Just after 7:00 p.m. H.S.T. on Sunday (3:00 a.m. E.D.T. Monday), departing Tropical Storm Darby was starting to pass by Kauai, the northernmost of the populated Hawaiian Island chain. At that time, Darby was about 100 miles to the northwest of Honolulu (island of Oahu) (Fig. 1).
As part of a luckily-timed vacation to the 50th state, I was thrilled to have sort of experienced a weak tropical storm pass by some five thousand miles from home. So much for “being lucky” and so much for a meteorologist on vacation!
Three hours earlier (around 4:00 p.m. H.S.T.), clearing skies in the wake of Darby allowed the convective process to begin in earnest on Oahu. At this time, surface sunlight heating and upslope mountain wind flow allowed towering cumulus clouds to build along the southern slopes of the Koolau Mountains to the east and northeast of Waikiki Beach (Fig. 2). To the east of this convective development, a lingering north-south feeder band, associated with Darby, was slowly moving to the west.
Around 7:00 p.m. H.S.T., these two convective forces converged and heavy thunderstorms (yes, thunderstorms) began in earnest. Within 30 minutes, streets in Waikiki looked like lakes. Water began rising to curb level and, in some cases, rose high enough (and ran off roofs fast enough), that at least one restaurant had to close due to flooding.
We left that restaurant (without even being seated), and raced to a restaurant just a street away. Umbrellas were hard pressed to offer much protection, as water splashed up on us from the sidewalk and water-covered streets below (Fig. 3).
Upon arriving at the restaurant and trying to dry off a little, my cell phone alerted me to the flash flood warning for Oahu. The warning talked about rainfall rates of one to two inches per hour. A storm report noted that water was one foot deep on Keeaumoko Street in front of a Walmart store (located about a mile and a half west-northwest of my then location).
As we ate a rushed dinner, sitting partially water-logged in the restaurant, we listened to the sound of torrential rainfall outside. Interspersed, came rumbles of thunder and flashes of lightning. That’s when a report from the local emergency office indicated that water had ponded so deeply on Interstate Highway H-1 (a major east-west route along the southern part of Oahu) that the six lane highway had been decreased to two usable travel lanes. The report noted that water was pouring in from nearby School Street. Minutes later, a report was issued about H-1 being closed due to water inundation and/or stalled cars in the water.
Returning to our room, we watched and listened as the rain continued unabated until almost 10 p.m. H.S.T. Then, rain continued, although at a lesser rate, until after 1.00 a.m. H.S.T.
It was clear from observing the movement of the lightning sources (and confirming this with radar) that Waikiki and the surrounding area experienced “training” thunderstorms. One storm moved through and was followed immediately another and then another. This is much like railroad cars moving across a fixed point on railroad tracks. I estimate that at least 5 or 6 thunderstorms passed over Waikiki between 7:00 p.m. H.S.T and 10:00 p.m. H.S.T.
In my more than 60 years involved in meteorology, and having lived in many locations across the U.S., including south Florida, I don’t recall having ever personally experiencing such a long duration, heavy rainfall, training event.
Rainfall reports were hard to come by overnight. However, recognizing the steeply sloped terrain to the north of Waikiki, it seemed that significant flooding had to have occurred in normal valley/canyon flood-prone zones. The morning 24-hour rainfall collective showed at least four sites across Oahu that reported more than 10 inches of rain (Luluku – 10.56”; Waihee Pump – 10.38”; Maunaloa – 10.31”; and Nuuanu Upper – 10.17”). It would not surprise me to find other reports with even higher numbers.
Stay tuned and we’ll find out tomorrow. Meanwhile, heavy rainfall, associated with a line of training thunderstorms was falling across northwest parts of Oahu this morning.
So much for an attempt at a meteorologically-correct vacation!
© 2016 H. Michael Mogil
Originally posted 7/25/16