Heavy rain with flash flooding is forecast for all the Hawaiian Islands, but residents likely won’t need to board up their windows because there is little chance that Tropical Storm Flossie will grow into to a hurricane, the National Weather Service said Saturday night.

Flossie will hit Hawaii island as early as mid-morning Monday, and Maui County by Monday afternoon, forecasters say.

When it is south of Oahu and Kauai on Tuesday, it will likely have weakened into a tropical depression, the weather service said.

The agency will have a better estimate today because the storm’s speed and direction could change.

The bottom line: "A slug of moisture is aimed at the state," said Michael Cantin, warning coordination meteorologist, adding that torrential rain is possible.

The windward sides of all islands could see 6 to 10 inches of rain through the duration of the storm, with 4 to 6 inches rainfall in other areas, and locally heavier rainfall in some areas.

East to northeast winds of 30 to 40 mph are predicted for Hawaii island, with gusts of up to 50 mph. Oahu may see sustained winds of 25 to 35 mph, with gusts up to 40 mph.

"These aren’t hurricane-force winds," Cantin said.

But he recommended tying down loose items and taking necessary precautions to avoid flooding.

At 8 p.m. Saturday, the storm was 775 nautical miles (891 statute miles) from landfall and was moving west at 20 mph, the weather service said.

Forecasters on Saturday issued a tropical storm watch for Hawaii and Maui counties and a flash-flood watch statewide, both effective Monday.

The main difference between a hurricane and a tropical storm is the wind speeds, and boarding up windows is unwarranted in the case of a tropical storm, Cantin said.

However, there is potential for power outages and minor damage. Residents are advised to prepare by having alternative ways to cook, flashlights and battery-operated radios.

Areas prone to the tradewinds may be hit hardest, including the saddle area of the Big Island, the Waimea corridor on the northern half of that island, channels of all islands, as well as leeward areas, Cantin said.

Hawaii and Maui counties have a 17 percent chance of tropical storm-force winds. Oahu has a 13 percent chance, and Kauai a 10 percent chance.

Flossie will also bring surf of 10 to 15 feet to all eastern shores.

State Civil Defense spokesman Maj. Jeff Hickman said the agency is monitoring the situation and will support the counties’ civil defense agencies.

The warning sirens will not sound for this event, he said. Sirens are sounded in the event of hurricanes, he said.

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Star-Advertiser reporter Michael Tsai contributed to this report.