Darby’s rainfall, wind and surf affecting the Hawaiian Islands (H. Michael Mogil, CCM, CBM, DMS)
Tropical Storm Darby continues to plod along to the west, refusing to make that anticipated turn to the northwest. As a result, instead of skirting the Hawaiian Islands on their northeast flanks, Darby has been trekking just to the southwest of the island chain (Fig. 1). This has allowed him to weaken a bit more quickly because the stronger northern sections of the storm passed over the mountainous Big Island of Hawaii. With mountain peaks topping 10,000 feet there, Darby’s low-level circulation has been significantly disrupted. Still, wind gusts in mountain passes and at higher elevations have been significantly stronger than winds at airports nearer the coast. 
For example, in the Koolan Range in the east-central part of Oahu, winds have gusted to 60 miles per hour. In the Kaupo Gap on Maui (south facing slopes of Mount Haleakala), winds have gusted to 51 miles per hour (Fig. 2). Other reports from the Hawaii mesonet showed sustained winds of 41 miles per hour, gusting to 55 miles per hour at just northeast of Kawaihae and 41 mile per hour sustained winds with gusts to 53 miles per hour at Kamuela (both on the Big island).
However, winds are not really the big story about Darcy, even though forecasts and advisories (including media stories such as the headline in the Honolulu Advertiser on Jul. 23, 2016 (Fig. 3) have been headlining the storm and its winds. Rather, rainfall and surf continue to offer the greatest threats.
According to several National Weather Service (NWS) issuances early on Sun., Jul. 24 (Hawaiian Standard Time):
- A flash flood watch remains in effect for today on the Big Island of Hawaii; through tonight for Maui, Molokai, Lani and Kahoolawe; through Monday morning for Oahu; and through Monday afternoon for Kauai and Niihau.
- A flood advisory has been issued for parts of Maui. During overnight hours, radar showed heavy rainfall (rates of two inches per hour) on southeast-facing slopes of Mount Haleakala and the west Maui mountains.
- A flash flood watch replaced the flash flood warning for parts of the Big Island of Hawaii early this morning. Still, heavy rainfall had fallen on southeast parts of the Big Island (with some locations topping six inches). Hawaii County Civil Defense has reported that Highway 11 near mile marker 59 was closed due to flooding.
- The NWS provided additional very specific details about the potential for heavy rainfall across the island chain – “… rain bands around Tropical Storm Darby will bring frequent showers and isolated thunderstorms … with periods of heavy rainfall and flash flooding possible.” For Oahu, storm total rainfall amounts of three to five inches are possible) with isolated amounts up to 10 inches in higher terrain. For other islands still in Darby’s path (Kahoolawe, Lanai, Maui and Molokai), 5 to 10 inches of rainfall is expected with local amounts of up to 15 inches. The Big Island, starting to move into a lesser heavier rainfall potential, can still expect 5 to 8 inches of rainfall.
- “In addition to flood prone areas…heavy rain events of this size may cause flooding in areas outside designated flood zones. Low spots in roads can become dangerous and impassible due to severe runoff. High amounts of debris in streams and gulches may clog bridges and culverts resulting in dangerous flooding (even outside of normal channels).”
The heaviest official rainfall report through early this morning was 2.11 inches at Hilo on the east side of the Big Island. However, there were two reports of more than six inches on the northwest part of Maui.
NWS radar estimates also show much higher amounts on the south-facing sides of Mauna Loa (Big Island) and along some of northeast-facing slopes of the northwest-southeast mountain ranges on some of the islands (Fig. 4).
Finally, and not to be forgotten, a high surf warning is posted for east-facing shores of Maui and the Big Island and a high surf advisory covers east-facing shores of Kauai, Oahu, and Molokai, both until 6:00 p.m. H.S.T. today. Waves of 6 to 10 feet are expected.
© 2016 H. Michael Mogil
Originally posted 7/24/16