CoCoRaHS 36"+, 24-hour Rainfall in Hawaii Confirmed

Lost in the coverage of heavy snow in the Midwest and wildfires in Oklahoma this past week was an incredible rain event that occurred in Hawaii over the weekend. The rain fell beginning late on April 13 and continued through Sunday, April 15. In produced severe flooding over portions of Hawaii, particularly over east Oahu on Friday night and across Kauai from late Saturday through Sunday.




The entire island was under a flash flood warning on Saturday and Sunday.Here are the 24-hour rainfall amounts from Kauai on Sunday morning, April 15.

Throughout the storm on the night of April 14 until the morning hour of April 15, the CoCoRaHS observer at HI-KI-24 made several trips out to the rain gauge to measure than rain before it over-topped the gauge. Nolan Doesken from CoCoRaHS headquarters contacted the observer to get more details on this incredible event. Here is a timeline of what the observer measured and reported.

HI-KI-24 Hanalei 3.0 W (Wainiha)
Saturday, April 14, 2018

Time HST	Precip, in.	Duration (hrs)
7:30 a.m.	2.96	24
2:20 p.m	5.00	6.8
5:30 p.m	7.55	3.2
7:00 p.m.	7.33	1.5

Local damage began to occur. After this observation the observer evacuated the house and stayed in their car which was parked up the hillside from the house along the road.


Sunday, April 15, 2018

Time HST	Precip, in.	Duration (hrs)
1:30 a.m.	Gauge was nearly full. Observer did not measure precisely at this time, but conservatively estimated 10.00 inches of rain since 7:00 p.m.
6.45 a.m.	6.55	5.25

Monday, April 16, 2018

Time HST	Precip, in.	Duration (hrs)
6:30 a.m.	2.14	23.75

The total Sunday morning from the incremental measurements was 36.43 inches. The final total of 36.49 inches includes an estimate of what might have been lost during to spillage during the incremental measurements (the observer does not have an extra outer cylinder). He made the measurements with the help of a friend, wading out to the rain gauge for the measurements and making them in the rain. Nolan indicates the observation is fully valid and if anything may be a bit of an under-report.

The observer commented that based on the location of thunder and lightning, the heaviest part of the storm seemed to be a few miles east of his location. This observation seems to be borne out by the radar estimated precipitation. The maximum amount on the Storm Total Precipitation image below is 27.5 inches.We know that the total rainfall was considerably higher. The location of CoCoRaHS rain gauges are plotted on the radar image.

Radar estimated precipitation for the 50-hour period ending
at 2:13 p.m.HST April 15, 2018.

While the observer's house barely escaped flood damage, there was damage close by. One notable comment from the observer was that somewhere around 7:00 p.m. the roar of the nearby Wainiha River began dominating over the sound of thunder to the east.Two nearby houses were washed away, and a large 18-inch diameter log ended up in a neighbors living room. The road was washed out in three places. Roads and homes were flooded across much of eastern Kauai, and as of April 18 more than 425 people had been evacuated.

 U.S. Coast Guard video of overflight of area near Hanalei Bay, Kauai, April 15, 2018

What caused this heavy rain was the interaction of an upper level low and tropical moisture. On Friday a trough was crossing north of Hawaii, but the bottom of the trough cut off and became a closed low. This closed low then drifted westward across Hawaii, tapping into low level moisture fueling the storms.

Here is a three panel image that shows the 500 millibar analysis beginning at 2:00 a.m. HST on April 14 showing the low drifting west, followed by a chart of the total precipitable water.

Total Precipitable Water at 2:00 a.m. HST April 15. The Hawaiian Islands are circled.

The National Weather Service in Honolulu reported that the rain gauge in the town of Hanalei collected 28.15 inches of rain from early Saturday morning into Sunday morning (April 15) before the gauge stopped working at 2:00 a.m. HST. The 24-hour record for that location is 28.54" set on March 5, 2012. It's highly likely that this record was broken.

In Wainiha, where our CoCoRaHS observer is located, an automated rain gauge recorded 32.35 inches of rain for the 48-hour period ending at 6 PM HST on April 15, 2018. This rain gauge recorded 19.54" for the 24 hour period the previous record for a 24-hour total in Wainiha was 16.70 inches on January 6, 1969. The confirmed CoCoRaHS total more than doubles that record. The total rainfall for HI-KI-24 for the 72-hour period ending at 6:30 a.m. HST on April 16 was a whopping 41.59 inches.

The 24-hour record rainfall for Hawaii is 38.00 inches at Kilauea Sugar Co. Plantation (Kauai) on January 24-25, 1956. It is an estimate, but is listed as the record because it was obtained from a reliable rain gauge and is deemed conservative by an inch or more.