The National Weather Service extended a flash flood watch to include all the Hawaiian Islands as moist and unstable weather spreads over the entire state.

Forecasters say there is a chance of heavy rainfall, possibly thunderstorms, through Thursday.

A winter storm watch is also in effect starting tomorrow morning for the summits of Mauna Loa and Mauna Kea because of the chance of thunderstorms and heavy snow.

Radar shows clouds have thinned and a few showers on Oahu and Molokai, and along the southeast coasts of Maui and the Big Island this morning.

Despite the flood watch, today’s forecast for Honolulu calls for mostly sunny skies.

"But any showers that do form could be locally heavy and cause flash flooding," said Bob Burke, a forecaster with the National Weather Service.

Burke said afternoon heating and heavy moisture in the air could create isolated showers this afternoon.

A upper-level trough, or low-pressure system in the atmosphere, is north of the islands and expected to intensify and move over the state late tonight and tomorrow, Burke said.

The system will create conditions that could lead to heavy showers and thunderstorms, mostly on Maui and the Big Island, he said. But the heavy rain could also spread to Oahu and Kauai.

Cooling tradewinds could return on Thursday. But Burke said another computer forecast shows the humid and hazy conditions continuing into the weekend.

There are tradewinds near the Big Island and that may keep some of the vog south of Oahu and Kauai, he said.

If the tradewinds do come back, enjoy it while it lasts. Burke says another low-pressure system may be developing that will bring variable or southerly winds and humid and voggy conditions next week.