Douglas could become only the third hurricane in modern history to make landfall in Hawaii.
There have been a handful of tropical storm landfalls on the islands, but hurricane-strength storms have been very rare.
The two occasions since 1900 when a hurricane has made landfall were Hurricane Iniki, in 1992, and Hurricane Dot, in 1959. Iniki was a strong Category 4 storm at landfall and Hurricane Dot a Category 1.
"It is fairly common for hurricanes to track towards Hawaii, but they usually dissipate or at least weaken considerably before impacting the islands," said Phil Klotzbach, a research scientist at Colorado State University.
"For example, both Lane and Olivia impacted Hawaii in 2018. Also, in 2016, both Lester and Madeline threatened Hawaii."
On Sunday, Douglas was a Category 1 storm situated just to the north of Hawaii's Big Island, and east of Maui. It is headed west-northwest towards the islands of O'ahu and Kauai, where landfall is possible in one or both locations.
Hurricane warnings are in effect for Oahu and Kauai County, and tropical storm warnings are in effect for Hawaii County and Maui County.
The main threats are storm surge, dangerous surf, gusty winds, and heavy rainfall.
"With the storm center moving along the north side of the islands, north-and east-facing shores will feel the brunt of the wind and high surf at first," said CNN Meteorologist Chad Myers. "As the storm continues to the west today, the winds and high surf will turn to affect west- and south-facing shores as well."
The northern islands will also see the majority of the rainfall. Widespread amounts are likely to be 2-4 inches on the Big Island, whereas rainfall totals will likely exceed 5-8 inches from Maui County to Kauai County. Some isolated locations with elevated terrain could pick up as much as 15 inches of rain before the system moves out on Monday.
Slow start to the eastern Pacific hurricane season
In a season that has seen early storm formation in the Atlantic, the eastern Pacific has been slower for storm development than in previous years.
"During the period of reliable records, this is the fourth-latest date in which the first hurricane of the season has formed," according to the National Hurricane Center.
A slow Pacific hurricane season, especially when paired with an active Atlantic hurricane season, is a sign of a La Niña event, which forecasters have predicted could occur this year.
Under La Niña, global convection wind currents yield sinking air over the eastern Pacific, and rising air over the western Atlantic.
Sinking air patterns increase wind shear, a sudden shift in wind direction, speed or both, which can rip apart hurricanes before they have a chance to grow. Rising air creates a favorable environment for tropical storm development, which is why all eyes are on the Atlantic this season.