Tropical Storm Niala could dump up to 8 inches of rain over leeward Big Island areas resulting in flash flooding throughout the weekend.
Between 4 and 8 inches of rain is expected to deluge already soggy areas of North and South Kona by Monday evening as the storm passes south of the Big Island, said John Bravender, a meteorologist with the National Weather Service in Honolulu. Since mid-August, leeward areas of the Big Island have seen heavy rainfall that has resulted in flooding and damaged property and infrastructure.
“With as saturated as the ground is with all the rain that you’ve had recently it won’t take much to cause flooding. And, with a tropical storm like this, you will get very heavy downpours,” he said. “And, if the runoff can’t be absorbed, you can experience flooding in areas that normally don’t experience flooding.”
The worst of the showers and thunderstorms should be expected in the afternoon and evening on Saturday and Sunday, Bravender said. A flash flood watch remains posted through Monday. A tropical storm watch is also in effect.
“Because of all of this moisture coming over with Niala, the airmass will be saturated, so, a lot of the afternoon showers could be heavier than normal,” he said. During summer months, Kona typically sees onshore sea breezes that ascend the mountain slopes and interact with descending trade winds resulting in localized afternoon showers.
As of 5 p.m. Friday, the tropical storm was located about 375 miles southeast of the Big Island, tracking northwest at 7 mph and circulating 50 mph winds that extended outward up 80 miles from its center, according to the Central Pacific Hurricane Center in Honolulu. Some strengthening is expected through Saturday. Thereafter, steady weakening is forecast as shear picks up and helps to tear apart the storm.
Showers, as well as surf, were expected to begin picking up on the east side of the island Friday evening. By Monday evening, windward and southeastern slopes of the island could see 8 to 12 inches of rainfall. Some higher elevations could see up to 16 inches of rain, Bravender said.
High surf of 8 to 12 feet is forecast to impact east and southeast facing shores of the island through Monday morning, Bravender said. Overnight and afternoon high tides of 2 feet will make wave run up slightly higher during these times, particularly in low-lying areas such as Kapoho and Punaluu.
No significant surf or wave run up is expected along the leeward coast of the Big Island, forecasters said.
Hawaii County Civil Defense Administrator Darryl Oliveria said the agency is in close contact with the its state counterpart, the Hawaii Emergency Management Agency, as well as the National Weather Service and Central Pacific Hurricane Center. As of Friday evening, some rain bands were beginning to reach East Hawaii, but there were no reports of damage, flooding or inundation of property, he said.
“We are standing by, continuing communication and coordination amongst all county emergency management and civil defense agencies, especially Hawaii County, to ensure that they have what they need in order to prepare for and respond to any impacts Tropical Storm Niala might bring,” said Vern Miyagi, administrator of the Hawaii Emergency Management Agency.
Niala is expected to make its closest approach to the Big Island early Monday when it passes more than 100 miles south of South Point as a low-end tropical storm. Southern-most areas of the Big Island would likely have the greatest chance of experiencing tropical storm-force winds. Current wind speed probabilities indicate a 1 percent chance of 57 mph winds and a 6 percent chance of 39 mph winds at South Point on Saturday through early Sunday. That decreases to a 1 percent chance of 57 mph winds and a 3 percent chance of 39 mph winds on Sunday through early Monday.
There is less than a 2 percent chance of tropical storm-force winds of 39 mph impacting Kailua-Kona between Saturday and early Monday, according to the National Weather Service.
By Tuesday morning, Niala is expected to be a tropical depression packing 40 mph winds about 200 miles south-southwest of Kailua-Kona.
Elsewhere in the Central Pacific, no tropical cyclones are expected to form through Sunday afternoon.
In the Eastern Pacific, forecasters are monitoring two areas of low pressure off the coasts of Mexico and Guatemala. Just one, located about 400 miles south of Acapulco, Mexico, is forecast to develop into a tropical depression within the next couple of days.
