The threat of heavy rainfall and flash flooding from a returning weather system that dumped dozens of inches of rain on Kauai in a 24-hour period last week prompted weather officials to post a flash flood watch for the entire state.

The flash flood watch, which covers all the isles from Niihau to the Big Island, is scheduled to go into effect Thursday morning and last through Friday afternoon.

An old, frontal boundary that has been lingering across the state, causing deep moisture, in combination with an upper-level disturbance passing by could cause heavy rainfall and thunderstorms, according to Derek Wroe, a National Weather Service meteorologist.

The heaviest rainfall is expected to fall from Oahu eastward to the Big Island.

“Right now, one of the main differences between last weekend and this is the tradewinds are a little stronger and deeper, which would typically cause showers to move through and not stay stationary in one area for a prolonged period of time, which is what happened last weekend,” said Wroe. “There’s not much of a chance of that happening. That said, the ground is saturated and it wouldn’t take a significant amount of rainfall to cause some flash flooding.”

A winter storm watch for the summits of Hawaii island, with heavy snow possible above 12,000 feet, also goes into effect from 6 a.m. Thursday to 6 p.m. Friday. Between six to 12 inches of snow may accumulate, according to weather officials, who warn that the roads to the summits may rapidly be covered with snow, resulting in difficult travel conditions.