded during the peak of this episode. With heavy rainfall covering most of the northern drainage basins, streams and rivers responded dramatically. Flash flooding in Hanalei River put portions of Kuhio Highway near Hanalei Bridge under 5 to 8 feet of water. Large flood waves in Wainiha River and Kalihiwai Stream inundated several properties as well. At around sunrise, the thunderstorm centroid shifted off the northern drainages and moved over northeast Kauai. While this helped lower water levels in the north part of the island, the shift resulted in damaging flash floods in Anahola Stream and Kapaa Stream, and flooding in other smaller drainages. Thunderstorms rapidly weakened then dissipated around 8 AM HST to end the second episode.
By 10 AM HST, it became evident that the event was not over as rain bands developed east of Kauai and moved over the island. The intensification of these rain bands marked the start of the third rainfall episode, and these thunderstorms became anchored once again over the northern slopes of the island. With the drainage basins already saturated, the resulting flood wave in Hanalei River was very large and put Kuhio Highway under deep water once again. Around 1 PM HST, the thunderstorms shifted to the northeastern portion of the island. Over the next several hours, the storm centroid shifted slowly southwestward putting intense rainfall of over 4 inches per hour over eastern then southeastern Kauai. Significant flooding occurred in the Koloa and Kalaheo areas, with homes off of Wailaau Road sustaining considerable damage. The last thunderstorms of the third rainfall episode dissipated around sunset.

Rainfall totals on Kauai were record-breaking. The highest 24-hour total from the real-time sites was 28.15 inches at Hanalei from 2:00 AM HST April 14 to 2:00 AM HST April 15. The data logger failed at the Hanalei site during the event so the total would have been higher. An observer from the Community Collaborative Rain, Hail, and Snow Network (CoCoRaHS) in lower Wainiha Valley reported 36.49 inches during the period from 7:45 AM HST April 14 through 7:00 AM HST April 15. A couple of days after the event, data from an automated rain gage in Waipa about 1 mile west of Hanalei were retrieved and analyzed. The results showed that this site recorded 49.69 inches during the 24-hour period ending at 12:45 PM HST April 15. This total, if certified, breaks the current U.S. 24-hour record of 43 inches at Alvin, TX on July 25-26, 1979, and the State of Hawaii record of 38 inches at Kilauea (Kauai) on January 24-25, 1956. The National Climatic Extremes Committee is in the process of validating this claim.

It was fortunate and amazing that there were no deaths or reports of serious injuries. The Hawaii Emergency Management Agency reported that 532 homes were affected by flooding on Kauai and Oahu. Damage estimates were already close to $20 million just for public properties. Estimates of damage to private and other properties were not yet available.

Overshadowed by the mid-April events were several other heavy rainfall episodes during the month. On April 2, an upper level disturbance triggered heavy rainfall over Maui County within large scale southwesterly winds in the lower levels. The east half of Molokai received 1 to 4 inches of rainfall which closed Kamehameha V Highway near mile marker 4. Leeward slopes of the West Maui Mountains were impacted by 2 to 4 inches which briefly closed Lower Honoapiilani Road due to a clogged drain. The anomalous low level southwesterly winds ended with the passage of a weak cold front on April 5 and 6 across the state. Rainfall totals from the frontal passage were mostly less than an inch with no reported impacts. While the frontal passage itself was not significant, a passing upper level disturbance tapped into its remnant moisture which produced enhanced shower activity along the windward slopes of the state through April 9.

The final significant rain event of the month occurred on April 26. This bout of heavy rainfall occurred along the windward slopes of Oahu’s Koolau Range as a cold front approached from the northwest. Gages along the slopes of the Koolau Range recorded 4 to 7 inches of rainfall during the event. Stream levels came up but there were no reports of flooding problems from the event. The cold front pushed slowly across the state on April 27 through 29 to close out the month with a cooler, drier air mass.

Island of Kauai : [April 2018 map] [year-to-date map]

All of the gages on Kauai reported above average monthly rainfall totals for April. The U.S. Geological Survey’s (USGS) gage on Mount Waialeale had the highest monthly total of 64.24 inches (170 percent of average). This was also the highest April total at Waialeale since 1982. The highest daily totals were from the April 15 event as discussed above. Records were broken at Anahola, Hanalei, Kapahi, Kokee, Lihue Variety Station, Omao, Wailua Experiment Station, and Wainiha for the highest April rainfall. Lihue Airport had its wettest April since 1971.

Due to the recent wet conditions, most of the gages on Kauai had above average rainfall totals for 2018 through the end of April. Mount Waialeale had the highest year-to-date total of 207.02 inches (166 percent of average). Note that this is already more than half of the annual average for this site.

Island of Oahu: [April 2018 map] [year-to-date map]

Most of the gages on Oahu logged above average rainfall totals for April. The Manoa Lyon Arboretum gage had the highest monthly total of 32.53 inches (231 percent of average). The USGS’ Poamoho Rain Gage No. 1 had the highest daily total of 6.59 inches from the April 26 heavy rain event. April rainfall records were broken at Ahuimanu, Hawaii Kai Golf Course, Kamehame, Luluku, Manoa Lyon Arboretum, Maunawili, Niu Valley, Nuuanu, Olomana Fire Station, Palolo Fire Station, and Poamoho. The Luluku and Maunawili totals broke records that were set only last year. Elsewhere, Waipio and Moanalua had their highest April totals since 1998 and 2001, respectively.

Oahu rainfall totals for 2018 through the end of April were near to above average at most of the sites. A few of the gages along the lower leeward slopes of the Waianae Range had near to below average totals. The Poamoho Rain Gage No. 1 had the highest year-to-date total of 105.51 inches (137 percent of average).

Maui County: [Maui April 2018 map] [year-to-date map] [Molokai/Lanai April 2018 map] [year-to-date map]

Almost all of the gages across Maui County posted above average April rainfall totals. The USGS’ gage on Puu Kukui had the highest monthly total of 54.75 inches (142 percent of average). This was also the highest April total at this location since 1998. The USGS’ West Wailuaiki gage had the highest daily total of 12.93 inches on April 9. Records for the wettest April were broken at Hana Airport, Kihei, Lahainaluna, Mahinahina, and Waikapu Country Club. The Hana Airport and Lahainaluna records were last broken just last year.

Rainfall totals for 2018 through the end of April were near to above average at most of the gages across Maui County. The West Wailuaiki gage had the highest year-to-date total of 110.56 inches (124 percent of average).

Island of Hawaii: [April 2018 map] [year-to-date map]

April rainfall totals were near to above average at most of the gages on the Big Island. The main exceptions were in the southern Kau District where several totals were in the near to below average range. The USGS’ Saddle Road Quarry gage had the highest monthly total of 48.54 inches (333 percent of average). The Pahoa gage had the highest daily total of 9.37 inches on April 15. Records for the wettest April were broken at Glenwood, Pahoa, and Piihonua. Kamuela, Mountain View, Pahala, and Waiakea Uka had their highest April totals since 2004, and Kamuela Upper had its wettest April since 1998.

Big Island rainfall totals were mostly near to above average for 2018 through the end of April. The few below average totals were scattered across various parts of the island. The Saddle Road Quarry gage had the highest year-to-date total of 147.62 inches (288 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