Darby Falling Apart as it Makes Landfall on Hawaii's Big Island

By: Jeff Masters , 10:57PM,GMT on July 23,2016



 

Tropical Storm Warnings are flying for the Big Island of Hawaii, Maui and Oahu as Tropical Storm Darby dashes itself against the high mountains of the Big Island. Saturday evening satellite loops showed that Darby was becoming misshapen and disorganized as it made landfall on the Big Island, and radar on the Big Island showed a highly asymmetric storm, with all of the heavy rains confined to the southeast side.


Figure 1. True-color MODIS image of Tropical Storm Darby approaching the Big Island of Hawaii at 21:30 UTC (5:30 pm EDT) July 23, 2016. At the time, Darby had top winds of 45 mph. Image credit: NASA.

Forecast for Darby
On August 8, 2014, Tropical Storm Iselle of 2014 passed directly over the Big Island, and the 13,000-foot high peaks of Mauna Loa and Mauna Kea basically shredded the storm apart. I expect Darby will suffer a similar fate, and will be downgraded to a tropical depression on Sunday. Heavy rains causing flash flooding and mudslides are the main danger from Darby. Widespread rainfall amounts of 3 - 5" will likely affect all of the Hawaiian Islands, with some areas of 5 - 10" on the Big Island. High surf of 15 - 25' that will cause erosion problems on the southeast side of the Big Island are another concern. Strong wind gusts will also be an issue, as the high volcanoes of the Big Island and Hawaii will act to create damaging wind gusts in some areas, even if the sustained winds affecting land are below tropical storm strength. In their 5 pm EDT Saturday Wind Probability Forecast, the Central Pacific Hurricane Center gave Hilo on the Big Island the greatest chance of seeing tropical storm-force winds of 39+ mph: 99%. Honolulu had a 39% chance, and Kahului, Maui a 31% chance. I think these wind probabilities are too high, and that no locations in the islands at sea level will see sustained winds of 39+ mph.


Figure 2. Composite radar image of Tropical Storm Darby taken at 6:30 pm EDT (12:30 pm HST) on Saturday, July 23, 2016. Darby's heaviest rains were confined to the southeast side of the storm.


Figure 3. Tracks of all tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 100 miles of the Hawaiian Islands, 1949 - 2014. Hurricanes approaching from the east typically fall apart before they reach Hawaii due to the cool waters and dry air that lie to the east of the islands. Only two named storms approaching from the east have hit the islands since 1949, an unnamed 1958 tropical storm and Tropical Storm Iselle of 2014, which hit the Big Island. Hurricanes approaching from the south represent the biggest danger to the islands, due to the warmer waters and more unstable air present to the south. The only two major hurricanes to have affected the islands since 1949, Hurricane Iniki of 1992 and Hurricane Dot of 1959, both came from the south. Image credit: NOAA/CSC.

Direct hits by tropical storms and hurricane are uncommon in Hawaii
It appears likely that Darby, though weakening, will officially be a tropical storm that makes landfall on the Big Island. This would make Darby just the fifth named storm since 1949 to make landfall on a Hawaiian Island. The others:

Tropical Storm Iselle, which made landfall along the southeast shore of Hawaii's Big Island on August 8, 2014 as a tropical storm with 60 mph winds. Iselle killed one person and did $79 million in damage.

Hurricane Iniki, which hit Kauai as a Category 4 hurricane, killing 6 and causing $1.8 billion in damage (1992 dollars.)

Hurricane Dot, which hit Kauai as a Category 1 hurricane, causing 6 indirect deaths and $6 million in damage (1959 dollars.)

An unnamed 1958 storm that had sustained winds of 50 mph at landfall on the Big Island. The storm killed one person and caused $0.5 million in damage.

Hawaii has seen a lot of activity over the past three years, which may be a harbinger of things to come--see my August 2014 post, Climate Change May Increase the Number of Hawaiian Hurricanes.

The winds at 13,000 feet on Mauna Kea
The weather on top of the highest point in Hawaii, the Big Island's Mauna Kea, elevation 13,796' (4,205 m), will be interesting to follow as Darby makes landfall. Winds have risen steadily today, and several of the six anemometers reported sustained winds in excess of 30 mph on Saturday afternoon. However, beware of the data from the Canada - France - Hawaii Telescope (CFHT). The Mauna Kea webcam page says that those winds are highly exaggerated due to location of the anemometer tower between two large telescope domes. You can see this tower on Google Maps.

Links
Weather on Mauna Kea
Live stream from KHON2 TV in Honolulu
Central Pacific Hurricane Center
2-km resolution WRF model output from the University of Hawaii for Hawaii
Storm surge maps for Oahu
Storm info from Tropical Tidbits
NWS Honolulu

Jeff Masters