Atypical weather conditions conspiring with a healthy dose of bad luck created ideal circumstances on Tuesday for wildfires to ignite on Maui and then spread, ravaging the island.
Even for an area accustomed to wildfire weather, Maui has been experiencing particularly low humidity and high winds this summer, and a drought has dried out its vegetation. The winds, strengthened by a passing hurricane and mountainous terrain, fanned the blazes.
Normally, the trade winds this time of year are moderate or breezy, and they move from high pressure toward low pressure. The greater the pressure difference and the closer the low and high pressure areas are to one another, the stronger the winds become.
This week, as Hurricane Dora passed hundreds of miles south of Hawaii, its wind field was not large enough to reach the islands. However, its pressure was so strong that it bolstered the trade winds, which reached 40 miles per hour in Maui.
Robert Bohlin, a meteorologist in the Weather Service’s Honolulu office, said that wind speeds could become even faster when they hit mountains.
When wind hits a mountain, it pushes over, then accelerates and crashes down the other side like a wave — not unlike when a strong water current strikes rock. That can add as much as 20 m.p.h. to wind speeds. The fluctuations, Mr. Bohlin said, make wildland fire fighting much more difficult. The West Maui mountain chain runs along the island’s western side.
In addition, the high pressure to the north of Maui created sinking air, which warms and dries, leading to lower humidity. The drier conditions acted as an accelerant, allowing the fire to spread more quickly. The pressure was also strong enough to transport drier atmospheric conditions all the way from California, which contributed to the overall lower humidity at the surface.
Then there was the drought.
As of Thursday morning, nearly 16 percent of Maui County, which encompasses the island of Maui and several others, was in what experts from the U.S. Drought Monitor consider a severe drought, up from mostly moderate drought last week. That likely contributed to the drying out of vegetation in the wildfire areas.
“The summer dry season has dried out many grassland areas in Hawaii, and dry fuels contribute to potential wildfire threats,” Mr. Bohlin said.
