Oahu and Kauai are not in the clear from possible heavy rain and flooding today, although conditions might improve, the National Weather Service said Tuesday night.

"Better weather is expected, but I will caution that there are early indications that this activity — unsettled weather — may linger" until today for both Oahu and Kauai, said National Weather Service lead forecaster Henry Lau said. "There’s a chance it may worsen for Oahu."

The Weather Service had a flash flood warning in place for Kauai until midnight Tuesday, and a flash flood watch was in place for Oahu, Kauai and Niihau until 6 p.m. today.

The weather service continued to assess conditions Tuesday night on whether the rain would spread southeastward to Maui County, Lau said.

A moist, unstable air mass will remain over the islands until today. The weather service said a passing upper trough is bringing the potential for heavy rain, thunderstorms and flash flooding.

"The air mass is pretty unstable, and heavy showers can flare up at any time," Lau said Tuesday night.

He cautioned that if heavy rain occurs in the Koo­lau on Oahu, water has to flow down to the coast, so streams and low-lying areas will be affected.

On Kauai "the Hanalei River had risen to where it has affected the bridge," Lau said.

Kauai County personnel shut down Kuhio Highway on Tuesday night in the area of the Hana­lei Bridge.

Although Hanalei is closer to the coast, heavy rain occurring upslope over the mountains was affecting the bridge area.

Low-lying areas, especially near streams, are susceptible to flooding.

As of 8:45 p.m. Tuesday, Kilo­hana, near Hana­lei, had a 24-hour rainfall total of 5.94 inches. Mount Wai­ale­ale had a total of 2.32 inches; Kapahi, above Kapaa, a total of 5.06 inches; and Wai­lua, 1.61 inches.