Dr. Jeff Masters ·

Above: Flooding in the vicinity of Hanalei, Kauai, Hawaii on April 15, 2018 after a record 24-hour rainfall of 49.69” inundated the area. Image credit: Facebook video by Petty Officer 3rd Class Brandon Verdura, U.S. Coast Guard.

The National Weather Service in Hawaii reported on Wednesday that preliminary data from a rain gauge on the north shore of Kauai at Waipa, one mile west of Hanalei, indicate that 49.69” of rain fell over the 24-hour period ending at 12:45 pm April 15. If verified, this would break the all-time U.S. 24-hour rainfall record of 43.00” in Alvin, Texas set on July 25 – 26, 1979, during Tropical Storm Claudette. The record-setting rains on Kauai were due to an upper-level low located to its west on April 14 - 15, combined with a surge of rich low-level moisture. This set-up brought radar-estimated rainfall rates of 2 – 4” per hour to the north shore of the island.

The National Weather Service office in Honolulu noted that the rain gauge where this new data was downloaded from "is operated by the Waipa Foundation which is a non-profit organization. Data from the gauge are not telemetered for real-time display and are used for watershed modeling and monitoring studies." In the coming months, data from this gauge will be reviewed by the National Climatic Extremes Committee to determine whether this instrument is reliable enough to accept as a new U.S. record.

The April 14 – 15, 2018 event also brought a 25.92” 2-day precipitation total for the perennial Hawaiian wet spot Mt. Waialeale, Kauai, and a 24-hour total of 27.52” to Hanalei, Kauai. Personal weather station KHIHANAL2 in Hanalei recorded 23.05” on April 14, and additional heavy rain on April 15 until 4:18 am, at which time the station stopped sending data. The owner of this gauge noted in their “Status” message, though, that they received 34.73” of rain on April 15, and it is possible that this gauge received a 24-hour precipitation total approaching the 49.69” measured two miles to its west, at Waipa. I contacted the owner of the gauge, and he commented that "having years of experience with the WU, VWS and my WMR200’s I am confident and stand behind my readings 100%. Being a surfer, we follow the weather world wide and especially in Hawaii waters for surf predictions."

Figure 1. MODIS satellite image from the Terra satellite taken on the afternoon of April 14, 2018, as an upper-level low located to the west of Kauai, Hawaii brought intense rainfall rates of 2 – 4” per hour and destructive flooding. A potential all-time 24-hour U.S. record precipitation amount of 49.69” fell at a location one mile west of Hanalei, Kauai. Image credit: NASA.

Hawaii holds multiple all-time U.S. precipitation records

Hawaii is no stranger to record rains, due to the warm tropical waters surrounding the islands, which can feed large quantities of moisture into thunderstorms that form over the steep topography. The previous state 24-hour precipitation record for Hawaii was also set on Kauai—38.00” at Kilauea on January 24 – 25, 1956. Kauai also holds the U.S. record for heaviest 1-month precipitation (148.83” at Mt. Waialeale in March 1982). Locations on other Hawaiian islands hold the U.S. record for 1-year precipitation (704.83” at Kukui, Maui in 1982), and 4-day and 8-day precipitation (62” and 82”, respectively, at Kukaiau on the Big Island). According to wunderground weather historian Christopher C. Burt, the 1982 record for 1-year precipitation on Maui was the last time that the U.S. broke an all-time precipitation record for a single location.

Figure 2. All-time U.S. precipitation records for a single location, as catalogued by wunderground weather historian Christopher C. Burt in his book, Extreme Weather: A Guide and Record Book. Note that many of these records are not officially recognized by NOAA, and the all-time record for most rainfall from a U.S. tropical cyclone is not listed here. In August 2017, Hurricane Harvey dumped a storm total of 60.58” at Nederland, TX, which was the heaviest single amount ever recorded from a tropical cyclone or its remnants in the U.S. The old record was 52”, set by Hurricane Hiki in Hawaii in 1950.

Second largest world 24-hour rainfall not from a tropical cyclone

The potential new U.S. 24-hour rainfall record will fall well short of the world record for 24-hour rainfall: 71.8” at Foc-Foc, La Réunion Island, in the South Indian Ocean, on January 7 – 8, 1966. Those rains fell during Tropical Cyclone Denise, which was a tropical storm with 40 mph winds at the time. However, the 49.69” that fell on Kauai on April 14 – 15 would be the world’s 2nd greatest 24-hour rainfall for a non-tropical storm event, according to Mr. Burt. The only greater 24-hour rainfall event not from a tropical cyclone was a 64.17" rainfall on November 2 - 3, 1999, in Central Vietnam. There is also a very dubious report of 55.12” from Muduocaicang in China's Inner Mongolia Province on Aug. 1, 1977. All of the other world 24-hour rainfall events greater than 50” have occurred during tropical cyclones. Here is Mr. Burt's list of all-time heaviest 24-hour rainfall events, taken from a 2014 blog post:

71.85” Foc-Foc, La Reunion (Tropical Storm Denise, Jan. 7-8, 1966 (the 73” figure for Cliaos has been discredited)

66.49” Belouve, La Reunion (tropical storm Feb. 27-28, 1964)

64.33” Isla Mujeres, Mexico (Hurricane Wilma, Oct. 21-22, 2005)

64.17” Truoi, Central Vietnam on November 2-3, 1999 (non-tropical cyclone related rains)

62.33” Aurere, La Reunion (tropical cyclone April 7-8, 1958)

118.13” in two days at Baril 1600 site on La Reunion on Feb. 27-March 1, 1993 (tropical storm related, figure not broken down into single day or 24-hour periods—but at least 59.06” on one day or the other)

55.20” Weiliaoshan, Taiwan (tropical cyclone Aug. 8, 2009)

55.04” Commerson, La Reunion (tropical cyclone Feb. 25, 2007)

51.85” Kaikawa, Takushima, Japan (tropical storm Aug. 1, 2004)

49.69” Waipa, Hawaii (non tropical-cyclone related April 14-15, 2018, if verified)

Note that Cherrapunji, Meghalaya, India measured 98.15” of monson rainfall in 48 hours on June 15-16, 1995. This figure was not broken down by daily totals or 24-hour period totals but it does mean 49.07” must have fallen on at least one of the two consecutive days, so it is likely that one of those days or 24-hour periods saw more rain than Waipa.

$100 million in damage on Kauai from April’s floods

The floods from the record rains caused numerous landslides and flash flooding along Kauai’s north shore, which blocked the Kuhio Highway and washed out multiple bridges. According to the weather.com write-up on the disaster, at least four houses were destroyed along the north shore, and Red Cross volunteers told the AP more homes were probably destroyed in Koloa on the southern end of the island. Dozens more homes were reported flooded or otherwise damaged. Hawaii lawmakers approved $125 million in disaster funding to repair infrastructure damaged in the April floods, with Kauai receiving $100 million of that funding. The other $25 million will go to the other Hawaiian islands affected by the floods.