(BIVN) – A Flash Flood Watch is now in effect for the entire state of Hawaii through 6 p.m. Sunday.

“An upper level disturbance interacting with an old front will result in periods of heavy rainfall and a slight chance of thunderstorms through Sunday,” the National Weather Service in Honolulu says. “Flash flooding may result.”

There has been a steady rain in Hilo, where participants are preparing for the Merrie Monarch Festival Royal Parade. Umbrellas are recommended.

From the National Weather Service forecast discussion:

Deep tropical moisture will remain over the islands through Sunday, with precipitable water values in the 1.8 to 2.1 inch range. Some drier air will then begin to push southward into Kauai and Oahu Sunday night, while conditions remain really moist over Maui County and the Big Island. The combination of the deep moisture and shortwave energy over the state will lead to a fairly wet and unstable weekend in most areas, with the potential for heavy rainfall and thunderstorms. As a result a Flash Flood Watch remains in effect for the entire island chain through 6 PM Sunday.

“A Flash Flood Watch means that conditions may develop that lead to flash flooding,” forecasters explain. “Rapidly rising water levels may also occur on area streams and significant flooding may occur in low lying and poor drainage areas.”