Hurricane Douglas satellite image
Infrared satellite image of Hurricane Douglas as of 1540Z (6:40 am HST) Saturday, July 25, 2020. (Image credit: RAMMB/CSU/Colorado State University)

Hurricane watches and tropical storm warnings were in effect for most of Hawaii on Saturday morning ahead of Hurricane Douglas, racing west-northwest across the remote North Pacific.

Douglas is weakening, as is typical for hurricanes approaching Hawaii, but the state could still experience serious impacts. Douglas is expected to sweep across or just north of the islands from Sunday into Sunday night as a Category 1 hurricane or a strong tropical storm.

As of 5 am HST (Hawaii Standard Time) Saturday, July 25, Douglas was a Category 2 storm with top sustained winds of 105 mph, according to the Central Pacific Hurricane Center (CPHC). The hurricane’s eye had clouded over, and the core of showers and thunderstorms (convection) was eroding on the south side. As it nears Hawaii, Douglas will encounter slightly warmer waters, but the twin effects of increasing wind shear and a drier environment will gradually take their toll on the hurricane.

Even so, Hawaii needs to take Douglas seriously. It’s uncommon for a named system to survive on a westward trek through the length of the islands. In fact, Maui and Lanai had never recorded a tropical storm landfall prior to Olivia (2018). With Douglas, there is high confidence on a west to west-northwest track on Sunday that will closely parallel the islands from Maui to Kauai. Along the way, Douglas’s intensity will likely be somewhere between a minimal hurricane and a strong tropical storm.

The main uncertainty involving impacts is whether Douglas’s track will stay well north of the islands or make landfall on one or more of them. As CPHC put it on Saturday morning, “Due to Douglas’ angle of approach to the islands, any small changes in the track could lead to significant differences in where the worst weather occurs. Even if the center remains offshore, severe impacts could still be realized over the islands, as they extend well away from the center.”

Douglas winds image
Probabilities of sustained tropical-storm winds (40 mph or greater) during the passage of Hurricane Douglas, as projected on Saturday, July 25, 2020. (Image credit: NOAA)

Depending on its track, Douglas could produce widespread sustained winds of tropical storm strength (40 mph or stronger), especially from Maui westward and particularly at higher elevations. Trees and power lines could be felled by the strong winds.

As those winds slam against the higher terrain on those islands, they will squeeze out torrential rain. Widespread totals of 5 to 10 inches, with even higher localized totals at elevation, may lead to flash floods and mudslides. Large swells will pound the islands starting as soon as Saturday.

7/24 Original Post: Hurricane Douglas to push wind, surf, squalls toward Hawaii

Hurricane Douglas
GeoColor visible satellite image of Hurricane Douglas at 1840Z (2:40 p.m. EDT) Friday, July 24, 2020. (Image credit: RAMMB/CIRA/Colorado State University)

After peaking as a formidable Category 4 storm, Hurricane Douglas is on a gradual downswing, but the Hawaiian Islands will still need to keep close tabs on Douglas this weekend. At 5:00 p.m. EDT Friday, July 24, NOAA’s National Hurricane Center (NHC) rated Douglas a Category 3 storm, with top sustained winds of 115 mph.

Douglas is cruising through the open northeast Pacific toward Hawaii at a relatively fast pace of about 18 mph. Forecast models predict that Douglas will slow down slightly and angle a bit more toward the west over the next couple of days. On that track, Douglas would parallel the islands from Maui westward on Sunday, perhaps moving over these islands or remaining just north of them. In either case, the storm will be weakening as it tracks west, so Maui appears to be the island most likely to receive the strongest initial impacts from Douglas.

A hurricane watch was issued Friday afternoon for the Big Island and for Maui County.  The farther south Douglas tracks, the more likely strong winds will affect one or more islands. If Douglas stays north of all the islands, then its winds will likely also remain below hurricane strength everywhere. In any event, wind gusts above tropical storm strength (40 mph) could be widespread, and sustained winds of more than 40 mph are possible. These winds could bring down trees and power lines if they are combined with heavy rains in the range of 6 to 10 inches – perhaps 10-15 inches wherever the strongest winds are forced up Hawaii’s steep mountainsides. Landslides and flash floods will be possible in localized areas.

Douglas is already generating large surf, and powerful swells will reach the islands on Saturday well ahead of the cyclone itself. Some coastal flooding can be expected with the high surf and swells, especially along east-facing shorelines.

Infrared image
Infrared image of Hurricane Douglas as of 1830Z (2:30 p.m. EDT) Friday, July 24, 2020, with Hawaii outlined at far left. Overlaid is the observed and predicted track from NHC as of 11 a.m. EDT Friday, with the yellow lines bracketing the “cone of uncertainty.” The numbered icons along the center of the cone show the projected strength on the Saffir-Simpson scale. (Image credit: SSEC/CIMSS/University of Wisconsin)

As is often the case, cooler waters en route to Hawaii will keep Douglas from posing a major hurricane threat. Douglas will pass over sea surface temperatures (SSTs) at or just below 26 degrees Celsius (79°F) from Friday night into Saturday before reaching slightly warmer waters near Hawaii on Sunday. Hurricanes typically struggle to maintain their central shield of showers and thunderstorms (convection), which serves as a heat engine and power source, when SSTs dip below 26 degrees Celsius. Increasingly dry air in the mid-levels of the atmosphere (relative humidity dropping from around 60% to 40-50%) will also erode Douglas’s convection as it approaches the 50th state.

Vertical wind shear will stay light to moderate through Saturday, so Douglas could remain fairly well organized even as its convection and top winds ramp down.

Hawaii’s history of hurricane encounters

Weakening hurricanes and tropical storms approaching from the east often pass by Hawaii or dissipate before they get there. It’s much less common for one of these to make a direct hit on the state. The first landfall on record in both Maui and Lanai occurred in September 2018 with Tropical Storm Olivia, which caused $25 million in damage. Olivia’s top sustained winds were 45 mph during those landfalls.

By far the most destructive hurricanes in Hawaii’s history are those arriving from the south, a path that allows them to remain stronger over warmer waters before final approach. The classic example is 1992’s Hurricane Iniki, which struck Kaua’i as a Category 4 storm. Iniki led to six deaths and more than $3 billion in damage. A hurricane that strong doesn’t have to make landfall to cause chaos, though. In 2018, Hurricane Lane churned south of the Big Island at Category 5 strength, then inched along an arcing path south of the rest of the state as it weakened. Along with $250 million in damage, Lane dumped 58 inches (1473 mm) of rain on Kahūnā Falls in the Big Island – the second heaviest rainfall on record from a tropical cyclone in any U.S. state, behind the 60.58″ dumped by Hurricane Harvey in 2017 in Texas.

Emergency kitEverything you need if a hurricane is forecast for your area

A study published in May in the Proceedings of the National Academy of Sciences found an increased frequency of tropical cyclones from 1980 to 2018 over a zone extending east and south from Hawaii, amid larger-scale trends that could be explained only by factoring in human-induced climate change. “We show for the first time that this observed geographic pattern cannot be explained only by natural variability,” lead author Hiroyuki Murakami said then in a NOAA news release.

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Bob Henson is a meteorologist and journalist based in Boulder, Colorado. He has written on weather and climate for the National Center for Atmospheric Research, Weather Underground, and many freelance...