The National Weather Service on Friday afternoon posted a storm watch for the Big Island for late tonight through Monday through Monday afternoon.

In addition, a tropical storm warning is in effect for all Hawaii offshore waters between 40 and 240 nautical miles offshore.

As of 5 p.m. Friday, Tropical Storm Moke (pronounced “mo-kay”) was centered about 595 miles southeast of Hilo, traveling to the west-northwest at 12 mph. Tropical-storm-force winds extended up to 70 mph from the storm’s center.

“Rainfall totals of 5 to 10 inches, locally up to 15 inches, are possible over the windward and southeast slopes of the Big Island,” NWS forecasters noted. “It will not take a significant amount of rain to cause flash flooding due to saturated grounds from recent heavy rain.”

Kevin Kodama, retired NWS Honolulu hydrologist, said the major concern of forecasters “is the threat of heavy rainfall and flash flooding” Moke is likely to bring in the wake of the damage done last weekend, especially in Ka‘u, by Hurricane Lala.

“The forecast amounts may not be as high as what occurred during Lala, but the southeastern portion of the Big Island is very fragile right now so any heavy rainfall over residually saturated soils could produce flash flooding quicker than usual,” Kodama wrote in his “Mauka Showers” blog. “Many drainage channels from Kapapala to Waiohinu are filled with rocks and debris, reducing channel capacity and the ability to carry floodwaters. This increases the risk of alternate flow paths down slope.

“Temporary roads spanning channels with washed-out bridges are also at significant risk from any flooding.”

Kodama said it’s unfortunate for Big Islanders to “have to deal with another tropical cyclone so soon after Hurricane Lala.”

Despite Moke’s eye path being forecast to track further south of the Big Island than the 30-mile distance by Lala, forecasters still expect significant rain impacts across the Hawaiian Islands due to the size of the storm, which has precipitation bands extending more than 500 miles to the north of the storm’s eye.

“The rain bands are exceptionally large,” said AccuWeather meteorologist Alex DaSilva. “In comparison to Lala, the rain bands extend about 200 miles further north from the eye.

“Given the intense rain impacts from Lala last weekend, this is a vulnerable region that needs to be prepared for more flooding.”

Although it’s rare for Hawaii to be affected by two tropical cyclones within in a week, it’s not unprecedented. In 2016, Tropical Storm Madeline passed just south of the Big Island on Aug. 31 into Sept. 1, followed just days later by Hurricane Lester, which tracked north of the islands.

“Madeline produced nearly 11 inches of rain in parts of the Big Island, wind gusts around 60 mph, localized flooding and power outages, and surf as high as 25 feet,” said Jonathan Porter, AccuWeather chief meteorologist. “Lester then generated a surf of 10 to 20 feet along east-facing shores, requiring numerous water rescues.

“Fortunately, neither storm caused significant damage or serious injuries in Hawaii.”

The county late Friday afternon said that Civil Defense received 544 reports of residential damage and 56 of commercial damage connected to Hurricane Lala.

In addition, Hawaiian Electric Co. crews continued to restore power to Big Island customers who were left in the dark by Lala. HECO has brought in additional personnel from Southern California Edison to kokua HECO’s employees and contractors on Hawaii Island who, according to the local utility, “have been working around the clock to repair equipment and restore power ahead of the new potential storm system approaching the state.”

Between Thursday morning and late Friday afternoon, HECO’s restoration efforts reduced the number of customers without power on Hawaii Island from about 13,000 to about 9,600.

Email John Burnett at john.burnett@hawaiitribune-herald.com.