Tropical Storm Flossie strengthened overnight as it headed toward Maui County and Hawaii island, where it could come ashore after midnight tonight.

Hawaii and Maui counties are under tropical storm warnings and a statewide flash flood watch is in effect as Flossie threatens to bring strong winds, heavy rains and potentially dangerous surf to all islands.

The warning, issued at 11 p.m. Saturday, means that tropical storm-force sustained winds of 39 mph or more are expected within 36 hours. Oahu is under a tropical storm watch, which means tropical storm conditions are expected within 48 hours.

A flash flood watch has been issued for all islands from Monday morning through Tuesday night.

The storm could drop up to 15 inches of rain to windward areas of Maui and Hawaii counties, and 6 to 10 inches in other areas, forecasters said. Up to 12 inches of rain could fall on windward Oahu and 4 to 8 inches in central and leeward areas. Kauai may see 2 to 4 inches of rain, with up to 6 inches on windward slopes.

"The rain will be widespread, will affect both the upper and lower elevations. Rock and mudslides caused by the rainfall will be possible around or near mountain slopes. The heavy rain will also fall over urban areas in the lower elevations, which will be more susceptible to flooding problems," the National Weather Service said.

Flossie’s maximum sustained winds increased to 65 mph overnight with higher gusts, forecasters said. The projected storm path is also a little bit north of what was predicted Saturday. The storm is expected to weaken as it moves towards and over the islands, but the current forecast track keeps it as a tropical storm through Wednesday.

As of 8 a.m. today, Flossie was moving west at 20 mph and was about 490 miles east of Hilo and 680 miles east southeast of Honolulu. Tropical storm force winds extended 175 miles from the center.

Flossie is predicted make landfall on the Big Island and/or Maui as a tropical storm. It will hit Hawaii island first and move over Maui later on Monday.

The heavier rain potential is on the northern half of the storm.

The maximum sustained winds of 65 mph are slightly higher than the  60 mph winds on Saturday, but below the storm’s peak of 70 mph Friday night when the storm generated maximum sustained winds just under category 1 hurricane status of 74 mph.

Honolulu forecasters repeated today that "abundant tropical moisture associated with Flossie will bring elevated potential for very heavy rainfall and flash flooding" throughout the state Monday and Tuesday.

Hilo could start experiencing the effects of Flossie Sunday as high surf generated by the storm hits east shores.

Early Monday morning, Hawaii county residents may see frequent showers, possible thunderstorms and 15 to 20 mph north winds and gusts to 35 mph increasing to northwest 25 to 35 mph with gusts to 45 mph.  During the day, Hilo can expect northwest winds 35 to 45 mph with gusts to 60 mph shifting to the south 30 to 35 mph with gusts to 60 mph early in the afternoon, then decreasing to 20 to 25 mph with gusts to 60 mph late in the afternoon.

Maui conditions will be similar, starting later in the morning.

Honolulu could  also start to see heavy rain and windy conditions during the day on Monday, with possible tropical storm conditions, including the chance of thunderstorms, Monday night through Tuesday.

Winds in Honolulu Monday night will be from the east at 30 to 40 mph with gusts to 50 mph decreasing to 25 to 30 mph with gusts to 40 mph after midnight. 

The storm will also bring high surf to east shores. The weather service marine forecast for Hawaii said Flossie “may bring dangerous surf to east facing shores."

Surf up to 18 feet is possible for east-facing shores of the Big Island and Maui County, while other islands could see up to 15-foot waves.

Forecasters and emergency management officials statewide say the public should be prepared and have stocked emergency kits on hand.