Tropical Storm Warnings in Hawaii for Darby

Tropical Storm Warnings are flying for the Big Island of Hawaii and Maui as Tropical Storm Darby steams westwards at 11 mph towards Hawaii. An Air Force hurricane hunter aircraft was in Darby Friday afternoon, and found top surface winds near 60 mph and a surface pressure of 999 mb. Friday afternoon satellite loops of Darby showed a modest area of heavy thunderstorms that were changing little in intensity. Wind shear was a moderate 10 - 20 knots, and sea surface temperatures (SSTs) were near 26.5°C (80°F), which just at the 80°F threshold needed to sustain a tropical storm. Rainfall from Darby should appear on radar out of the Big Island by Friday night.


Figure 1. Latest satellite image of Tropical Storm Darby.

Forecast for Darby
SSTs and wind shear are predicted to remain about the same through Saturday, so no significant changes in Darby's intensity are likely to occur as the storm approaches the Big Island and Maui. Slow weakening is likely beginning on Saturday evening as Darby interacts with the high terrain of the Big Island, and I expect Darby will affect Hawaii as a weaker storm than Tropical Storm Iselle of 2014, which hit the Big Island with sustained winds of 60 mph. Iselle caused about $80 million in damage.

Heavy rains causing flash flooding and mudslides are the main danger from Darby. The 12Z Friday run of the HWRF model predicted Darby would bring widespread rainfall amounts of 4 - 8" to all of the Hawaiian Islands, with some areas of 8 - 16" on the Big Island and Maui. Strong wind gusts will also be a concern, 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 Friday 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: 63%. Honolulu had a 33% chance, and Kahului, Maui a 51% chance.


Figure 2. Radar image from the South Hawaii radar at 7:49 am EDT August 8, 2014 of Tropical Storm Iselle near landfall on the Big Island. The radar beam was being intercepted by the high mountains of Hawaii, and could not "see" to the northwest.


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
Tropical Storm Iselle made landfall along the southeast shore of Hawaii's Big Island on August 8, 2014 as a tropical storm with 60 mph winds. Iselle was only the second tropical storm on record to hit the Big Island, and was the strongest. The Big Island's other tropical storm was an unnamed 1958 storm that had sustained winds of 50 mph at landfall. Iselle was just the fourth tropical storm or hurricane to make a direct hit on any Hawaiian Island since accurate records began in 1949. However, 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.

Eastern Pacific ties a July activity record: 7 named storms
The Eastern Pacific's record parade of July tropical cyclones continued this week, with the formation of Tropical Storm Georgette and Tropical Storm Frank. Both storms are expected to reach Category 1 hurricane status, but neither is expected to affect land. Georgette is the seventh named storm to form in the Eastern Pacific this month, tying the July record for named storms set in 1985. Since July 2, we've had Tropical Storm Agatha, Category 4 Hurricane Blas, Category 2 Hurricane Celia, Category 3 Hurricane Darby, Tropical Storm Estelle, and now Frank and Georgette. This puts us well ahead of climatology: the Eastern Pacific usually does not see its seventh named storm until August 7, its third hurricane until July 27, and its second major hurricane until August 19. An average season has 15 named storms, 8 hurricanes, and 3 major hurricanes.


Figure 4. VIIRS visible satellite image of Tropical Storms Darby, Estelle, and Frank, plus Tropical Depression 8-E, taken at 8 pm EDT Thursday July 21, 2016. Image credit: NASA.

Atlantic still slumbering--but for how long?
As is usually the case when the Eastern Pacific is active, the Atlantic is quiet. This inverse correlation in activity occurs because the conditions over the Eastern Pacific driving this July's bounteous activity--surface low pressure and rising air--creates a compensating area of sinking air over the tropical Atlantic, creating surface high pressure and dry weather--the antithesis of conditions needed for tropical cyclone formation. There are no tropical cyclone threat areas in the Atlantic to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming five days. However, for the first time all month, NHC is not highlighting any new areas for tropical cyclone development in the Eastern Pacific over the next five days, and the GFS model is not predicting anything new will form. That may mean it will be the Atlantic's turn to get active during the last few days of July, and a number of recent runs of the GFS model have been highlighting the possible development late this month of a tropical wave predicted to come off the coast of Africa around July 27. There will be plenty of dry air and dust from the Saharan Air Layer (SAL) around through the end of the month, though, which will likely make it challenging for any tropical waves to develop (check out this animation of the 10-day African dust forecast from NASA.)

Jeff Masters