Mauka Showers

It’s early Sunday morning on July 20 before the feral chickens start crowing. The text notification chime on my phone awakens me. I check and it’s a WaterAlert notification from the U.S. Geological Survey (USGS) telling me that the Hanalei River water level went above the threshold that can result in the inundation of Kūhiō Highway. What the heck? I actually used different words. I realized that after retiring from the NWS, I forgot to turn off the WaterAlert notifications for several key USGS gages. In a previous chapter of my life, after receiving the notification I would have looked at the hydrograph for the alerting gage and checked the radar data to see if the alert looked valid. If it looked valid, I would see how nuts the weather situation looked. If it was bad, I would go into the office to help the duty forecasters monitor the situation and issue warnings and advisories as needed. However, that chapter closed so I went back to sleep. It’s nice to be retired!

I checked on what was happening over Kauaʻi after I woke up at my normal time. By then, a Flash Flood Warning from the Honolulu Forecast Office was already in effect and Kūhiō Highway inundated and closed. Welcome to Hanalei River, your platinum status frequent flash flooder for the State of Hawaiʻi. As far as I know, there is no other basin across the state that has a higher frequency of significant flash flood impacts. The closure of Kūhiō Highway effectively cuts off access to the rest of the island for hundreds of residents and visitors west of the Hanalei River Bridge, so a Hanalei River flood is a big deal locally.

The image at top, from the County of Kauaʻi on the morning of July 20, shows the bridge in the upper left, and the inundated highway extending to the center right. It’s a fairly robust flood event if the highway is inundated all the way to the slope leading up to the bridge. This can also be seen in the hydrograph below from the USGS. The Hanalei River Bridge water level sensor is actually out of the water under normal flow conditions, and only starts to pick up the water level height for significant flows. When the stage level at the bridge reaches 7.3 feet, it usually means that the river will begin to overflow onto Kūhiō Highway several minutes later at a low spot roughly half a mile west-southwest of the sensor. The stage value peaked at nearly 12 feet over three hours later. The upstream gage showed that the river level exceeded the alarm threshold at 3:45 AM HST and stayed elevated until around 8 AM.

 USGS hydrograph for the Hanalei River Bridge gage. Around 4:20 AM, the water level reaches the sensor and steadily climbs and peaks a little over three hours later.

 USGS hydrograph for the Hanalei River a couple of miles upstream from the bridge gage. Flood waves at this gage take roughly 45-60 minutes to reach the bridge.

So what was the nature of the rainfall that produced such a “hefty” flood event? The graph of hourly rainfall from Mt. Waiʻaleʻale (below) shows a total of 10.71 inches during a 24-hour period. Even though Waiʻaleʻale is one of the wettest spots in the state, this amount for a one-day period is still quite respectable. Enhanced rainfall occurred through the sample period, but noticeably intensified after 10 PM on July 19. Rain rates remained elevated then abruptly ended after 7 AM on July 20.

Hourly rainfall from the USGS’ rain gage on top of Mt. Waiʻaleʻale for the 24-hour period from 8 AM, July 19 through 8 AM, July 20, 2025. The hourly amounts appear at the top of each bar. Times on the x-axis indicate the end time for each 1-hour accumulation period.

In terms of the weather pattern, a low pressure trough aloft was northwest of Kauaʻi, with a subtropical jet axis above 30,000 feet over the state. Large-scale rising motion from these features may have helped destabilize the atmosphere by eroding a mid-level stable layer and increasing the depth of the lower level moist layer. This can be seen in the changes in the temperature and dew point profiles from the Līhuʻe Airport balloon sounding data from 2 AM to 2 PM on July 19. Low- to mid-level winds from the sounding were mostly from the east to east-southeast up to around 10,000 feet. Radar data from the Kauaʻi WSR-88D showed rainfall moving generally from the east-southeast, consistent with the winds from the sounding. The most intense rainfall cores (orange and red) are mostly missing the Hanalei River basin, outlined in black.

Plot of data from the Līhuʻe Airport balloon sounding at 2 AM HST, July 19. Vertical lines are temperature (right) and dew point (left). Wind barbs at various levels are plotted on the right. The red box highlights a stable layer that erodes over the next several hours. Plot downloaded from the Univ. of Wyoming Atmospheric Science Radiosonde Archive.

Same as the previous figure, but for 2 PM HST, July 19. Note that the mid-level stable layer (red box) has eroded and the moist layer has deepened. The blue box highlights the area where the wind direction changes over the next 12 hours.

Loop of composite reflectivity from the South Kauaʻi WSR-88D radar. The loop covers the period roughly from 10 PM HST, July 19 to midnight, July 20. Red and orange pixels indicate the most intense rainfall areas. The black outline in the north center of Kauaʻi is the Hanalei River basin boundary. The WSR-88D data are from the National Centers for Environmental Information (NCEI) radar archive.

By 2 AM on July 20, the sounding plot below shows south winds extending down to about 6500 feet. This change in the southerly component in the cloud layer winds is reflected in the change in the radar echo motion. The radar data loop from 3 AM to 5 AM shows cells aligned in a more north-south direction and directly over the Hanalei River basin. The cells are also more intense (more red pixels!), with echo tops (not shown here) reaching above 30,000 feet. The shift in rainfall orientation over the basin and the intensification of the rainfall cores resulted in an increase in flow rate that ultimately inundated Kūhiō Highway.

Same as previous sounding plots, but for 2 AM HST, July 20. The blue box shows the heights where winds veered to southerlies over the past 12 hours.

Same as the previous radar loop, but covering the period roughly from 3 AM to 5 AM HST, July 20.

The shallow eastly winds veering to southerlies within the cloud layer aloft shares similarities to the record-breaking April 14-15, 2018 north Kauaʻi flash flood event, though this July event was not as unstable and not as persistent. In the figure below, from a paper that examined details of the April 2018 event (Corrigan and Businger, 2022), note how the vertical wind profile in the frames share similarities to the profile from the 2 AM HST Līhuʻe Airport sounding on July 20. It’s these types of subtle details that can determine whether or not a flash flood event occurs in the Hanalei River basin. It keeps forecasters on their toes, and can be quite challenging to forecast in advance with enough lead time for key stakeholders to take meaningful mitigation actions. And they’re not just watching Hanalei River. They have to keep their eyes on conditions across the entire state!

Figure 7 from the Corrigan and Businger (2022) paper that dissected the conditions during the April 2018 record-breaking rainfall event on north Kauaʻi. The focus here is on the vertical wind profile.

Reference:

Corrigan, T. J., and S. Businger, 2022: The anatomy of a series of cloud bursts that eclipsed the U.S. rainfall record. Monthly Weather Review, 150, 753-773.